Dec 20, 1990 - life. Ma> 'j-js i-.esis be i. rir..i^ c:~.pensation to them for beLng away from home. Special thanks go ...... The first approach requires extending a relationalstorage system to accommodate: ⢠the ..... [Demers SS] alcor/j---r. allo-»>.'s for a ...... Tne foundation ofthe system is the Version Manager,which is used to ...
V^'^^'^ o' T^^
03^^
HD28 .M414 no .3131-
[DEC
20 1990
J
TECHNICAL REPORT
OBJECT SHARING IX A MULTI-USER
HYPERTEXT SYSTEM M.S. Thesis
Melvina H. Tarazi
CCSTR
^ 101
CENTER FOR COORDINATION SCIENCE
Massachusetts Institute of Technology Sloan School of Management Cambridge, Massachusetts
,^^3,3Cj
OBJECT SHARING IX A MULTI-USER
HYPERTEXT SYSTEM M.S. Thesis
Melvina H. Tarazi
CCS TR^
101
^^^f^3l5^
'ft
^l^"
,30
\
OBJECT SHARING
IN
A MULTI-USER HYPERTEXT
SYSTEM
McK-ma H S
P
Tarazi
Co'-r-uie: Sc:e-ct and Engineering
Massa;.". jsens insurjte of
Technoiog>
(19&S;
SL'BMITTED TO THE DEP.^RTMENT OF ELECTRICAL ENCOs'ZERING ANT) CONIPITER SCIENCE IN" ?.\r~^M. FLIFILl-MENT OF THE REQLTRENIENTS FOR THE DEGREE OF
MASTER OF SCIENCE a;
ihs
NL^SSACHVSETTS INSTTTITE OF TECHNOLOGY
Ccp>Tigh: Tr.t 2_'_-.r: nereby grar/.s
cones of
(ci
MTI
iT-s ihes:s
1985
-
Me!'.
-j-.a
H
Tarazi
perrrussior. to reproduce
document
in
whole or
and to distnbute
v. rar..
Simanirc of AuLhor E>eparmcnt of Eiectncal Engineenng and Computer Science J'onc 20. 1989
Cenified b%
\M^jgy%
mI— Professor
Tnomas W. Malone Tnesis Supcr^•lsor
Accepted
b)
Professor .Ajiniir C. Srmin Chairman, Comrruoee on Graduaxe Students
OBJECT SHARING
IN A
ML LTI-LSER HYPERTEXT
SYSTEM MeivLTiE
Subrrjne-d tc the Depa-trr.er.: of
Science
or. J jr.:
2C. ]9^'^
ir.
K
Tarazi
E!emcal EngLneerinc and Computer
rar.ia] fj-I:TLimen: of the
requiremen*^ for the
decT&e of Masier of Science.
Abstract Obieci-onenied da-^ba^es
2.-ic
r.\-penex! sysier-.s are emergir.g
appiiactior, developers for Lncreased. less sn--crkired modelir.g
ir.
response :o a
power
m
demand by
database systems.
prcMdir.e the necessary tools and facmues for building Such sy5;err.5 succeed coopera::\e v.ork arr'.ications bu: are luTuted ui providing the appropnate object sharing envirormentl'-.
propose to adcL'^ss the object sharj^g requirements for one such system - Obiec: Lens Object Ler.b ir.:erra-.e> fearjres z: r.;.T^r.ex: system.s. object-oriented systems and ruiebased agents. 'V^'e
We
evaluate various approaches
tc
object sharing (includL-.g
message passing, centralized
and various scnemes for concurrency conrrol timestampung with respect to the charaaenstics desired
object server and dismr^ted objec: servers
and update propagation (lockiLng. m Object Lens (e g.. speciaLzatior hierarchy, user-mterface. object linking, version control and combination of long Lnteract;\e transactions and short automatic ransacuons). >
We
propose a nev. scheme for j'jtiating object sharing through the exchange of electronic mail messages. Object protecticr. is achieved by a hybnd scheme oi access control lists and capability systems.
Analysis of the transacuons
in
Object Lens reveals rwo sets of transactions; (1) interactive weak consistency requirements and relatively flexible automate transactions that require stna concurrency control
transactions that require relatively
concurrency control, ^.d. ;I; Vv"e propose a hybnd locking and version control concurrency control scheme that accomodates the two types of transacuons.
requirements.
Thesis Superv'isor:
Profes. or
Thomas W. Malone
Title:
Panck
McGovem
J
Professor of Inform.aaon Systems
Dedication
To
My Parents
Acknowledgements
I
vi.o_id Il^s tc tharik
guidance, and suppon
my
thesis super\isor. Professor
Tnomas MaJone.
for his
ir.s:gr.:.
His ide^. o?ser\'aao.i5, and suggesuons have deeply contributed to
the success of mis uo.-i..
I V.
culd als: iixe to
'S.zr^.
His sugges:;?r.s. cnucues
M> needed life.
Kf %
comments
l-.
Crowston
v.ere
parents. Reine and Kar„. deserve
ne—
Ma>
ar.d
Nl:
'j-js i-.esis
be
i.
rir..i^
exremeiy helpful
more than
ar.c for ell -jr.^s .:.r. affecrion.
.
for reading several drar.s cf iiis vk'ork.
I
can express for being nere
guidance and encouragemen; throughou;
c:~.pensation to them for beLng away from
Special thanks go to m.> bro'-her. Ram^i. for listening to
throughou: m.>
Fmall)
.
s:a> a:
I
uould
Ml
when
I
my
home
m> gnpes and complaints
7
like to
acknowledge Ibrahim Saad, Nazhiri Zarghamec, Ala Alry^es.
Emar. Hashem and Ahjnad Tabari for their constant friendship throughout
my
s'^ay ai
M J.T.
Table of Contents
Abstract
2
Dedication
3
AcknowiedgemenLs
4
Table of Contents
5
List of Figures
7
List of Tables
8
1.
9
Introduction 1.1
Object Sharing in Object-Onented Databases and H\-pcncxt Systems
10
1.2
The Proposed Objea Sharing Scheme
1
Objec: Lens Thesis Afjproach 1.5 Thesis Oauine
11
1.3 1
2.
.4
1
14
Architectures for Object Sharing 2.1
2.2
15
Message Based Object Sharing 2.1.1 Advantages of .Approach 2.1.2 Disad\an;ages o: Approach Cenc-alized Object Shanng 2.2.1 Adsantages of .Apjproach 2.2.2 Disadvantages of Approach 2.2.3 Improved Performance using Caching 2.2.4 Lmpicmentanon of Ccntrai Data Ser-er
Database Sever 2.2.4.2 CentraJ Objea Sever 2.3 Distributed Object Shanng 2.2.4.1 Central
Objea Server Objea Server and Manager
2.3.3.1 Distributed
3.
Object Sharing Features
ai each Workstailon Relanonal Database Systems
in a Distributed
Environment
3.1 Replication
3.2 Consistency and 3.3
Update Propagation
Concurrency Control 3.3.1
18 18
19
20 20 23 23 25
Ad\antagcs of Approach 2.3.2 Disadvantages of Approach 2.3.3 Imolcmcntauon of a Distributed Data Server
2.3.3.3 E>istnbuted
P
24
2.3.1
2.3.3.2
17
Synchronization Techniques Based on Two-Phase Locking
2PL Implcmentanon Pnmarv Copy 2PL Voting 2PL Centralized 2PL
26 26 28
29 32 33
35 35 36 37 40
3.3.1.1 Basic
40
3.3.
41 41
1
.2
3.3.1.3 3.3.1.4
Locking 3.3.1.6 Deadlock betecuon 3.3.2 S>Tichroruzauon Techniques Based on Timestamp Ordering 3.3.1.5 Granulann, of
42 42 42 44
1
-6-
3.3 2
Basi: TLTiestamp Ordenrc
;
3,3.2 2 MultiN ersior. Tunes'.arr.p Orderj^z 3.3.2.3 Conser^atlve 3.3.3 S\T.:rjor.:zuUor.
Tunesiamr Oraenng
Techniques Bas&d on Qptimisnc Cor.roi
3 3.3.; Ma'cr.r\ Cons-er.s'-!;
Dismbuiec Cenificaucn
3 3.3 2
Tnree MeihcKJologies OLher ConcurrcncN Control Schemes 3.4 Secur.p> and Protecuor. 3.3.- Ccrnz;a-~-^on cf the
3.3.5
3.4.1 Capabi]ir\ Sys'^rr.s
3.4.2 .Access 4.
Conro!
List
System
Literature Surve%
4.3 Mui-j-use: H\7>ertext
Systems
An example
Multi-user Object Lens 6.2
72
TemiLTolop
6.3 ObieriS in Distnbuted Obiect
Lens
Personal Obieas Public Objects 6.3.3 Local vs. Remote Objects 6 4 Creation of a Shared Object 6.5 Deletion of a Snared Object 6.6 Protection cf a Shared Oc^^?:: 6.'' Modification of a Shared Objea 6.'.1 Transactions in Object Lens 6.7.1.1 Interaaive transacuons
Shared
6.3.1
vs.
6.3.2 PnNate \s
L2 Automatic Transacpons ^ Concurrency Control
6.7.
6.7.2
6.7.2.1
Time-St^mp Ordering
74 75 75 76 77 77 81 81
85 87 88
90 92 92
6.7.2.2
Optirmsuc Concurrency
93
6.7.2.3
Two-Phase Locking
94 96 97
6.7.2.4 Version Control 6.7.2.5
7.
64 65 65 66 69 70
'"3
Decisions
6.1 Early Desipr.
55
64
Objec: Lens as a Hv-penex! S\-s:em 5.2 Objec: Lens as an ODjeci-Onented Database System 5.3 Obtec: Lens Specific Features 5 4 Using Object Len^ 5.5 Objer. Lens Axchitecrurs 5.1
6.
52 59
Object Lens Re\isiled
5.6
46 4b 46 48 4f 49 49 50 50 52
4.1 Distributed Reiat;ona; Database Systems 4.2 ODiec:-Oner.ted DataDS-se Systems
5.
a^ ^5
Proposed S>Tichroni2ation Technique: Hybrid of Locking and Versiorung
Conclusion
106
Work
7.1
Summan-
7.2
The Proposed Distributed Object Lens
1 1
7.3
Direcuon of Future Vvork
113
of
106
List of Figures
Figure 2-1: Ccr.ralized Orjec: Server Figure 2-2: Dismbuied Object Scr\er Figure 2-3: Objec: Ser^cr and Manager ai each Wortcstaiion Figure 2-4: Distributed Relauonai Database Figure 3-1: Example of Conflias in transarjons Figure 3-2: Deadlock Siruation Figure 3-3: Wait -for Graph Figure 5-1: Person object in Object Lens Figure 5-2: Aichitecrure of Object Lens Figure 6-1: Object DefiniDons Figure 6-2: Two-user Dismbuted Object Lens Figure 6-3: System W;de L'ruque Object ID Figure 6-4: Interactjve Transacnons Figure 6-5: Rule execunon transacuons Figure 6-6: L-.terleaving of Rules Figure 6-7: Resoi'. L".g Modif>-.Modif\- Conflict using Hybnd Im.plementauon Figure 6-8: Resolving a Rcad-Modif\' the Hybnd Conflict using Lmplementaiion Figure 6-9: Rule Execution Deadlock Figure 7-1: Distnbuted Ooject Lens
19
29 33 3-i
39 41 45 67 69 75 76 ''9
88 91
92 99 100 101 111
List of Tables
Table Tablt Table
4-1;
Dis-j-.ruiec Database Featjres
Fearures of ODier:-Oner.:ec 4-III: H\-?er.ex; Features 4-11:
Dauiascs
5^
60 63
Chapter
1
Introduction
Conventional record-onented rclauonal daxabases are rescricted they
manage.
car.
of mformanor.
The\ proMce da;a modeiing and cransacuon management capabii:i:c>
thai are wcli suited to business daia processing but
supponed coopera:;Ne ccmputer-ajded
in the i^-id
\«-ork
desirr..
appucations
sofrAare
intelligence knov. ledge bases.
Sjch
which are not adequate
L-.creasingI> important
for
compuier-
apphcanons include
developmen:. documema:ion authonng, and anif.cial
Tnese applicauons
Lmstructured objects and comp:ex relauonships
are T.teracnve
among
daia, people
l-.
narjre and deal
wi±
and scned'ules.
Object-oriented databases and hvper.ext systems anempt to provide the platform to support
models requL'ed
the semantic and data
89]
IS
one such system.
provide a system that
It
may
Object Lcjis [Lai 88. Maione
integrates fearurcs of hvTxrtext and
be used to
complex information such
for these applicauons.
as
v^xite specific
objea-onented system.s
to
appLicanons for retnevmg and browsing
meeting scheduling, project management and document
authonng.
However,
the current
implemcmauon
of Object Lens
cooperative applications; Object Sharing.
Our
goal
is to
Object Lens
fails
is
to
suppon
a
key fearure of
currently a single-user system.
propose an object sharing scheme that accommodates the key characteristics
of Object Lens (long interactive transactions combined with short rule-based transactions,
complex objeas, naive
users, and version maintenance).
,
-10-
1.1
Object Sharing
Object •
ar.c
in
Object-Oriented Databases and Hypertext Svstems
ir.fomanor. sharjig
Multiple
jser.- snc-l::
s-.
orjsc.-onented daubases and h>-pcncx; systems implies:
dc airie
;c
access documents and destrr.s created
:?\
oihe:
users. •
CoUaboratL-.e u?e:s should be able to viev. modi5iCaiions shared docjmer.t^ c: desirns (update p:opagat;or.
• D^'fere-.". useri-
others
To
work
update propagator,
(such
as
implemented
sc.-.err.e
and
and
ar.
s\stem>
transactions Ti^nc \s s>
a
concurrency control scheme, an
object protection scheme. car.
timestampmg
fcr dtsr-.butec databases
supported b> these
anv cf Lbe
uorkir.: ccr.currer.'jy should not be able to mterfere v.il- each
r.-.-ptr.tx:
locidng
tc
(.concurrenc> conirol;.
surpor. ob'ect shar_-.c these systems need tc have
databases and
made
shon, and the
not SL~.pl\
and
Unforronately object-oncmed
adopt the concurrenc\ conrrol schemes
update
propagation
The major problem^s
schemes proposed and
are the different narure
of the
of Lhe objects (complex vs. simple)
dif'^'"rnt nature
stems anc traditional database systems.
Hov.e\er these schemes can
be used as the basis for m.ore adequate schemes for object-onented databases and hv-pertext
systems"
Many
object-onented
database?
prototypes [Purdy
variations of locking for concurrency control.
protocol to disallcv.
87.
Fishman
Some systems
m.Jttple users from accessing
8".
Homick
87]
adopt
use a checkin-checkout
an object concurrently.
Existing
distributed h\-pcnext systems use either locking or version control to resolve possible conflicts from, concurrent users.
Most
of these systems do not support replication (multiple
copies of an obiect; or version maintenance.
-1!-
1.2
The Proposed Object Sharing Scheme
V,e
a-'fjc
proposed
ihc
thai
for
fnendliness
cradiuona]
concurrency concrol schemes
obiect-onenisd databases
are
not
adequate
v^eli
as
may
be
ihcLj
v.hert
systerr^s
for
of major imponance, and (2) modificanon
is
as
made
to
vananis use:
(I
the
object
nteractne!) o: automatically.
User fnendliness implies intolerable.
that
redoing user performed modificaiions
(ai
to
an object
disallowing users to access objects for long pcnods oi iimc
Cb'i
is
is
again
intolerable.
We
propose a concurrency control scheme thai
is
This scheme makes no restncnons on replication,
scheme would
be applicable.
sell
It
also
a hybrid of locking
i.e..
accommodates
and version control.
objects m.ay be repLicaied and the the existence of different versions
of an object.
We
feel
scheme
that this
is
emphasize user friendliness and an
object
form
and
and hypenext applications
suitable for object-onented that
automate
combine
interactive
modiiicauons through editing of
through
modifications
that
the
execution
of
ruie-bastd
cxnsacaons.
1.3
Object Lens
Objea Lens
is
an mtegraied (hypertext, object-oriented daiabase, rule-based) environment
for developing coopcraiive
knowledge (using
work
applications.
It
combines the fomial representation of
strucrures such as frames, inheritance networks and production rules) in
knowledge-based systems with the informal representation of infomiation (unstructure or graphics) in hypenext systems to present a scmiformal middleground.
scmanuc
structure to
hypenext nodes
that allows the ease
of
text
Object Lens adds
summanzanon of objea
-
-i;-
anc
ccr.:e.-.:>
objerts
I:
brrv.
a.-:"'
rs.a:;or.>r.:r>
ar.d
cf ccmr'.ex
documeniatior.
irj'rr-r.a:!??..
of a coliecjor,
of
de\elop~.sr.:
(r:>T>er.ex:
..
schedding. projec: management and
meet'-nc
including
by
objects such as difiertr:. —.essaze types, pecp'.e. task,
Tne basic primitives
1.
n-.ampjlatior.
^e LS-rc :? de^e.?r sr>er:f;c applicanor.s lo: L'lformatior. shanr.c. rer-.f-L".:
r-.a>
5L-.L
search
2u:?rr.a:;:
ar.c
a\ ailabie lo die
programjner
knowledge on various
CTpiici:
mcedngs. produr^ and companies.
to build his
applicauons
and object types Lirough
Facilities i?: creatine object instances
are:
a tem.plate-
basec jser interlace modi-f> objects by editLte the
2- Facilities tc
window
display corresponding tc
the object create r_e-t:2sed agents
3. Facilities tr
'Jiat
automaucaliy process Lnfomianon
on behalf cf the user. 4. Facilities
collect
tc
^T
Featti'''*^ C*^ ar' c*e""
We
toge-ner
objects
into
folders.
The
folders
be
m.a>
maintained by an agent.
autom.aticall;.
r
c* O'^'er* I_en^
feel that tne follcv. j-.g charactenst^cs of
Object Lens liighly in£uencc our choice of an
object sharjig scheme •
Naive Users
makes
Orject Lens
user-ir.tsrt'ace
is
a m.ajor
geared toward non-programjming users.
component of
the system..
model u'iiuences what aspects of object sharing •
Combination of Objects
may
interactively
•
\ersJon
.>\utomatic
ei'jier
by
a user
be
.Maintenance
applicauons, Ob'ect Lens of objects.
As
exposed
Tne uscr-intenace to the user.
ancT Interactive Modifications of Objects
m.odtfied
edimg
air
Tms
automancally by "rule-based agents" or
a display
a
tool
wLndow corresponding for
shodd supp>on
the
unplemcntL-.g
to the object.
coopcradvc
work
Maintenance of different versions
-13-
D:s?:bu:ed Ob'?:: Lgr.s
Dismbuted Object Lens should suppon object sharjig bcnveen various ncrwork.
should suppon locanon rransparency, rephcauon traiispar;r.c\
1:
trar.sparcncv'
Users any where
and possibly access transparency
in :he
.
concurrenc>
ncrwork
able to reference and dereference objecis located elsewhere in the nerwork.
provide a mechanism for object protecuon.
It
sho'uld
conflicts that anse because of concurrent access tc
1.4 Thesis
main concern
and the possible
solutions
"instances" in Object Lens.
not be
sho'uJd be
It
should
guard against or resolve possible
object by a
number
of users.
Approach
In this thesis, our
object
ar.
on the
users
to
exp'.onng the different issues
these
V.'e v.-ii
r\-pc defjiitions, i.e. the
concemed with
15
issues.
assume
PERSON
all
We
are
in object
sharing
concerned with sharjig object
users have the
objec: universally
mvohed
same
means
object hierarchies and
the
same
object type coercion. [Lee 88] explores the various
Lhuig.
We
will
ways of doing
type translation.
We
explore
number
of dismbuted
objea sharing is
is
achieved.
Most of
to
examine how
these systems are prototypes and exjxnmental.
Our
on Lhe architecture configuration of the distributed systems (centralized or
distributed), the concurrency control upxiaie propagation
We
systems (distributed relational daiabases,
h\pcnext systems and distributed object-oriented systems)
distributed
emphasis
large
a
schemes used, the protection schemes used and the
schemes adopted.
specify the requirements and fearures of Distributed Object Lens and then study the
suitability of the various techniques for the
Object Lens environment.
\^e
recommend
Obiec: Lens
ar.c
1.5 Thesis
Outline
Chapter in the
2
examines the
frameuork of
Chapter
3
ar.:
Chap'^r
most appropriate
the
var,o-js arch:tecnires for
ir.
cor.:t>.:
of
aesigr, for Distributed Obiec: Lens.
objea sharing
in a distributed environmien:
schemes
to
a
dismbuted
acme\Lng object sharing
in
system-.
I;
a distributed
surveys the
vanous
envuonmen:.
The
fearjres are conc-rrenc;. conrrol. update propagation and protection.
-i
preser.-.s
a
bterar^re
s'jr%e>
of various distributed
systems from relarional
databases to objec:-onentec datar^ises to h\penext systems. Tne emphasis
on
the
SerNer-Chen: Model.
the
descnDe> vanou> features of
aJgcr/Jrris and reieN
Lradeoffs betueer. the difieren; techr.iqjes
the
S'jr-jr.ar.ze
fL^.a^i)
the conc'jrrenc;.
conrro! m.tcharj.sm.s adopted, the propagation
m
the surve\
is
methods useJ and the
version control schemes explored.
Chapter
5 describes the fear^res
and characteristics of Object Lens.
Chapter 6 describes the requirements for Distributed Obiect Lens.
shanng and protection
in
possible transactions in
Objea Lens and accordmgly examines
the context of the Object
concurrency control describing the tradeoffs. Object Lens
is
A
It
Lens envTronment.
descnbes object It
defines the
the different techniques for
hybrid scherrie best suitable for Distributed
proposed.
Chapter 7 summarizes the key tradeoffs between the various choices and schemes and then presents Lhe best appropriate approach for Distributed Object Lens.
15-
Chapter
2
Architectures for Object Sharing
Users of centralized systems such as maLiframes are accustomed
and objects
to share da*^
With
oLher users of the system.
w-ith
with, the idea
of being able
the ad\ent of personal
computers, workstaiions and nerworts, Lhe essential schemes and algorithms to suppon
fcamres of object shanng needed
The
to
'c>e
revised to v».ork
Allow muluple in
2.
from one user
Multiple copies of an object
ensure that in time.
Lhe
3
all
may
is
to all other
may
CProtection').
(i.e.,
The system need
to
the same") at any point
copies (UpHJate Propagaiion).
the illusion that the user
is
multi-programmed fashion while preserving
executing on a dedicated system.
To
attain this
system should prevent modifications made by one user from
the
with
interfering
reads
or
The concurrency
Control).
system because: different
these objects
achieved by propagating changes made to one copy of
Perrrut users to access object in a
goal,
)
to
exist in the system.
copies of the object are consistent
Consistency
objea
The access allowed
i.
another {Read, Wnte. Delete
to
are:
various sues tc access objects created b\ any one user
a:
system (Shared Access
lhe
differ
users
such dismbuted environments
system
essential features to object sharing in a distributed
1.
ir.
sites
mechanism another sue.
ai
(a) users
in
one
a
updates control
may
problem
is
by
another
(Concurrency
exacerbated in a distributed
simultaneously access objects stored
distributed
site
performed
system
and
cannot mstantaneously
(b)
a
concurrency
know about
a:
many
control
interaction
at
-16-
Tne.T
approache;;
three
aj-f
ic
cb;ec: sharing
l".
a
dismbuted environmer.:
Tr.?
three
approacnes are :Tie5sace-ba5ed, centraiizec shared object space and distributed snared object space
assume
V.'t
A
focus
\^'e
tr.i-
oii:
anention or the locauon and maintenance of shared obiects
perscni! object> are located
message-based approach suggests
of specialized
electronic
information sharing
is
mail
a:
Lha: object sharing
To
messages.
done thjough
the local site for pjcrsonal use only.
a
could be done through the exchange
large
extent
the use of electronic mail.
in
distributed
The suggestion
is
systems that this
could be extended to incorporate exchange and sharing of objects.
Tne cenral_zed approach suggests
Lniv;dual v-ori^staticns nttd
nerworx.
approach
Tr.2S
The
is
shared objects be located to direct
at
a central
sue
Users
h:gbJ> dependent on the availability of the central
at
different sues snouid
'oe
ir.
the
shared object access to the centra] sue. site.
distributed approach suggests that shared objects be distributed at different sites
nerw.o:k
Vv'e
Lhat
m
the
able to access objects located at each sue.
present the centralized and distributed approach to objea sharing in framework of a
Model
Serv'er-Cltent
The
client
is
locate
The at
ser\'er
provides the required object m.anagement and objects.
each woritstation
to
direct
object requests.
In the
centralized
may
be provided
approach, one object server exists in the distributed system. Object caches a: the client
sue to improve object access. In the distributed approach several objea servers
exist
Tnesc may be dedicated machines as
caches
ai
the client workstation
may improve
in the central
approach, in which case object
performance. Another option
is
to locate both
the scr\'er and the client processes at the workstation. This eliminates the need for a cache.
We
examine
in this
chapter the various possible implementations of each of these schemes
in the context of an object-oriented,
knowledge -based, hypertext systems
Lens.
^preaches adopted by
O'ur sur\e\
fearures to keep in
is
a SNTithesis of
mind about
these systems arc:
similar to Object
different systems.
The major
•
Complix and
•
Maiupulanor. ofobiecis perr'ormed ihrough rjie execuuon or use: inie:aci;or
•
Triggers :hai activaic processes
•
A
scrru-s3'jc:ured objects.
uhen
stored objects are modified.
of long ar.d shor. irar.sac'.ions
rr.ixtiire
Tne next chapter
is
reser.ed to
cxammng
vanojs aigonthLTis and schemes p'c>posed
concurrency control, up>da:c propaganon and protection.
dismbuted systems indjcanng
v.
control and update propagation
for
Chapter 4 presents a survey of
hat object sharing approach they use, and
'-he
concurrency
schemes adopted
Message Based Object Sharing
2.1
This approach suggests that objects are shared ber*een different users by embedding them in
electromc
m.ail
messages
embedded
either literally
that are then
into the
mailed through the ner*'ork. Tne objects could be
message or embedded
requires that the receiving end extraa the object
li link
descnpuon and
The
form.
the unique
first
objea
method
identifier
suprphed by the underlying system, and buCd a local copy of the object with the appropnaic
unique object
message
tc the
objea by mailed,
Seoion
2.1.1
id.
The second method implies
objea he uishes
explicitly asking for
who
15
to share.
ii
if
It
that Lhe
is
up
sender insens only
to the receiving
decrr»ed useful.
The question
end is
a reference in the
to dereference the
once the objea
is
responsible for maintaining consistency of the different objea copies.
4.3 gives a clearer account of this method.
Advantages or Approach
T7)e advantages of the
message-based approach
• Fits well with distributed culture
are the following:
IS-
Fo: sys-^m?
C-.i:
L"jrrma:.or..
share
Moreover. na:\
e
m.esiages
u?L-g
may
users
fee'
2.1.2
sr.are
objects
more comfonible wuh
extensr.e use of messages to
seem.s
tc
be
a
evoiuuon.
narura^
explicit (exposed
shanng of objects
Disadvantages of Approach
disadvar.tages of the messagr-based approach are the following: • Lirr^ted tc L^jna-.i-.g object
Limited
'c :r.::iar.ri
Tnis approach to
:c
make
V.VJ: c-ar. sparer,: snaTiT.t
•L-har.
The
are ee2_-ec -.ruardv na:%e users a-nc L-a:
omers
1:
rr.^irl)
~
ec' shs'':r£
focuses
or. Liitiating
arises of maintaining
l-.::
me
i?— hutec
approach
to object sharing
Sharing
This approach dictates tha:
2II
the network.
utformauon (objects) deemed sharabie.
person has access to the
The
objects") m.us:
overall conng^jration
This coriiguranon
falls into Lhe
is
be located
server.
workstation. accesses.
The
aa
With
prol ably be due to processL^g
remote
is
the
a:
(i.e..
accessible by
at
may
the
sites.
central
which
is
site
from
server
local area
their
slower than local
be useful to explain to the user
mcreased speed of
end
site acts as a
as clients requesting objects
system, hides the details of the remote access
Indicaung that an access
and updatr.g these
a central dedicated site in
at
Server-Client Model, where the central
Users will remotely access objects
client the reason for the delay.
would
accessible
depicted in Figure 2-1.
of shared objects and the other v.orkstaaons in the nerworic
from the
making an objea
problem
2.2 Centralized Object
I
i.e.,
the
a; CLfr'eren: sites,
Tr^^ evol'-es
more than
object shanng.
does nc: address the maintenance of the shared objea. Once there are several
copies cf ihe oDjec; objects
^
shanng
ai
the
networks the delay
-19-
Figure2-1: Cenoaiizcd Object Server
2^.1 Advantages of Approach
Tne advantages of
the cenc-alized
approach
•
Simple concuirencv' control.
•
No overhead due
• SLTiple Riile
to
are;
update propaganor..
Rcsoluuon
Simple concurrence convol All accesses to shartd objeCTs must go through the cenc'al server. centra! ser\er to control a
and coordinate concurrent access
to the
It
same
mechanism vanes from locking (checkout, chcckin). timestampL-.g
object.
.
Of
detail.
interest in this
approach are the mechanisms
for the
T^e choice of
optLTustic control or
Chapter 3 and Chapter 4 discuss the various mechanisms
version merging.
amount of
would be easy
in
a great
thai pertain to a single
copy of the objea.
No
update propanahon overhead
All upxiaies are directed to the central server.
objea
exists
and
is
Only
always obtained from the central
overhead encountered
in the distributed or
a single accessible site.
cached approach
copy of a shared
Hence, any ujxiaied propagation is
completely avoided.
-20-
Sinr.e Rule Rcs'lur-^r
Simple quenes
ca.-.
csr~L approach silt
be lorrr.ulatei
aI'.cv->
b\ lookiTiC ror objerti
2.2.2
r„:e5
-j-.e
'>r.a:
l-.
kt.ou ledge -based sysierr.s usir.g
p^r.zjj-.-jr.z
IF-THEN
shared objects tc be resolved
ic
rjles.
the
a:
Tne
cenraJ
maicr. and perfonr. the appropriate manipuianon.
Disadvantages of Approach
TTie disadvaniaces cf the cenralized approach are the following: •
Contention
a:
Cenn-a! Site
• AvailabiliiN c: •
Rerr.cis access
Cenral a:
S;:e
eNer. snared ocjtct acces.
Cortenrion or Cc.r-c' S:u
The
may
central site needs to process object requests
lead tc
a
decradatior
significar.:
from
all
workstanons
The
perfom^^ance
ir.
in the
network. This
server should be
dsyTicbj-onously accept object requests and pass ±ie object ids to the disk
rerieval
Once
Ayai!ab:l:r\
Tne site.
ar.y
o'^
the objer.
is
available
it
would be returned
able
manager
to
for
to the requester client.
Cerr-g! S:re
entire systerr.
's
access
;c
shared objects
is
dependent on uhe
availabilirv of
one central
In case of cental s:te failure, users at various worl^^tanor^ will be unable to access
shared objects.
Remote access Even- time
or
Tms
is
not a very desirable feature.
fvf\ shaded obec: access
a shared object
to the central server
is
accessed
demanding
a:
any workstaaon
the object.
accessed has not be«n modified since the
Delays
this results in a request
are unacceptable
last access.
when
being sent
the shared object
2.2.3
Improved Performance usmg Cachmg
This problem of unnecessars successive remote accesses to
by desicr.atir.g
ar.
As long
objec.5.
area lt memory-
wnte would be done
issue
caching
at
'^"le
m
the
cache
'iien
w-nnen through
to the ser%er.
is
A
Rules and
locally on the cached copy.
what granulanry
level of an object.
One simple opaon
caching performed.
is
Tne other opuon
segments tcolleaion of
objec:s,.
A
basic
is
to pierform
contained such a cluster of objects, then
objea references
in the
perform
noaon
in
caching of larger units such as
object-onentcd
would be accessed together should be clustered
because of a decrease
is
Hence, objects are the unit of transfer benAeen the central
server and the cache.
objects that
users, us state
remevsd
Csrre
at
is
cached copy and
to the
methods may be {jcrformed
One
ine individuaJ \>.orkstanons to cache remotely
a:
objea has not been modified by other
as Lhe
same object may be solved
Future accesses to the object will be directed to the local cached copy.
Lhe valid siaic.
G'-aru'ia'-ir. o'
'j-.e
stems
together.
approach could lead
this
s>
to
is
that related
If the
segment
improved performance
number of remote accesses performed, because successive same segment.
are to objects in the
VaLdation of Cache
Moreover, the system should guarantee
thai a user will
system should propagate the changes made are
cached
1
.
ai
woricstanons. There are
The server maintains If
an update
is
a
list
always access the
to an object to
number of ways
ail
the copies of the
list is
are notified of the update.
The
objea or prompt
do
the user to
The
objea
that
ai
each workstanon.
checked and the appropriaie workstations
client
so.
copy.
for propagating the updates:
of the objects thai arc cached
received, the
latest
A
may
automatically fetch the updated
cleaner implementation would be to set
tr.erer- i:
checked cj; cbjecis
update p:oces>
2.
1:'
ob;ec:
ar.
is
l:
L-^.e
cbiec.
upda'.ed.
ir.e
l=.
the ser%er thai
a;
It IS
3.
frorr. ir.t
refetched from ihe
Timesiamp?
are
ser\e: sends messages tc
cache
stale of the object.
L:
the objcc:
maintaLned for objects
timestamp
If
retnevL-g the objec:
ar.
is
valid
me
caches that hold
The cbent anempts it
is
a
to
reramed otherwise
scr%'er.
the times'.amr of the obiec;
loca^
mgce:
automatical]},
updated.
cop> of the obiec: nva^ica--ir.g the access the cbier.
wUi
a
the central server
At each access.
retnev&d from the server and compared to the
is
the object
differer.:.
as
at
Tms
whole.
is
retrieved.
Tnis
is
faster thar.
requires the maintenance of a larger
number of tme stamps than previous method. Method 2 required nme stamps for
seements and used cbects
for
all
orjecL-
a*
the ser\er zr.d
ir.
ir.
the cache.
This method requires timestamps
the cache.
Repiacernen: of cached unirs
Tne
size
Reneved
of the cache
1.
mucr. smaller
thar.
the size
LRU
.Ar.
object replacement straiegv
Geast reccnt]\ used
may
of memor>-
The cache
objer.s car. no: be cached mdefinitely.
managed
to be
is
is
required.
s
the object
a:
server.
Lrnited memory- space needs
The options
are:
be used to select infrequently accessed objects
from the cache.
The
2.
FIFO
3.
Random, may be used
(first-in-first-out)
server needs to
'oe
may
to
be used to selea the oldest object in the cache.
sciea the object
to
be replaced
at
randomly.
informed of the cache's flushed enincs so thai
it
updates
its list
of
ob'ccts and the coiTespondL''.c caches the
compiexKies associated
rropaearion
These
is
bclou
ir.
me
v.
ith
object replicatior..
will also be
Concurrency control
one such complexirv-.
are discussed
objects art cached our system v-U! have rr.an> of
d:snbuted object ser\e:
a
v>.il-.
If
Upcate
more complicated
context of distributed objea servers.
2^.4 Implemenuucn of Central Data Server
The next imponant question two main options:
the actual
is
a central relational
implemenuiion of the
database server and
There are
central scr^'er.
objea
a central
sen-'er.
2^.4.1 Central Database Sever
A
database server build on top
implement a central data
o'^
a convennor.al
reiauonal daiaoase
Shared object instances would be stored
server.
Converting complex objects into a relation
relations.
may
is
be used to
terms of
in
not always a one step task.
Difficulnes anse because of the network strucrure of tne object space (objects u:th links to other objects).
network.
A
Put in relational database terminology,
Moreover, converting
more
This again
to
difficult.
is
Another
±e
object
of rclauons to ensure
a field
not easy to
map
another important feature
in the
anses
of a Lens
Objea could be
into a relarion.
systems
are stored as relanons ai the server
can only be performed
ckscnbed
at
m
in
4
1
we
are
a
combination of
Mapping hypencxt-Uke power of is
hypertext.
an exception^).
concerned wiih.
methods can not be performed
the client side
Secoon
mapping semi-sructured objeas
difficulty
database systems do not provide the trigger fcarure (R*
is
number
normalize
normal form makes objea (clustering related objects together for
relations wiih pointers to flat files again diminishes the
'This sysiem
difficult to
and ensure mutual independencies between different aimbutes.
The contents of
into relauons. links.
is
given object will need to be represented by a
atomicity of domains
fast access)
it
where objects are formed.
text
and
objects into
Most reiauonal This again
is
Moreover, since objects at
the server.
Methods
Tne advan:age>
ci usir.r a da-.aba^t ser. s:
provides the techr.olor> lor
:2^^;
i$
iha:
:t i5
from
easiJ\ obLainabie
the markei.
li
data access ar.c concurrent) connrol (aiihouzh thus rrugh:
no: be adequate
Tne database uorkstaijons
l"
queries tha:
resiJ;
processing
set
be
the nerv.o:k-
I:
is
abie
ine
:o
of the
d_"_
Tne
sues
accept
in the
from vanous
requests
fomi of
a set of relanonal
appropnaie relauons thai correspond
to
the
of the cuent end to translate object requests into the
of reiational qjenes.
a: Dotr.
asvTichronousJy
Tnese requests may be
retrieval
the
ir.
requested object,
apprcpnate
must
ser%er
client
Notice tha:
this requires
a
tremendous amount of
end should also extraa the necessan' informauon from
the reruTned njpies to build up the requested objea.
Tne commun: cation
betv.eer. the serv'er
and rerumec ruple:
Tnt
management, concurrency
systerr.
and the client will be
uiT
re;>-
in
frmi
of relational quenes
on the rclanonal database
control, authonzarion
transacuon
for
and recover)'.
22.42 Central Object Sever
A
cenrai object server ma) be used to implement the central data server.
returns objects Lhat are
database serve:
remms
m
the format of objects in obje
tuples that need to^bc laier
a -oriented
decomposed
systems.
An
object server
(Recall that the
at the client side to
form the
objea). Messages bcrvteen the client and the server would be in the form of objea requests similar to those tha: access local objects.
Unique Objea IDs or objea queries
(eg.,
description or selection rules) uill be used to access objects at the central objea server.
Tne
central object server will be built
on top of a storage manager. The storage manager
could either be an extended relational storage system or a persistent objea relationship between an object server and objects filescrver and files.
is
analogous
store.
to the relationship
The
between a
The
first
approach requires extending
a relational storage
system
to
accommodate:
d;ifferen: concurrency concol requirements Lha: result from, transacaons character^uc of objcct-onented hNperxext appucauons.
• the
• vcrsior. •
the
long
maintenance and concrol.
mappmg complex
efficiendy
and
h\-p)cnex!
semis tructured objects
into
a
rclaiionai storage system-.
tnggermg of processes upwn
•
The second approach
Tne
•
the
modi^cauon
of stored objects.
requires building or acquiring a persistent
obiec: store accepts unique cDjec: ids or object
objea
store.
quenes and rerums the
corresponding objects.
concurrency conixol can be customized for the Objea Lens environment.
•
• version
m.aintenance and control.
• version
merging.
•
recovery and resiliency.
• protection
and authorization.
• triggers.
The advanuges
of the
the market place.
The advantages of
first
approach
Feamres of the
is
that a rsiaaonal storage
fast access, protection
second approach
is
thai the
The disadvanuge
the applicauons of interest.
is
system
may
be acquired in
and recovery- are already provided.
objea
store
the effon
would be
needed
tailored to best suit
to build
such an object
store.
23
Distributed Object Sharing
This approach proposes that shared objects be distributed Objects
may
be dismbuted in a
manner such
at
various sues
thai:
1
one copy of the objea
2.
mulnple copies of the object are present (objects arc rcplicaicd).
3.
a copy of the
objea
is
is
present.
rcplicaicd at each site (fully redundant).
in
the nerAork.
-26-
Repbcanon
leads to
because
requires
it
L-T.provemen:
ar.
,
uTcreased
I
memor>' space
Licreased overhead to ensure that updates go to
Moreover,
consistent.
are
traditional
deterioration in performance
u*'
all
to
It
also
may
is
is
copies of an objea and that
performed
at
expensi%e
hold replicated objects and. (2)
concurrency control schemes lead
replication
In the case of no rephcanon, ob'ects
and throughput.
avajJabilir.-
ir,
more than 4
to
all
copies
an overall
sues.
be allowed to rmgraic from one sue to another.
This means Lhai object manipulation m.ay be performed locally.
2J.1 Advantages of Approach
The advantages of
the distributed
approach
•
Increased availabilirv' of shared cata.
•
Lmproved access p>erformance.
•
Lessened contenuon
at
are:
storage sue
AvaHab'.'.ir.
In a distributed system access to shared data
one at
site,
since the data
vanous
sites
is
distributed over a
is
no longer dependent on the availability of
number of
udl make data access msensiuve
sites.
Moreover, replication of data
to the failure of a
paruculax
site.
Improved access Replication will allow objea access to be directed to the closest the objea. V»'nat site
thiis is
depends on the implementaiion. (See Section
Lessened coniennon qi single srorgge
improvement
machine.
in
contains a copy of 2.3.3).
site
In the distributed case, object requests go to one of a to an
site that
number of objea
performance because objea access
is
no longer
servers.
This leads
restricted to a single
2
J. 2 Disad\anta^es of
Tne d:sadvar.:ace> rf
Approach
Lnt cLsr-.^uted
•
mere
•
upda:; propiCa:.?r. c^ernead
•
complex
•
complex recoverN
aprroach
are:
corr.piex concurrenrx control.
rule resolunor..
Complex Concun-enr\
Corp-:'!
Ailovkuic object rephcanon complicaies concurrency control because concurrcn: accesscrs at
different sites are harder to detect than concurrent acccsscrs ai one sue.
to access the closes: ccr;- of
ar,
object and perforrr. his changes.
to detect
such confiiris and provide
preser.:>
a
sccuon
mechanism
mechanisms appropriate
is
able
The system should be
able
for resolving them.
of su;h detection and resolution
detailed s'ur>e>
A presents
a
Each user
Chapter 3 section 3
mechanisms and Chapter 6
for Distributed Object Lens.
Updaic Prnpa£a:ior AlloNMng replication introduces the problem of maintaining consistency between the various copies of an object
at
the various sues.
For traditional database systems
be guaranteed tnat he b reading tne latest committed cop> of an objea. that thus requirement
it is
suffiaent to
ma>
pve
be relaxed in an
cnvironmem such
a user access to the lattst local
not be the latest copy in the distributed system.
We
Chapter
will see later
as Object Lens. In
copy with an indication
must
a user
some
thai
it
cases
might
3 section 2 presents a detailed
account of the various update propagation strategies and Chapter 6 section 5 describes jjropagarion of updates
may
be performed in Distributed
how
Objea Lens.
Complex Recovery Rccoverv- and reliabilin- are partitions
may
Inconsistencies
result
mav
in
much more complicated inconsistencies
between
in a distributed
various
be detected but are verv hard to resolve.
copies
system. ai
Network
various
sites.
-28-
Complex Rule Resolution Since objeas are dismbutfrd over
a
number of
sues, this requires thai a rule be applied
m
here the objects in question are stored
the sues
v.
the rule.
There are three possible ways
to
handle
order to search for objeas that match
this:
The collection of objects being considered may be
l.Subrules.
divided mco subcoUecuons indicating their location sent to each of the objects.
The
back
the
:o
rule.
The
A
message
is
then
rules fire at each of the storage sites and the results are directed
sue.
initiating
This
is
analogous
rule
is
at
simple: level to query
a
relational databases.
The required objects may migrate
Migration.
2.
site.
virrually
appropnate sue containing the rule and the considered
m distributed
execution
at
to the site
of execuaon of the
then fired and the appropriate results returned. The rmgrated
objeas could be returned
to their
onginaJ
site
then or
a:
a future time
when
the\ are requested.
3.
Replication.
In the fully redundant case,
already present
need
to access
at
his
objea
sers-er.
an remote objeas.
all
objects accessed by a user are
This means that any fiXed rule docs not
This of course requires an increase
amount of memory space needed and an increase
m
m
the
the overhead inairred
because of update propagarion and complex concurrency control.
2JJ Implementation There
are three
of a Distributed Data Server
main approaches
environment. TTie
sites
may
can store local as well as
system
may
also be
to distributing
shared objects over sites in a distributed
be dedicated objea servers or extended super workstations that
commonly used
shared objects.
The
object-oritcnted hypertext
implemented on top of a distributed database system.
the requirements and implications of each of these approaches in turn.
We
will address
-29-
2-?. 3.1
Distributed Object Server
n:
Thii- approacr. irr.z'.isb
m!Eh:
no*,
ex:sier.ce of several object se-. ers ui
supper. repl::a:ion of daia
.'.
tz:
Tnc
me ner^oiK and migh;
aciua] confifurauor. of the
ncrvork
is
depicted
or
ir.
rerejf
LAN
Figure 2-2:
me
In terms of
A
client
server.
Tnc
chen:-ser.er model, there are a
serMce
We
w
E>istribuie'
mentioned
in section 2.2.3 the
unplicauons of caching.
is
to
manage
the
-31-
Duties of Obiec: Server
Thf
duties of the object ser\er car.
sainmanzed
Accep: objec: rsques:s
•
Protection and Au'jionza'jor. of objects.
•
Version Conrol
•
Recover
•
Clusiennc
•
Tngce:>
•
Update Propagarion
•
Concurrenc} Conro!
o: queries
as foIio>AS
and rerum objects,
•
Accer: cbier:
The
x
reaue^^:^ or auer.es
object se:^ e: processes the uruque objea id and renims the requested object.
also be able to accept a quer% indicating specific attributes of an
objects tha:
match
Ir.dexir.z sc.z other feauires
need to be used
objea and return
to
It
should
the set of
improve access time.
Protect! en and Author.t:a::?r of O'necis
Tne
object
Given
an.
manage:
is
responsible for mamtaining concroDed access tc shared objects.
object access request and the user's id the objea
authonzed
to access the object in the specified
capabiiirv -based
mechanism
or access control
manager
verifies that the user is
mode. Protection may be performed using lists.
a
Sec Section 3.4 and Section 6.3 for
further details.
Version Control
The objea manage:
is
responsible for maintaining different versions of the objea and
alerting the user to the existence of the different versions.
Recovery
The
object sever
is
responsible for ensuring the persistence of the objeas.
sound recover\ scheme
u^
case of sue failures.
It
should have a
Clustgnng
If
segments
used as the
are
'jrut
of transfer berv-esr. ihe ser-c: and the client cache
of cachx.g), then Lhe object ser.er
segments stored
is
fir,
case
responsible for clustering related objects together mto
the server.
ai
Tnggers
The objea
objea manager
server should be able to send trigger messages to the
have been updated or
if
links have
if
objects
been added.
Concurrencv Conrc'. and Update Prcraeatior
It
is
difficult to classif>-
important funcuonaliues
2JJ2
in the context
is
is
highly dependent on the concuirencN- and propagation
local
in
Chapter 6 regarding each of the
of Object Lens.
at
each Workstation
an extension of the distributed object server approach.
distributed over the its
It
Object Server and .Manager
This ^rproach
have
fall.
uhat end (object server or objea manager) each of these
Implementation notes are made
schemes adopted. schemes
at
memor,- space of
the workstations.
Each worksution
Objects are is
extended
objea manager and an object ser\er talrcady present). Object servers
longer dedicated machines but implemented
at
responsible for protecting objects
manager, which
in
rum
at its site.
Objea access
directs a to the appropriate
Communication occurs between objea managers and objea server as
in the earlier
objea manager and
the
server approach.
Each objea server
each workstarion.
Objea managers
responsible for local objeas that could be shared or personal.
directed
is
objea manager as
opposed
to
aie
if
first to
the object
is
to
no is
are only
the local
not local.
between object manager
approach to access an objea. Otherwise, the dudes of the
objea server
are
more
or less the
same
as in the distnbuted object
-Si-
Figure 2-4: Disnibui&d Reianonal Database i
ii
3.
Caching
No cachmg
Objec: ser/er and object manage: clien:
a:
each workstation
a Rephcaiion b No rephcauon Nligrauon i
ii
The other dimension on top of
No migrauon
c: thjs
is
how. the object scr\-er
a relational storage systrrr. or
is
mpiemented. Tne
:\ Control
also look
a:
system
to
schemes proposed
provide object sharing
to the
capabilr^-ies.
var.ous protection schemes that p. -vide differing access rights tc sharers of
a shared object
3.1 Replication
Data
replication
means
that
representatives at several sites •
Reduced commurjcaiion
a
given
logical
daia
can
have
several
The advantages of such an arrangement traffic
dis'jnct
arc:
and improved response tame by providmg local
representanves.
•
Accesses
to the
same logical data by several users may be serviced
in parallel,
improving system, throughput.
• Increased
system availabUiry by allowing a given data object to be available for
processing as long as
at least
one replica
is
available.
stored
-56-
SupportLng
hcAe'-er. corriplica'.ss ih: deiails of data rerieval and TiodLfica::cr.
r:-p'.:zai
and Update Propagation
3.2 ConsistencN
.A-".
of
update to
Consistency of
ob'er..
's.s
logical da:a object rr/jst be propagated to ail stored rcprcsentanves
e:'-fr.
a.-:>
representatives
th.e
Tv-o degrees of
be ens'jred.
n-.-js:
consistency are possibie:
• Perfect
same
consistency L-nplies thai '•alues
initial
upcatec
Lnstantly
l- i-.e ia.—.:
•
a: the
representatives
sacr.
•*a> to
because scm.e
difnciil: to ensjre
una\a:Iab".e
and ^nsr
all
-"
.'.
~
'~ •
~
?.'
r-—.
Update
after
a
is
arc trj-ee basic me'^hods for
problem,
This
cim.e
ma\
is
be
i-.at
all
representatives v-iH evenmally be tr.e
same value
a:
some tme
even
-s.
pnm.ary copy.
A
phmary copy.
It is
the update to
ail
m
a
a
singie-uscr
disthbuted
muln-uscr system
may
system.,
.
Ail
One representanve of vi,-ntes
vtnte
is
done
the data object
to the data object
complete as soon as
it
is
uhere
be ignored. There
pcrformang update propagauon:
Ccpv Update
the prjT.ary copy.
may
are
updates have stopped.
Propagation
Pnraar.-
r^t^resentatr-ss
time of updais
concurrencv' control consideratior^s uhat anse
1.
the
.
contariing the repre>er.:arves
updated, so tha: the representatives converge to al
?.
remain identical throughout
sites
Lcoser fom^.s of consistercy im.pA
Lnter%
diia cb;ec: have the
:: L-.e
designated as
must be directed
to Lhe
has been applied to the
then the primary copy's site's responsibility to proragaie
oLher sues that have reprcscntahves of the data objea. Reads
be directed to any representanve of the data object. Tnc disadvanuge of
Lh:s
me'jioc
i:s
is
solution to ih:
ihe
Dismbjtec U?da:e ^u-nte-aU of
data
the
A
..
applied to
ail
A
unavailable (disconnected., 10
3.
i.e.,
the
A
is
may
be directed to any is
applied
complete onJ\ when
difficulty
would
site is dov..-.. a
rule).
unte operation
u-nte
a uTite
pninan
i:
to
all
has been
anscs when some sues arc
fail if
not
all sites
are available
approse w.s update.
Ma'?r.r\ Co-.^er-ji object,
WTue
is
[Demers SS]
however
,
orl]~
Acii--^ the uTite are directed to ai! representatives
a
of an
majority of sues need to approve the update before the
com.~:r.ed.
alcor/j---r.
allo-»>.'s
for a looser consistency control.
differe-: replicas evenrjally converge.
cns'ure tha:
outdated copy of
3.3
Tne
representatives.
Once
read operanon
object.
represen;a:ues of the data obje~.
.
some voLing
ele::?^ (according to
site is
represen:a::\e
site
One
ar. ob;.s::.
Background processes
Hence reads may be direaed
to an
Tnis migh: be acceptable for some applications.
CoDCurrenc} Control
In a multi-user system, different users
Concurrency control preserves the system.
access the same logical data object concurrently.
illusion thai
each user
is
executing alone in a dedicated
Concurrency control prevents data modifications performed by one user from
interfering with data
problem
may
is
remevals and ujxiatcs performed by another. The concurrency control
exacerbated
• Users
may
svstem.
in a distributed
access data stored
system because: in
many
different
computers
in a distributed
-38-
•
Users
rr.a>
daia objec!
concurren-Jy access differen: representai:vcs of die same logical at
differer.t
One sue cannot
sites.
insian'^aneously knov.
about
LT.eractior.s at oiher sites.
Conflicts that
[Couiouns
rr.ay
anse
fall
into
two caiegones: version conHias and senaJizable conilicts
88].
Version Conflias
These
resuit
\».hen
users
independent modificanons
access
representanves
different
to different fields or slots in the object.
To
resolved using version control and mereir.g
illustrate the
consider a fue accessed by rv.o concurrent transacuons. The at
the top of a f^e and the secor.:
merged
to
of an
mclude both changes
—odjies
te\: at the
object
and perform
Such confnas may be
notion of version confhcts.
first
bortom of a
transacuon modifies text file.
Tne versions can be
Su;h concurrency control schemes wlC be discussed
in
section 3.3.5.
Senalizable Confjcts
Tnese
cor.f.icts
modificauons
anse when users access an objea concurrently and perform dependent objea.
Here
concurrency
achieved
adopting
a
syTichroruzaaon technique that ensures that the outcome of p*-o interleaved transactions
is
to
the
control
is
ry
equivalent to execution of the rwo transactions in serial order [Bernstein 81].
techniques prevent or detect conflicts serializable conflict, consider a data
when
To
they occur.
illustraxe
the
objea x and two transactions Tj and T
.
These
nouon of a If T, issues a
read and T. issues a wrue. the value read by Tj will differ depending on whether the read
precedes or follows the wnte (rw conflict).
Similarly
if
both transactions issue write
operations, the value of x depends on which write happens last (vvw cor^ict). Consider the
case
w here
transactions Tj performs x = x
+
1
and transaction T; performs x = x
-
1
.
Both
need
car.sariior.s
'.c
the reads and wTites
reac and
m2\
l>e
Be;c—
wnie
x
mteriea'.ec
IC
(1
K
f:
T T
R!t
"
R'l
10
"
Ru
T T
(3
Figure 3-1
Execution H^-
)
is
W(xj R;x
IC
R.J
;C
T
.
the
Figure 3-1
ir.
Acq.!
ijixnoii
10
10
*
IC
10
•»\(it9
W(x, V,
X
1111
IC
c
Example of Conflicts
:
,1
11
IC
and T, are concurrent.
in transactior.s
and T. Execution (2)
Execuuon
illustraies a
(3) illustrates a rw-
rv.
and
a
and
a vi-w conflict if
"lost
update" or an
are con:-rrent.
conflicts
transaction
v^v,
ar.d
"inconsistent read all
^
::
I
the senalized execution of T,
corJ":ic: ifT.
Tj and
^vi
H
11
R
T^
as shovk.T.
l^
I,
W(x
IC
ua\s
Lhree
ir.
L.
I.
transactions are exeruied concurrent] %
If bo'J:
m
corj~;r:s
Note
.
that v.e
anomalies such as a
car.
lead
wiL
see ia;e: that senalizabilirv'
tc
design and h\-penexi systems.
It is.
might not be required for
however, of paramount imponance
in
database s>'stems.
SvTichronizaiion techniques
ma> be categorized
as optimistic or pessimistic in narure.
"Pessimistic" assumes thai concurrent transactions accessing the conflict and hence
makes one
this
approach
object will lead to
transaction wait for another to complete before
Synchronization lechruques based on two-phase locking disadvantage of
same
is
the p)ossibility of ckadlock.
fall
A
into
this
deadlock
it
is
started.
The
category. arises
when
a set
of transactions are waiting for each other to comrrut before they can proceed.
"OptuTusnc" takes the view conflict.
Transacuons
are
that concurrent
allowed
to
transacuons accessing the same objea
proceed
until a conflict is detected.
may
not
S>'nchronization
-JO-
techniques based on timestamp ordenr.g and optun:s:ic concurrency control
The disadvantage
caiegor.
approach
of this
is
the
overhead
incurred
fall
into this
in
redoing
transactions once conflicts are detected.
Tne rcmairong of
sccnon
this
dedicated to examining the various synchronizanon
is
techniques thai have been proposed. Pan of the classification
3.3.1
Synchronization Techniques Based on Two-Phase Locking
Tvi-o-phase
lockL".g
preventing
conflicts
transacnon must
The ouTiersmp 1.
f2PL) sv-nchronizes reads
and writes by explicitly detecting and
ov.ti a
of locks
read lock on is
x.
governed by
Before venting into
is a
Once
a
write loch, or one
is
x,
it
must oun
a write lock
x.
a
on
x.
tv-o rules;
Different transactions cannot simultaneously
other
Before reading data object
'DCPAeen concurrent operations
conflict if both are locks on tne
2.
based on l^emstein 81].
is
same data
wnre lock
own
conflicting locks;
item, one
ar.d the
other
is
is a
transacuon surrenders ownership of a lock,
it
two locks
read lock and the
a
write lock.
may
never obtain
additional locks.
The
describes the
first rule
first
phase (lock acquisiuonj and the second rule describes the
second phase vlock release).
3J.1.1 Basic
2PL Implementation
One cop\ of obiec: Associated with each
site is a
Lock Manager (LM) responsible
and releases for objects stored
objea X requests
the corresponding lock
granted, the ojxradon
on
the waiting
at that site.
is
A
for processing lock requests
transaction wishing to read or write a data
from the LM.
If the
placed on a waiting queue, ^lien a lock
queue of the object x are processed
requested lock cannot be is
released, the operarions
in a first-in/first-out order.
-41-
Redundan- cones o^ oh tec:
opcra::or.
rr,2>
read
However.
reads.
must obiam wrue where
the
an operanon
loclj,
read lock
for
or.
is
updaung
acquired and a
is
j:.
then
repressr.:a::ves of
al!
v.-^
copies
redj-idaTi!
ccp> and need on^> ob'-iin a
ar:>
\i
correc'J)
v-o:;:^
irr.piemer.tai.o-
TTiis
it
or.
Lhe cop\ cf
must update
A
;!:.
confLc:
the
re'a T^.
The existence
v.a:t-for grap".,
deaiock
0: a
siruaiior.
Ficj~ ?-3 shows me ua:t-for graph
ar.
edee from
waiting for a resource iockec
is
corresponds
a ua::-
v.iu-,
to the existence of a cycle in the
fo: the
deadJock cxamr'.e
Figure
in
3-2.
Figure 3-3: Vvaii-for Graph
Two
genera] techniques are a\ c—able for deadJock resolution: • C>ead!:^ct;
Prevenuon
aboned and
restar.ed
basic approach to decide
Tr.ii-
.
v.
cautious scheme in which a transaaion
hen Lh° s>stem detects
to assign
l^
a
is
pnonues
(possibly
two
aboned and
are
wait for T, n T, has lower pnoriry then
edge
m
no cycle can
T
to transactions
(alread>
owns
lock).
If
This prevents deadlocks because,
lower priority then
itself
T
.
and since Tj can not have lower
exist.
restarts
altogether.
prcdeclaraiion of locks (each transaction obtains
This
is
This all its
a deadlock avoidance
technique
requires
a transaction is the
the
locks before execution).
Data items are numbered and each transaction requests locks one
numeric order. The prioriry of
and
For example, Tj could
restarted.
prcordenng of resources.
avoids
for
the wait-for graph, T, has
path from a node to
is
T
deadlock might occur. Tne
nmesLamps)
whether T^ (requesting lock) can wait
the test faiii. one of Lhe
for ever\-
thai
is
at a
time
ntimbcr of the highest lock
in
it
owns.
Slicc
no deadlocks
•
Deader: •
onJy u
a rar.sactior. car.
car.
occjt.
De:e:'-:or.
•Ji:s
L".
.
scheme. cran.sacuoni wa:t for each other
unconn-oLled manner and are only
Deadlocks
by
are detected
and picking
a
for trar.sa:::or.s with hughe: pr.onr\-.
ii:
transacuon
aborted
in
cycle to abon.
Lhe
Moreover, transactions may
be performed
at sites
a
ar.
deadlock acruaUy occurs.
cor.sr-jr.ir.e wait-for graphs, searching for cycles
distributed environment because transacdons sites.
if
ir.
where data
'5e
may
Tnis
complicated
is
be executing
in
a
different
at
divided into subransacaons tha: ne*d to
stored.
is
3.3^ Synchronization Techniques Based on Timestamp Ordering T'jnestamp ordering n^ansaction execution
is
is
technique whereby
a
if
X had
restar.s
wTote las:
'iie
'-".e
These techniques allow
it.
been
w.T.r.en
transaction.
ser.alizarion order
forced :o o'ocy this order
each piece of data has the timestamp of thai last
a
a n^ansaction to
3 J.2.1 Basic
to
ensure the uniqueness of the
it
stamng time and
a
and the n^nsaction
read or ^ritc a data object x only
by an older transacuon; otherwise
m
read
last
it
a diszr.buted
rejects the operation
system
than in a c«nralized system because of the lack of a global clock.
been proposes!
selected a priori and
Each ransacuon nas
transacuon that
Generating timestamps
is
dmestamp [Lampon
is
Many
more complex
tecrjuques have
7S].
Timestamp Ordering
One cop\ of Obiea The
basic
1
dmestamp mechanism
Each transaction receives
2 For each data objca
wntc operanoD
a
applies the following rules.
timestamp TS when
x. the largest
are recorded
-
omesump
indicated by
it
is initiated at
of a re*d
RTMa)
tbe site of origin.
opennoo and
and >^TM(x).
the largest
and
omesiirap of
a
Fo" readi.
3
i
Lransacuoc
the
e
Dme5ia.T.p. otbcr^ise the read
Fo-
if
w.t;i£s
V*TM
X
TS
restar.ed
is
x
o:
»-.tr
a
TS
is
issuing tra.isa::ior.
txie
RTM;xi
executed and
< V>TMiXi. tbe
is
operanoc
vt-nie
ofu higber times'^Tip.
is
resians^ uiil
a
i
maxfRTMvXATS)
set to
rejeaed and the issuing
is
otberskTse tbe
be pjenormed to the founh rule using the
car.
Tins s:a:es iha: instead cf rejectinc
Hence,
tn-uig ic read da-.a t^a: «tas wntier, b-
»Tit£
is
executed and
TS
IS sc: IC
opuinizauon
RTNl
reading the version of
of R-ls.
A
read
is
the
oper^non
is
x.
.r
^itb tbe
larges'.
nnesuTTip
TS.
new
if
vereioo of x,
any R-ts(x)
lies io tbe
.
is
created
A
read ignores
read
A
is
write
all
versions with timestamps greater than that of the read, hence the value
identical to the value
is
it
would have read had
processed by creating a
timestamp has occurred. In
this
new
it
been processed
in
timcstamp order.
version of the objea unless a read with a greater
case the write
is
rejected.
-16-
3-3. 2J
Thai
ConservatJNe Timestamp Ordering
IS
tschruque lo:
a
elirnir.a:L".g
•-".e
elirr.ir.aur.c
fiiT.damenta] idea
performed
until
possib:l:r>
of cor^'icung ojxrauor.s and henc;
r.eec :c restar. rar.saCwCr.s '^eca-:5^ of such corJ"iic:s rBerr.steL".
approach however provides
The
me
concurrency than Lhe basic T/0.
less
underlymg conservative timestampmg be guaranteed that
car.
i:
Tr^i
S*"'].
it
is
No
simple:
operation
is
ever
can not possibly cause a con£ia (and therefore a
restan) at any time in the future. Ln oLher words, a request for an opcrarion from transaccon
T
is
delayed undJ
n-ansacuons.
would otherwise
'iiat
siruaaon m.ay
classes.
system knows that no confliring requests are received from older
Conflicts are eiiminaied Dv scnalLang
Lhose operanons
TT-.I5
's.c
'dc
all
opjcraaons
each
at
conflict.
improved considerably by mroducmg
T.'ansactirn classes are defined b>
read se: and
a
a
the concept of ransaction A.-.-.e
se:
to be
performed
a:
vanous
case
irj.s
Ir.
operations are delayed until no m.ors ope.-anons in the readset or writese:
timestamp need
not jus:
site,
vi:*_h
ir.e
a smialler
sites.
3.33 Synchronization Techniques Based on Optimistic Control The
basic idea of optimusuc
methods
conflicting operations, like in
However, wntc operations test is
passed
vaiidanon
at
test fails the
the
foUowLng;
Lns'.ead of
suspending or
or T/0, always execute a transaction to
performed only
arc
transaction
2PL
is
commit time
in a local
are the
workspace. Only
wnte applied
if
rejectu-.g
complcaon.
the validation
to the database.
temporary writes are ignored and the transaction
is
If the
restarted.
3JJ.1 Majority Consensus This algonthm [Thomas 79] assumes that a copy of each data objea
Hence each transaction of this algonthm read.
is
is
executed completely
a
that are
stored at each
site.
origin.
Another assumption
wnnen by
a transaction but not
the site of
no data objeas
that Lhcre are
Each transaction receives
at
is
unique timestamp when
its
it
starts
execution, and each
r^p^esen•.a•.:^e
vk-nn^r.
ir.'x
Transactions e\e;u-^
i;
transacuor.
each data ob;e:; cames
c:'
produced
:>
tLT.esiamp of the
rue phases. In
ir.
home
the
a:
me
s:te
It
the first phase an update
Ir.
and tne tLTiest^mr of the transaction
the
rule
second phase, the update
foUowed
each
b\'
positive votes, a
message
are only applied
i:
the
otherv.ise the update the
transaction
rransacaon
sent tc
is
ignored
rejected.
is
If the
all sites lo
A
?.\SS
\
mentioned
are
nmestamp
ote prevents
A
deadlocks
site
If a
REJ
votes
in
is
list
PASS
pending request
The request
Updates
of the data object,
informed and hence the upxiaie
of a potenua] deadlock situation.
sites
The voting
gets back a majonr>' of
site
carlierj.
the request's base va-iables conflict writ these of another
Lnform oLher
the cransarjor.'s v^nie-
the updates to the database.
less than the
is
(this is the 'TV>"R
aH sues
onginaung
commit
timestamp of Lhe request
disregarded
:s
i.^
m
sent to ever>- site to validate and vote on.
list is
for Lhe
itself.
shour. belou.
site is
list
received, for
l*:
order to
quesuon can continue
Votinc Rule
Conra.'e
the
omesiimps of
ibe reqjes; reid-se; witc ihc correspoDviiii^
tunesiamps
id the local
daia.^asc cop>. 2.
3.
Vote REJ Voif
OK
if
any value
m the
rcad-sci
Vote
obsolete.
and mark the request a5 penduig
requesi does ooi confLict with 4.
is
PASS
if
each variable
aii>
m
if eacb viriible peeling requests,
the read-sei
is
5.
Oiherwose
f
is tbe read-set
pnont>
(earlier
concurrent confbct, louver pnont>), defer voang and
reconsideranon.
is
current and the
cooiLas (has omestimp).
current but the request
read-sei vanables) v^itL a pending request of higtier
that
only uher.
be considered until sufficient P.A.SS votes accumulate to prevent majoritv- consensus.
1.
hai-
contains the data objects in the transaction's
read-set wrJ-. theL- timestan-.ps. Lhe nev. vaJucs of the daia objects set.
transacnor. uiuch
las;
remember
common
the rrques! for liter
to
3J.3.2 Distributed Certification
One copy Tnis
IS
of die data
a
dismbutec
u.-ne
-based aigo-Lhrn which operates by exchangu:g cenificauor.
informauon during the commit protocol [SLiha
UrJikc the previous algonthm
85].
it
allows
distributed transactions ;a master transaction at the iruaatmg site and subtrar.sactions
A
different sitesV
may
Transactions
workspace
read timestamp and a wnte timcsLamp
stamp associated with
certified
if
it
is
used
v.
the
to
This timestamp
message, and
For each read, tne transaction must remem'5«: the wr.te
time.
the item
and have reponed back timestamp.
is
hen
was
read.
certified and subsequently
Man\
cories of the data
Tms
req-ires
sues
was read
is
cemficaiion. Updaiers simply
all
transacnon
subrrans actions are com^pieted
is
assigned a glooally unique
read and untes as follows:
.A
commit"
read request
Lhe c-jxrcnt version and no \*Tite with a
still
request
.A v.r.t£
committed and no
-Aith
Vv"nen
sent to the subtrans actions sue in the "prepare to
to locally cer:if% all its
the version that
Lhat
it
master, the
tLmestam.r h2s been locally certif.ed
3.3.4
maintained for each data object.
read and update data item^ freely, storing any updates mto a local
comm::
until
is
at
later reads
representatives
certifv' the set
of
certified
is
if
no
later reads
is
newer
have been
have been locally cemfied already.
c-n
the
also
of wntes they receive
at
participate
commit
wrJi
the
time.
Comparison of the Three Methodologies
The locking approach may detection.
easily result
The timestamp approach
another expensive process. conflicts arc rare
is
in
deadlock requiring deadlock resolution or
deadlock free but
The optimisuc approach
is
may
require transaction restan,
based on the assumption that
and therefore most transactions are validated (few rollbacks).
Performance analysis of the 2PL, Basic T/0 and distributed certification was conducted by
-49-
[Ca-T>
L-
SS'aj
rse coniex: of di'^r-iojied database systems.
Tne foLouinc conclusions
vkhere reached'
•
;PL
and
d:sribjted
certification
dominated
Basic
T/0
m
tcmis
of
2PL uas
the
fjcrformance. • NMier.
the
cos:
of sending and receiving messages uas low..
sur>enor performer cje to Lie
• V."nen
message
its
cost
avoidance
v.as
o:'
transacDon restart.
and
high
data
was
replicated,
distributed
2PL due to us ability to exchange the necessary information usmg only the messages of the two-phase commit.
cen.ificauon outperformed SNTichror.izanon
3.3.5
Other Concurrency Control Schemes
Many
systems CNeptune, Gv-pss! that are characterized b> long transactions use version
merging
tc resoJve \ersion conflicts that
The meihods proposed
transacDons together
These schemes
siruauons.
fail
to
take
ma\ anse because
of concurrent long user-dnven
require the user's intervention to
nto account
merge versions
the need for senalizability
ir.
some
SoniC fue system.s provide mechanisms for automatic merging of versions.
Various strategies have been proposed to resolve merging confuas:
• Conf.ic: • •
resolunon rules
Data Fiov. [Reps ^T,
Some
made, bu: a comment is also included indicaung the nature of descnpuon of the nature of the confliCT is placed in a separate documcntchoice
i?
the conflict, or Lhe
•
The user
3.4 Security
is
interactively asked to decide
to keep.
and Protection
Implcmcntaaon of protecrion mechanisms [Salt2er75]: • Capability
•
which version
Systems
Access Control List Systems
thai
permit sharing
fall
into
two categories
-50-
Systems
3.4.1 Capability
A
capabilia
operaiiofLS
on
is
an identifier for a shared objea thai also grants nghts to perform ccoain
Capabuuies
the object.
are
of a unique identifier for the object and capabilirv'
ma\ be presented
rspicaUy represented b\ a a specificarion
system
to the
b:t
sequence consisting
A
of access rights to the objea.
an object.
in return for
Capabiliiies m.ust be
secure and unforgabie.
The
capabiiiry
flcxibiLty.
system has as
chief vurues
r.s
Efncienc) comes from
"Jie
its
inherent efficiency,
simplicity
and
ease of tesung the \al:d:r> of a proposed access,
the acccsser can present a capability, the request
is
The
valid.
simplicity
comes from
i:
the
natural correspondence beru'e«n the mechanical properties of capabiliues and the semantic
properties of addressing.
The
potential
problems with the capability system are revocation of access, 'jncontrolled
propaganon of access and
some one u cannot be
inability to
disabled.
that they expire after a given
It is.
penod
review access.
Once
however, possible Ln time.
to put
Capabiliues
without the knowledge of the creator of the object.
a capability has
been given
to
time limits on capabilities so
may
be distributed to other users
If the user id is
incorporated in Lhe
capability then only the users with the appropriate user id can use the capability, asstmiing thai
one user can not impersonate another user.
3.4,2 Access Control List
System
An
list
or
access control
domains
that
list is
a
have access
maintained for each objea to the
in the
system indicating the users
objea and the operations pcrmined
for each user or
domain.
Access control
list
systems overcome the problems of a capability system but has
-fl-
LTiplemer.-.anor.s probierr.' o:
cor.sumL-.L
chance
u-.
proces-
;er;rJ-.. car.
;is
.\jc:tr\er
be
our.
Search through ar access coniro; hs:
alocatior. of space
2 forrr^idabie
I'o:
access conro!
LTiplementation probiem.
rr.a\
lists,
oe a tune
uhirh
car.
Chapter 4 Literature Survey
Our
litcramre survsy covers a wide range of dismbuied sysiems.
into
three
dismbuied
categones:
h\pcnext systems.
Our concern
:s
reiaiional
to
Tne systems
are divided
ojject-onenied databases
daiahases,
explore the approaches
to
and
object sharing adopted by
oLher systems, the concorrency conn-o! schemes used and the update propagation
schemes
followed.
4.1 Distributed Relational
Database Systems
Distnbuted-INGRES Distributed-INGRES [Stonebraker "9]
v.
as
developjcd
a:
•j'.e
Uruversirv- of Caiifomia at
Berkeley, as a disnbuted version of the relanonal database system
INGRES.
phase-lockjng for concurrency cono-ol.
and resolved wld^ a
centralized deadlock detector.
multiple copies of data
Note however,
thai the
m
Deadlocks
is
Discributed-ENGRES. The basic approach
commercial product ENGRES/'STAR
It
at
the
IBM
resolution consists of aborting one of the transactions
suppon data
replication,
however, an alternative
primary copy 2PL.
1987 did not suppon
m
San Jose Research Lab
in
Deadlocks arc detected and
uses 2-phasc-locking for concuirency control.
resolved using a distributed deadlock detection mechanism. its
as of
is
such an enhancement in the future.
to
a prototype system [Haas 82] developed
California.
detected
uses 2-
" describes the concurrency conaol and consistency of
Data Replication. RTI has commined publicly
R*
are
It
When
a deadlock
the wait-for cycle.
to replication
was introduced
is
detected,
R* does not in the
nodon
Snapshots
cf snapshots cor.s:ste-.-_'>
re
— e'-e:
pressr.:
so— e
a:
£
ra-s:
vjfv.
iir-.e
of the database (possib!>
which
ar.d
peno conn-ol
of transacuor.s.
POREL
approach
ic
car,
no: be granted at one site until
are
Update transactions need
propagate updates.
updated
b>-
t:^
other sues
~ansacuon which
differsr.:
a
condiuons are favorable or v-hen
At aU other
a read transaction
there
all
is
Umvers:r% uniil the
is
end
a toul ordering of
the
pnmars copy
to lock exclusively the
sites,
o:
higher pnonry locks ar?
vanauon of
Tne> also create an "intention
there.
contains the updates tc be sent
the
at
done b\ 2 -phase -locking, and locks are kept
is
supports data rep.icanon and uses a
copy and pen'orm updates
Copies
database systerr. developed
In order tc avoid deadlocks, locks are ordered:
sues, and lock request.-
granted
distributed
a
is
list'
a: the
pnmar\-
pnmarv' site that
copies are marked invalid.
started
either
when woridoad
must make a "consistent" read.
SDD-1 The SDD-1 was in
protor>-pe [BemsteLt 80], de\elopcd
the tlrs: prototype of a distributed database
SDD-1
is
at
the
Computer Corporation of America.
management system. Concurrency
achieved using the conservative timcstamp method.
deadlock fre< and the usefulness of'conflict graph analysis not been demonstrated.
propaganon
(write
alJ).
SDD-1
control
This mechanism
in a rcaJ-life
is
not
environment has
supports data replication and uses distributed update
This means that
alJ
copies of the data arc updated before the
transaction conmits.
ORACLE/STAR ORACLE/STAR
[Gref 87]
is
a
marketed by Oracle Corporation.
commercial distributed database system developed and It
docs not suppon data replication.
It
uses 2-phase
.'nJ
locking
at
allowed. '0
±c
I.e..
record level to rcsolv- uTiie-u-nte conf;;c:s.
Read-Wnie
conf.:c:s
wntsrs do no: block readers. Oracle/Siar uses deadlock deteccor.
'Sit
relevan: fearures of the discrlbuitd relarionaJ daiabases above.
Repu:ancD
Chstnbut£d
Conairrencv'
D5.»^1S
Dismbuted-INGRIS
oppose:
^esc;^e ceaiocKS-
isadlock rr?\
Table 4-1 summenzes
as
ar;
:
es
Phase -LocJcag
L'pdaie
Dudlock
PropagiDoc
Detecnoc
Prjnar>'
Cenrabze'
Corv
DtadJock Deiec'kOr
GR£S'ST.\R
Nc
2 ?bisc-LoD-1
res
Cooser. anve
T
Dismbuted
Deadlock
UpcU:e Propaganoo
Prjv-nted V-aj-.-for
AnaJvsu
ORACLE'
-55-
4.2
Object-Oriented Database Systems
There
ruo genera! approaches
are
c«nnralized o: disnbuied.
foliow
L".e
tc
impiementmg
Present pro:cr.7>es
falJ
shared object space (hierarchv):
a
one of these genera! caiegones
into
no; ar e>J".2usuve s-zr\f\ of prescn: protor\-pes.
IS
the vaner>- of protors-pe s>s:ems bcL'ig
however
i:
Tne
gives a flavor for
implemented.
GEMSTON'H GemStone
Ser\io Logic's
[Purd\ 87. Maier 87] adopted the centralized approach to obiect
shanng. Tne Gerr.Si-r.: objec: ser. network. ser^ers.
It
ciirren*J>
Memoc
GemStone
server or
GemStone
Stone
b;-
to
vanous workstations
database to be distributed
a
thiiough a local area
among
several
GemStone
be performed either by executing messages remotely on the
car.
copying an object's
state
the
to
workstation local cache for
Objects are represented b> depuues that decide whether to forward messages
marupiiiation. to
does no: allow
execution
connects
e:
or cacht
provides
ne
object state and execute the
secondary
management,
storage
message
locally.
concurrency
control,
auihonzailon,
transaction m.anagement. recovery and supp>on for associative access of objeas
Gem
sits
atop Stone and adds the capabilities of compiling methods into b\iecodes and exccuang thai code, user authentication
and session control.
Pan
of the
Gem
layer
is
the virtual
image: the collection of classes, methods and objects supported by the objea -oriented system.
Stone suppons multiple concim-ent users by providing each user session with a workspace thai contains a
shadow copy of
the
object table, called the shared table. table adds
new nodes
optimistic control
objea
As
that are copies of
scheme
is
used
to
table derived
from the most recently committed
objects are changed in a session, the shadow object its
shared objea table with the proper changes.
check access conflicts.
'VMien
Gcmstone
An
receives a
-56-
commi: message,
i:
notifies
any modified cached
state
all
depuues of
to
GemSior.e.
conflicts with cransacuons Lha: have
objea
there are none, the shade the enz^es thai have not
shared table.
Each
commit.
intent to
iis
dcp'-r> tncr.
Stone checks for read-^nte and
commir.cd since me ume
table of the session
been modified, and
is
\>.-nte-viTite
Lhe ransaction began.
treated as
if
it
overlaid on Lhe most recent version of the
is
merged with
are
those of other transactions thai committed after Lhe commirting transacuon began.
transacnon told
fails to
commit.
invalidate
to
its
Lhe
changes
cached
in its
shadow
Tne
sute.
I:
were "n-ansparent" on
way, the changes made by the committing session
In this
Gushes
If the
table are discarded ar.d each deputy
developers
exploring
currentjv
are
is
Lhe
implementation of locking and versioning on top of the shadow scheme.
IRIS
Hevs-lett-Packard Laboraior.es*
implements the
models
that
L-is
[Fishman S"] also chose
Data Model, which
suppon
bjgh-ievel
generalizatioa'spwcialuaiion,
However,
L-.s
the Ins Storage
and
Manager
capabiliues supported by the storage relations, transaction
falls
ur
aggregation, a
well
as
convenaorul
manager include
management, concurrency
It
Lhe general category of object-onenied
such
abstracuon,
structural
is
Lie csr.ral-zed approach.
classification,
as
absn^cuons.
behavioral
as
rclaiionai storage subsystem..
The
dynamic creanor. and deleuon of
the
control, logging and recovery, archiving,
indexing and buffer management.
The developers transactions
arc actively exploring extensions to the storage
common
in
design applications.
A
version control
the basis for concurrency control in design applications.
out versions for extended manipulation. others.
A
higher level locking
Al-based applications.
mechanism
manager
to support long
mechanism
proposed
proposed is
Users would be allowed
This prevents further access to is
is
to
this
to
check
objea by
sujjpon concurrency control for
This provides a hierarchical lock structure with intention locks, as
well as conventional read and wr.te locks.
Disirib'jtej Obier: Ser^ c:
Texn-onLX La^oraiones's [Pone: 88] adopi&c the dismbuie-d approach. b> impicmcnung lou-le\e! disno'^ted object ser\e: cuer.i workstations
intended
to fcrrr.
from
the user
uhjch
and objects migrate
me
the
ir.
the gjobaj object space
to the sites
where they
iov.es; ia>e: of an object-onented
system
discnbated across
15
This server
are used.
tha: wiii
a
completeh
is
insulate
pnrmtive mode! of objects provided by the server. The server provides a
shared object space of persistent objects. The ufjper layer of the objeci-onented system wiE
augment
this
model
to
provide such concepts as
ty-ping,
message passing and mhentance.
Tlie genera] architecrure of the s\siemi consists of a network of workstations.
stored to a
workstations where the> are used
at
name
server.
Objects migrate from one
Shared objects site to
control for shared objects
is
anamed
must be obtained before
11
can be accessed,
manager
(loczl cr
object
blocked
conn"ol
Tne\
15
m
are located
are
by communicating
another for manipulation. Concurrency
via a variant of two-phase locking. i.e.,
the object
it.
A
shared object
must migrate from a disk
remote) into the session's object server memory. until the session relinquishes
Objeas
Access
to this
The system provides low
shared
level version
terms of historical objects that are immutable and maintain time of creation.
are explicitly created to capture the state of the
objea
a:
any
moment
in
ume.
Distributed Smalltalk
[Decouchant 86] descnbcs the design of "k distributed objec manager which allows several Smalltalk-80 systems arc present.
to share objects
Remote access
is
over a local area network.
Single copies of objects
done through the use of symbobc links "proxy objects" that
contain information about the locauon of the object
Objects then migrate for local
manipulation and associated proxies are updated indicating the objea's new design docs not address concurrency control to pjcrmit simultaneous access
site.
to objects.
The
-58-
ObServer Observer [Hormck 87]
is
store thai
is
currently used
backend of an objcCT-onented database system
as the
manager
for an
transfer of the enclosing segment,
were used
to
replicanon
reduce
Bro\vn Uruvcrsin.-
and
as
Lhe
ai
Objea access by
ObServer.
storaee
a client results in the
Segments
not already present at the client workstauon.
if
communicanon
facility, i.e. ar. objec:
may have
clients
(ENCORE)
at
objea-onented interactive programming environment (GARDEN).
Objects arc clustered into segments
all
objea
a persistent
traffic
may
m
object transfer.'
more than one segment. Moreover, rue
be placed in
copies of the same segment.
ObServer suppons an object
The system guarantees automianc
copies of an object in the vanous segments.
It
ujxlatc to
also guarantees thai a client always
accesses the latest commirted copy of an object through the use of timestamps. Timesiamps are given to transferred
is
segments and used objects
updated, the changes are sent on
to Lhe
the copies of the object to indicate the
in the
segment by ObServer.
ObServer. Timestamps are sent
new commit
to be
If
an object
anached
to
time. If another client tries to access an
outdated copy of the objea (difference in timestamp of object and scgmcnil, the oirrent
copy
requested from the server.
is
provide concurrency conffol.
The
locked in a range of locking and notification modes.
The
clients to access (read, write) an object in a restrictive or
non-
ObServer uses a comprehensive locking scheme
scheme allows objects various
modes permit
restrictive
made
to be
to
manner. The non-restrictive mode allows cUents
to an
objea. Communicaiion
mode
allow
s
to share
unconunitted changes
a lock holder to be noufied of the status of
an objea, including requests from other clients for that objea or a committed update from another client.
^Dne
of the basic ideas for
ire clustered together
unproving performance
m physical memory.
in
objea-oriented programming
is
thai related objects
•59-
POSTGRES [Sionebrake: 86. Rov.e S6j take the approach of stohng a shaiec object ruera:cr.\ gensrai:or. reianor.a: database mar.agemsn: system to the
RDMS
means of supporting complex
L-.cIude
POSTGRES POSTGRES's objects, of allowing
of supporting aleners. tr.gger- and general rule processing.
advantage of elimmaung Lhe need
The use of an
implement an object manage:
to
new
l-^.
a nexi-
extensions
daiar\-pes
RDMS
and
has Lhe
that supports shared
access, maintains data integnt> and resiliencN, controls access to objects and maintains data
consistenc\
.
As
objects are referenced by the application, a run-time system retrieves
from the database. Objects retrieved from the
them
databsise are stored in an object cache in the
application process so thr subsequent references to the object will not requLT another
database rctncval. that
Object updates are propagated to the database and to other processes
have cached the object. .AJeners nozf\ the system that an objea has been ujxiated.
Table
4-11
^ummar^es
the relevant feamres of objea-orienied databases above.
4.3 Multi-user Hypertext Systems
Note cards Notecards [Tngg 86]
is
a hv-pencxt-based idea structuring system.
electronic notccard containing text/ graphics and images.
connected
to other notecards
The
basic
objea
is
an
Individual notecards can be
by arbiuarily typed links, forming nerworks of related cards.
Distributed Notecards allows users simultaneous shared access from their workstations to notcfiles
residing
on any machine
contention resolution
at
the
in
nerwoik.
the level of individual cards.
Distributed
The system allows any number of
users to simultaneously read and display of a given card.
modifications to the card notified
is
when modifications
restricted to
arc
made.
one user
ai a
Notecards provides
However, permission
time.
to
make
All readers of the card are
Readers are provided xhrcc levels of modification
-60-
Obiec!-On--.:e- collisions a: 'Jie
SL-nply allocaied for an urJirr-ied per.od
no suppon
for versions.
Nerr.r-ie
Neprune [Delisle 86] effons.
A
context
is
dcfi-ies
'-he
noaon of ccniexis
coliecdon of nodes and
different contexts togc'iier to create a
histones of objects ar.d
Y.cr.zt
new
links.
context.
to
support muln-pcrsor. cocperarive
A
mechanism
is
provided
to
merge
Contex*^ arc used to enclose version
suroor: concurrent access.
.A
context
is
use-d as a
rnvate
-6;-
workspace
fcr
master' con:e\:
Conflicts
would be obscured detect
me
updaies. the \o:zl updates wiL evenruaUy be
rr.2.-^ir.c
arise iS & modificatior.
i:
a
is
uas made
to store, organise,
supponed
environmen:
another
mas:e: contex: Lha:
Tne system
able ic
programming suppon environment build on top of an
obiect-
is
up
to the user to resolve
them.
the Version
is
and selecnvely retrieve versions of objects.
V»'orKspace Manager.
the
b>
v,ith.
is
onenied operating system. Tne foundation of the system used
to the
were merged with the master
loca] updates
l*"
conilicts bu:
Gs-psy [Cohen 8S]
rr.a>
merged
'iie
o'^jects.
MuJuple users
The Workspace manager provides
is
arc
protected
a
Objects are clustered into workspaces that
for v.orking on pr.\ate versions
define user's access r.ghts to
Manager, which
Objects are not replicated but various version and
version branches for an object exist. Versions created inside a workspace are private to the
workspace and
To
pubhch'
are no:
\
isibie until
create a nev. private vers]on. a user
lock
a
branch o^
workspace
who
a
version group.
Houever
additional
must
they are released from the workspace.
first
be anached
OrdmarUy only one
workspace, and then must
user can be attached to a given
workspace access controls
take advantage of this flexibility have to
to a
f>cnTut concurrent access; users
depend upon synchroruzauon mechanisms
outside those provided by G\-psy (e.g., informal messages indicaung
Qose and weak models of
the
workspace
concurrendy. access rights
specify
to
Weak
of coopjcrauon are provided.
whether
authorized
Qose users
who
is
using what).
cooperation allows the creator
may
access
the
workspace
coojscrarion allows user to access private versions with specified
This allows coworkers
to get a
sneak preview of a needed object before
it
is
released.
Merging of versions
is
not presen'Jy implemented in Gvpsy.
The design proposes
to
-6;-
suppor
t>-p)e-spec:fic
rr.erge
handle conflicts.
Poss:b.e
quen-'ing 'asking
user
L-.e
v.
ca.-a.-netc.iz;d operations thai 'jse various srratccics tc
;•_-.
s-aieg:es
L^clude
cont'ict
resol'uucn
r^es or
Lnteractive:\
Notes
Even though Notes
is
not
interesting system to look
used by people
on
a
ai.
h\-pcrtcxt
Notes [Greif 88]
numenc and
to share textual,
a local area ner^». ork of
system, is
suppon of
its
a
group commurucaaon system
graphical uiformauon.
personal com.puters.
Notes
is
The
exist.
a:
sites
comjnunicate
the nev-er version
is
to
latest
document
The
in all replicas.
cop.es of ahe daiabase:
to
if
not
obtained from the remote database.
Optimistic concurrency conn-ol
used
is
to detect
and signal occurrences of multiple updates
version mstance of a document (in the same database
to a single
is
size.
arbiira.'y
different sites allowing rephcas of each
\erjH ihat Lhey have Lhe
that
based on document manager that
Notes replicanon ensures eventual consistency of the documents
database
an
it
system, operates
provides perm.anent storage lor free-form and serru-strucrmrd objects of
These databases m;a> be replicated
makes
replication
)
and fjiaUy converge the
versions to a SLngle version of an updated document (by asking the user whether he wants to
overwrae an existing version).
document most
ai
often.
another d ax abase
Access control
The system resolves site
lists,
by choosing
may
the version Lhai has
been edited and saved
be used to control concurrent updates
ways: (1) to ensure thai changes can only be made thai
conflicts with different versions of a
to a single mastfir copy,
in a
number of
and (2) ensure
changes can only be made by authors of the document.
Table
4-III
summ.arizes the relevant features of the distributed hypenext systems above.
-6?-
Concurr:r,c\
\ trsior
Conrc:
Locking
NoiecanL-
No
No: apjphcibie
N; RepbcaucT ConiLcisi
Nepcuiic
DismDJted Nc Rephcauor
\ srsion
Performed
Yes
Conn-o!
b\
Use:
Ehfference deiecaoc
proMckd
G>7?>
Lockinc
Cencraliiei
Yes
Performed bv User
No; appbcabie
Versioris
Note?
Distributed
Opumisn:
Versions
RerLcaoDC
ConcurrcncN
crcaied
CoDcrol
iDiemiUy bur oo;
mainuined
Table
4-111:
H\-pcnexi Fea*
j-es
-6^
Chapter
5
Object Lens Revisited
In :his chapter v-f rr.airJy present a descripdon of Objer. L«ns.
For those readers viho art
familLar with objea-onenied database and h>7>c.-:ex: systems w-e
these systems
Lens
'iha:
W-
Objer. Lens shares.
major
that will play a
roie
'si
wlI
indicaie lear^es o:
then present fear^-es specific to Object
\x-AL
our choice of the appropnate object sharing scheme for
Disn^buted Object L^ns.
5.1
Object Lens as
Object Lens
fits
[Conk^iTi 8"].
basis".
fo-LL'
Tne
• "Vi'indo'^s
Hypertext System
a
of the
s-j.
fearjj-es
of an ideali.zed hyper.ext sys'ism enumera'^d Ln
feardres present in Object Lens arc:
or :h£ screen co'resporj :o nodes
Tne nodes
m
Object Lens are
in the
database on a one-to-one
objects and objects have display
trie
windows.
•
"U'ii^'oMj can contain any
number of
other nodes in the database". fields
and values
link icons.
m
Objea
these fields.
Gicking on
'-he
link icons v>hich represent pointers to
display
The values can be
Imk icon causes
objea and open an object display window
•
• 'The
database
frt€ text
a
number of
combined with
the system, to find the referenced
for
'The user can easily create ney> nodes and are easy to create
windows contain
it
on the screen.
links".
Object types and instances
through a tempi axe -based user interface.
may be braised". The
user in
objeCT space by following links and opening
Objea Lens may
navigaie the
windows successively
to
view the
-65-
A
conten:
user ess. ajer\
5\stem for objects
lis
v.ith
par.icula: arr-.bjte
\a.i:;
S2 Object Lens
Objcc:
as
an Object-Oriented Database System
proNides
Lcr.s
m:erface
ar.
to
an
objea-onente
of increasingly specialized type with each
t\T^ mher.tL-.c r.slds. ac::?r.^ and other propenies from us parents.
Moreover. Objec:
Lcr.s provides a simple v.a> to
agents tha; scar, the objects folder
5-3
The
rj'es
m
m
perform datasase quenes; users can create
one folder and inscn links
to selected objects into
another
the agents specif\ the cnteria for selecting objects.
Object Lens Specific Features
In addiuon to the
objea-onented features and hypertext feamrcs of Object Lens, the
following specific characterisucs influence our choice of an Object Shanng
Scheme
Distributed Object Lens.
1.
Naive Users.
makes
Objea Lens
is
user-interface a major
geared toward non-prograrrcrung users.
component of
the system.
model influences what aspeas of object sharing
2.
Agents.
are
The
exposed
This
user-interface
to the user.
Object Lens supports the creation of rule-based "agents" that
for
-66-
process
mformanon
automatically on behalf of the user
TTiese agents car. be
triggered by everii- s-ch as the modificauon of a particular object.
3
Each object
Semistructured Objects
however
values,
may
the user
different fields as they desire.
from
4.
free text to a link to
m
fill
:s
a
much
as
coUecuor. of fields and field or as Utile
informauon
Fields are not typed and the values can range
another objca to a combinanon of both.
Customizable Folders.
Objects
may
be collected in customizable folders.
These folders can be m.aintained manuiliy or by an agen: executed on the objects executed on tables
5.
Version
the folder in the
in
Ln relational
Maintenance.
applications.
Object
in the
Lens
are
quenes
are
as
databases.
tool
a
.As
same manner
Quenes
should
for
umplementing cooperaave
suppor.
the
Maintenance
of
work
different
versions of objects.
5.4
Using Object Lens
Object Lens, like us predecessor Lnformation Lens [Grant 8"], makes extensive use of
mformanon exchange
(Ir^eral
using
text)
specialized
semistrucrared
electronic
mail
messages.
Creating semistructuied objects requires "defining a collection of fields and field contents.
The
fields are defined
by creanng
a
new objea
will inherit fields
renamed or
their display supjprcsscd.
Once filling
the object type in
the
fields
PERSON'), hence
is
in
from
new
its
object type in the rvpe hierarchy.
parent
in the
hierarchy.
Additional fields
may
The
Note
that, the
inherited fields can be
be added.
defined (for example PERSON*), objea instances can be created by a displayed
object
creating, for example,
window corresponding
to
object type (e.g.,
JOH.N and TIM. The contents of an objea's
filed
-6-.
mc> be
a
\-J\t.
to
anoLher obisct
o'-'fr: L- ihr er.:".?;L".r objec:.
d:spia>
corresporidiT!; i?
'i-.e
order of minutes to hojri. shov.r. across the trr of
incluGLng:
L'.e
Tnis
11-^:
r;a>
be expanded to appear
object
Tms
interacnve ediur.c of objects
mav
Actions ma> be invoked on the objea from the displayed object windov.
actions that appl\ tc the editing
to execute
ar.
Object msiances ma> be modified b\ ediur.r
.
embedded
me
vk
take
ir.dou
or.
Lhe
command
bar
There are several kinds of actions.
window such
actions that apply to the object such as save o: add link.
subseconds
a;;
as close,
move and
These acuons take on
shape, and the order of
Figure 5-1 depicts a displayed object form of a Person object
Ob'ect Lens.
•,»
i-„-.
ac-e>s
workstation,
An.y updates
made
ic
because of incorrunc messages appear for objects such as a V^'e
fee!
without
It.
Aat '_he
r.
PEIRSON
v.ould be
need for user
tc the folder
could brow.se throcch and read Lhe
in the
that the user
more appropnate
inter-^ent::^n
BUG-MESSAGE
svstem,.
folder
is
opened
a:
a user's
BUG-MESSAGE-COLLECTOR
opened form.
Note
thai earlier v^e
suggested
should be prompted to refresh the object form.. in the case
snce folders
the next chapter v.e discuss the requirements
Lens
ai:
Liser.s therr. in:o the shared folder
folder by the agen:
tr.e
fiiiers
and
ina: are re--.2_L-.ir.c ic bLjc fj^es
Nov. consider the case uher? the
messages
BUG-MESSAGE-COLLECTOR
acen:
are
m
of folders, for links to be added
most cases background processes
and features of object sharing for the Object
Chapter
6
Multi-user Object L«ns
Lens
Object
Distributed
allow
v.ill
users
workstations to objects residing on any
may
be replicated
vanous
at
sites to
Distributed Multi-User Lens •
A d:s~buted
lU
v.
simulLaneousIy
to
machne
improve performance and
x
Objects ma>
may
Lens users.
Hence, objects may have links
access
the
iocaied
same
Ir,
a
multi-user
availability.
at different
sites in the
to other objects located
object.
Trus
may
lead a
elsewhere
links.
one or more user
s\'Stem..
The system should provide
modiScauons.
their
some cases be tTansparen-> accessible by remote Object
The system must suppon such
• Concurrer.i u se rs
from
attached to the local area network. Objects
network and
in the nerw.ork.
access
have the foUou ing basic feamres:
obiec: space:
u-.
share
to
loss
of
mechanism
may
concurrently
infornuuon
and
for guarding against
such anomalies.
• Protection of
shared Objects: Access
to objects vars-
depending on the authonzaiion of the user. read the objea while another
We
will first explain
we
define
Distributed
the
some
is
We
Objea Lens with emphasis on
•
Shared object creation.
•
Shared object deleuon.
•
Shared object protection.
user
may
to another
be allowed lo only
allowed to read and modify the objea.
early design decisions
terminology we use.
One
from one user
made
for Distributed Object Lens.
Next
then present a design for object- sharing in
the following features:
•
Shared obiec: mod;f:ca:ior.
The goal cf
the firs: proio:>-pe of
Tne choice of
for objec; sharu".:
thus
assume
ar.
Dismbuied Objea Lens
users
bcLng on the same
v.
orkstanon.
We
to ac; as a prjr^tive object ser^e:.
acquire one tha:
manager
fits
to
is
architecture that requires the least effon to
Tne configuranon rely
Tr^
the spec J"; cations
is
n
of sccondarN
imponance
implement and
that of the server
most
that
and the
client
on the local disk manager of the underlying system avoids the need to implement an object se:%e: o:
menuoned
ir
chapter
2.
chapter 2 will
We
Many
provide the features of object sharing.
assigned to the objec: server
proposed schemes
to test the
is
the architecrure a: this stage
resembles the curren: configiiranor.
objeCT
accommodaies concurrent
Design Decisions
6.1 Earl}
We
iha:
need
to buiid the chen:
of the duties that were
nou be performc
be divided into sets of private objects, each
LENSFOLDER
to the folder.
Distributed Object Lens objects
nerwork.
may
Shared objects
may
the nerwork).
This
Objea
Shared Object
6.4 Creation of a
In this section
wc
\>.lLI
descr.'De use: mterface for
C'dnr.'".? access [c a
Shared Ob^e:
The
created
objea
actual
L«ns
using
(i.e..
is
edi:o:
ir.e
the object type;.
Once
m
•
same marjier
the
~^
:c
1.
may
be done
The
-".
the private object
one
:ac
o:
creator sends a Ler.s
The receiver then
2.
The
\_\rk_.
allowing
mm tc
created, the
owner may decide
that other users
Informing or granting access to the
to the object.
message
hnk
a
folder) in bus object spavi the
is
to
a>s-
v.
inser.s
as objects are currently created in Ob;sc:
Si03 of a serru-structured tennplaic corresp>onding
w-.e
\r.
should also have some form of access object
crcaung shared objects.
T:^.z
:o ano'Jner user,
to that
receive-
with a link to the object.
object into another ^.^-^
L.-'.sr..
objea
a
(e.g.,
o: a: a future time resolve
v;ev. tr.e contents of the obiec:.
creator insens a link to ihe objea
m
an object Lha:
already shared.
is
system can possibly notify other users of the addiuon of special case, and possibly the
most comjnon, would be
diis
the
The
object.
A
mseraon of
the
new
link into a "shared folder".
The former of
We
the
vtill illustrate
User
B
rwo methods the
from the
TASKX
TASKMANAGER
TASKX.
object User
TASKMANAGER
reference to the
appropriate for initialing sharing of a parent (root) objea.
rwo methods using ihe example
creates a task object
TASKM.ANAGER
is
objea
A
User
Figure 6-2.
m.anages
ai!
tasks
therefore needs to have access to
objea.
User B therefore mails
objea (Method to
A
in
I).
User
A
a
by using
his jjcrsonal
TASKX through
message
to user
A
a link
with a
can then adds a link from the
T.ASKX. The reference may be
later resolved.
.79-
Tne
LESSFOLDER
fc'.ce:
tha- car. be reacr.ed b\
is
shared b>
from
tracLnc iLiks
users (A ar.d
all
a lini.
share the b\
ir
ine ob'ec: unstance
neu objec: instance
inenng
a Ilix to
it
:?
Lisened
Tne
JOHN
This Lmphes
,,
the folder arc shared b>
joins the croup, a nev. person objec; instance
and
B
l".
(JOHN; the
is
all
iha:
users
If a
objects
neu use:
created corresponding to the user,
LESSFOLDER
folder.
All user?
person object was made accessible
from an already shared object
al".
LESSFOLDER
uiT nov.
to other users
{Method 2).
Resohine Links Resolving •
a link
Creating a local object instance representative of the object.
becomes
•
could imply one of rue things depending on the desired implementation.
a loca. object
and future accesses will be made
Remotel\ rerieN'ing the object from, object and furore accesses to
uiU
it
iti
sue.
Tne shared object
to the local copy.
Tne object remains
also result in remotely
retnevmg
a
remote
it.
L'^PLE\fEMATIO\' Each object may have different sues.
A
a
num'^er of object instance representatives that are located
at
unique object identifier should encompass information about the locanon
of the particular object instance representauve and ihe objea confiruranor. for the system uide object id
c:
t:
is
showed
m
it
corresponds
Figure 6-3.
to.
A
possible
.80-
The [Time
representanves.
on the objec;
carjiot rei>
CreaietTi fields allows for differer.: v;rsions to ex:s:.
generated by the underiyung system
id
to
the underiying system. aJlcus an object id to be specified, then
mennonec above
unique object ids is indirection
where object
wide u.'uque object
.Anoiher ait£m.an\e
generated by the system arc
:ds
fit
our requirements.
we choose to
is
mapped
to spccifS
build one
to the
level
\\e If
Lhe
of
proposed system
ids.
Resolving an object reference translates Lnto a read request direaed to the object manager. If the object
is
not local, the ,'rr^chine created or' field
the appropriate object
manager across
Lhe
network
is
at site
used
to direct Lhe read request to
[machine created on].
SnorTCC'^.:>-£ C' P'O'OCO'.
In essence a user
is
granted access
to a
some shcr.con-jnes
that object. Trus ".hanng protocol has • Unintentionai Ljnks.
regards as personal. is
his
workspace,
User
.A
shared object by providing the user with a link to
:
might unintentionall> provide links to objeas he
For example,
however,
in
A
grants
doing
B permission so,
A
to include
grants
B
objea "x"
permission
to
"\" uruntentionilly include .A's personal object because of the link berween "x"
and
"v'.
• Propagation of
an object.
Access User .
For exam.ple,
turn might give
C
if
A
A
has no control of grants
B
a link to thai object,
who
pwrmission
else
might get access
to
B
in
to access object "a'\
hence allowing
C
access to objea "a"
without A"s consent.
•
Revocation of .Access
gave out a
luik.
.
User
A
cannot revoke access to an objea to which he
6.5 Deletion of a
Lr.
Shared Object
obiect-onenied systems obiecis are removed from memors- using a process called
garbage coLJectior.
Tne
his links to thai object
efrec: of deleur.c an object from, a users object space
OrJ>
v.her. there are
obiec: ge: garbage coI:ec;td
Multiple users
lr.
ma\ have Imks
Hence, the reference count
that
to
a
Tms
remote references
to an object will the
d;s~Dutec envL'onment, marters
an object
Many
are no:
of these links are from remote sues.
The
object
is
garbage collected when
might be unnecessary for the iruual protonpe.
An
Trus
is
hard
alternative simpler
tc
the
impicmen: and
scheme would be
However, the object manager has
use the current underlying garbage collector
maintain local representatives (m the fonn of links) of remote objeas pointing
These links
bo'J~.
are zero.
implies rev.r.:Lig the underlying garbage colleaor.
shared objects
simpie
as
maintains the n'jmbe: of local references to an objea should
be extended to include remote references local and
no more links pomnng
removing
is
are
dropped when the
clien^LS
to
to also
lu the local
indicate that they have
dropped
these links.
6.6 Protection of a
Thus
far
Shared Object
we have explained how
a user
may
specified the type of access the link provides.
controlled access to an object
enforced
get a link to an
objea but we have not
Distributed Object Lens should provide for
The question
is
how such
infomiation
is
declared and
?
In Chapter 3, section 4
we saw two models
of protection, one independent of objea naming
(Access Control List) and one dependent on objea naming (Capability System).
examine how each of those models would be environment.
tailored to the E^istribuied
We
will
Objea Lens
CapabiJ:r\ Svsien
The object LID descriDed earhe: object (perhaps containing one V.'hen a nev, object
is
created
v>.ould also consist of a spec •J": cation of access
bit fo:
b>'
each class of operations applicable
a user, the object
object with a full set of access rights.
,:.e.,
with a specificauon of the nght3 required.
ne-A
OLTD
set
of access r.ghts
co'^ild
be
m^^ed
t
tc the
to ihe object
OUID
the
i.
of the
The modificauon of access nghts may be made pan
of the object manager duues. The objea
diminished
manager constructs
nghts
the
OLTD)
ls
The new
presented to the objea manager
OLTD remmed would
otherwise users wrJi few nghts could apply for
tc i_fferer.: ^ser?
±a: may
now-
have a
more
>.
have rcsmctive access
This to the
objea. This scheme as
it
Users would do
stands assumes that users do not forge links or impersonaie another user.
this to uicrease their
not authorized to get access
to.
access rights or to get access to objects that they arc
For our purposes of designing a research system assume no
malicious intent on the par. of the users.
could cleanly be added
to
a
An
authenticanon mechanism such as Kerberos
producnon system
to
guard against possible forgery and
impersonation.
Access Concrol
A
list
IS
List
associated with each objea specifying the authorized users and the operations
permitted for execution by the users.
Such information
is
separate
fr
om
the
QUID
maintained by the objea manager. Hence, when a user requests access to an objea specified
mode,
the
returning the objea.
objea manager
From
verifies thai the user
the user's point of view,
when
is
to specify a set of rules that
govern access
to the
is
some
allowed such access before
creating the objea he needs to
specify the users thai have access to thai objea and their access rights.
would be
in
and
object These
Another option
may be
easier to
formulate and have the effea of defining protection domains for the object. This however,
may
be inefficient because
it
requires resolving the rule every time the objea
is
requested.
Comr2.-:sor, o: CarabL'-.r. -Based and Access Conr-o! L;s: Svs:ems
A
model does no: guard
C2pabil:r>-bis.ed pro:er::or.
Lne earlier
semen
m
objer.
Tms ma>
a lot
unether
a
o:'
overhead
a lirce
ir.
'J".a'
the hierarchical strucrure of the object space.
child object inhents the propcnies (access nghis'* of the parent
some
be appropriate in
overv-nie
default, but expliciij>
Given
cases.
shoncomir.gs mentioned
model incurs
Ar. access conrroi bai.ed proiecuor.
migh: be unnec€ssar> Lhe c'jesnor. arises
agair.s: ihe
cases but not
all
by describing a new.
it
It is
set
possible to allow this as a
of author^atior rules for the
child object.
Hvbr.d Mode!
It
more appropriate
IS
permissive sense
Tnc access control
allowed access
not
to use access control lists in the restnctive sense as
to
the
lists will
A
object,
system..
It
obtaLned
a
ls
access conn-ol links
•
may
'tx
overcome
to 'the
list
Hidden
objea.
links to personal objects
that denies access to all users
protected
m
rwc
cn'ierent
and hence can sec
pcTTTUttcd to
it
its
icon form.
may
is
view ing the contents of
TOM person objea.
are
hidden
be proteaed using an
not pcrmirted to get
To
The user however,
Tom
the
is
a
more information about
Hence, when the unauthorized user looks tc
is
not
follow on the example in
oblivious to the existence of the objea.
folder he will not sec a link
Hidden
the existence of a certain
be permitted to sec that
of the Lens group, but
is
have somehow
except the creator of the objea.
view the contents of the objea.
The unauthorized user
may
who
modes:
in
Figure 6-2, an unauthorized user
most adequate
the
is
the deficiencies of a capability -based
The unauthorized user may have knowledge of object,
•
implementation
important to guard the object against undesired users
Il-tI;
to the
contain informauon regarding users that are
hybrid
Restrictive access control lists are used to
opposec
TOM person objea.
at the
i.e..
member
Tom
by
the links
LENSFOLDER
-8-;-
The laner mode
is
much
harder to Lmplement.
It
requL-es that the object ser\'er performs the
following additional step? before retumLng the objea.
•
FoUov^s
ar.d
• verifies if
resoives ans
access
is
Shared
.Aeer.ts
Agents
in
rencve
restricted trom.
ar.
objects.
to the
agent that operaies on tne
seemg some object
have the same access nghts to
LENSFOLDER
agent ar.d hence can tr.gger
the triggered agent should differ
it.
embedded
have,
Object Lens are implemented as objeos, and hence can be shared like any other
and user B have access
analogous
to these
may
the links.
object instance. Consider
IS
pcnruned
requested object
kept intact.
• if so Lhe links are
• cLherv-ise
'.irJcs '--.e
ir.
the
it
to another.
to the objects as the user that
It
will
.A.
on the folder. Hov>.ever, user
A
LENSFOLDER,
from one user
an executing program.
Both user
folder.
namely TTM. The outcome of
Hence,
a triggered agent should
mggered
;t.
A
mggered agent
have the same usend as the user
who
is
triggered
Vvlien the rules are fired and the acuons arc executed, the objects thai are read or
scanned
arc these that
.Another option
is
to
creator of the agent.
triggered
it.
can be read by the user.
suppon agents with parameter^ed userids
The
default
is
Hence, even
if
may
be
set
by the
for an agent to assurnc the userid of the user that
Otherwise, the uscrid of the agent
(e.g., himscifj.
that
the agent
is
may
be
set
by the creator
tnggered by another user,
objects that are accessible by the specified uscrid. This
is
to
it
be some userid
scans the set of
helpful in giving a user access to
the services of an agent to run a particular query without giving
him access
to the
ennre
underlying folder. The user will only have access to the objects in the folder thai satisfy the query.
-85-
Use:
Ir.ierface
accessing an obiec a use: Me-J-s the corresponding
I.-,
users
access
cxa.-nplt.
ar.
the
r.z'TM.
td::
delete conrr.ar.d
appropriate
conmand shou shoui orJ>
i:
Specif>'ing restrictive access
6.7 Modification of a
Objects
in
Objec: Lens
change
con^jnands are sh?v.T.
orJy
lT
'j-.s
the user has the
lists is
car.
comn^and
Fc:
ba,'
nghts
to delete the object.
the object
created.
is
be modified either interactively by the user or automatically b>
in the objec: space: save,
ar.
can also be applied manually
;.
add-Imk, move, close. Users
Vv"ner. a rule is
executed on the object.
commands
may
item to a folder. Tnggered agents apply a
set
applied, a collection of objects
look for objects thai match the description of the is
the
Shared Object
and save the chance > or add
the rule
or.
Lhe
or.
use: has the nght to modif>' the obiec; and a
command when
an extra
DependL-.g
d:spia>
Objec: Lens ccrrentJ> supports the foUowLng user-dnver,
triggered agents. result in a
s
windc*
rule.
If a
match occurs,
These actions may var\
m
the
that
edit an object
of rules (rules is
scanned
aaion
to
part of
complexity (sec secuon
6.7.1.2 for further detail;.
L-.
a
distributee rr.-jl::-user environmer.:
imponance.
consistenc\
Distributed Object Lens should provide
The remainder of
this
and concurrency are of paramount
suppon
for each of these features.
chapter will be devoted to exploring various
ways of proNiding
concurrency control for simultaneous users and achic\'ing consistency between different objcCT instance representatives
(if
present).
Consistencv
The
basic premise
is
given time period to
that all
all
changes made to the objea should be made visible within a
authorized sharers of the objea. The question
is
whether the update
propagation should be exposed to the user or done transparently by the system. This
is
only
-86-
an issue
;f
Tne
inzr^ are different cbjec: instance representative of the object.
tm-o options
are.
S'^'ste":
-
Guaranteed Once
an.
object
'.s
mod'.fied, the system, ruarantees that
all
sharers of die ob'ect evenr-ally see &ie change Goose consistency conirol).
H?u
trjs
implementfid depends on the shared objea implementation
is
we
choose and ihe concurrency control desired.
E.T:;osed to user
2.
chancrs
.
The modifier of
tc all sharers
of
±e
'?;sct.
the object sends a
I: is
'-".en
up
message indicaimg the
to the sharers to modif>' their
objea instance represcntaiive. ConcurrenrN-
The system should allow any number of time.
It
is
m
viizl
such
each o±ers operations.
•
A
a siruaiion tc
Two
users to access the
ensure
'jiat
same shared objea
the
ai
same
concurrent acuon do not inicrfere wiuh
kinds o: ;onfucts ma>' ansc:
Version Conflict that
res'ults
v.
hen different users are accessing
their object
instance representatives concurrently, but are modifying different slots or fields in
the
object.
Version
conflicts
can
resolved
'oc
by
merging the two
representatives together.
•
A
Serial 12 abUiry
allowed a
occurs
access the same slot
modify or when two users
on each
A
to
ConfliCT
m
arc
when two the object
or
more concurrent
and one or more of
users
are
the accesses
is
pxrforming modifications thai are dependent
other.
concurrency control scheme should be adopted thai
the concurrency control can be possibilities anse:
vrill
resolve such conflicts.
done by the system invisibly or visible
Again
to the user.
Two
-g-.
S'-srerr
1.
Hence.
Tne
Guara-nieei ar.>
transactions
system
iha:
res'Jt
should
conT.ici
a
ir.
g^jaraniee
resolvng
should be
conflicts
abcned and
restated
sysiem.
6.7.1
Vv'e
Tne use:
User AJerred
2.
It is \^'p
Transactions
ulL use
alerted of a conf.ic: a5 soor, as
is
to the user to
in
A
model
either a
COMMIT
transaction
operation or a
successful tcrrrur.aticr. 'the unit of
used -
to signil
1.
'jie
is
the rw.o types of modificaiions possible
defined as a unit of work consisting of a sequence
BEGIN_TRANS ACTION
ROLLBACK
work has been
operation.
operation and ending
COMMIT
successfully completed).
is
used to signal
ROLLBACK
':>e
read;.
types of modifications are formal!) descnbed in terms of transaction as such:
User-dnven modiiicanons
are
performed by the user editing the objea form.
We
wiH term these interacuve transactions
and
may
the
objea form, making
be days. redo.
last
from minutes
to days, i.e., the
the changes
and
Moreover, these transactions
Interactive transactions
.
These transactions
are long-lived
time that elapses between opening
finally closing the object
are very
form
may
expensive to roll-back and
wiU overlap more frequently because of the
duration of the transactions.
2.
m
is
unsuccessful termination (the unit of work carjiot be successfully completed
a data object can net
The rwo
knovkT. to
the actions to resolve the conflict.
descnbe
to
of operations. becLnrung with a special
w ith
is
Object L€ns
the transaction
Distributed Obiec; Ler.>
undo
i:
Agent-dnven modifications
arc
performed as a consequence of a
Rules are either applied by triggered agents or by users. automatic transactions
.
We
Tliesc transactions are shon-lived,
fired rule.
will tem:i these
i.e., last
no longer
ihar.
feu
a
Automatic
seconds.
m
processing n-ansactions
V^ e
iniuaHy
V. lLI
hou
exarrLir.e
database
ihe
transactions
comparable
are
to
daia
systerr.S-
various
s\Tichronizanon
techniques
proposed
for
dismbuted database systems, C.\D ssstems and H>-penext systems may be unpiemented
to
achieve concurrency control for mteractivc transactions. Accordingly v-e wd] examuie hov. these techniques
may
be used to achieve concurrency conffol for automanc transactions.
6.7.1.1 Interactive transactions
An mteracuve
transaction could either consist of a simple read operauon (opening the
object form) or a read op^erauon followed by edicmg the object form and then closing the object form hence
savmg
transition for O, to 0,^j
Lie
is
changes
We
will
on
Op
solely based
assume thai
Lhat
m
case of mod:5icaticns, the
no other objects need
is.
Figure 6-4 shous the ruo possible sets of Literactive transacuons.
objea
IS
considered as a separate (not a nested) transaction
embedded
object
feci that the
is
no different from opening any other objea
two transactions
other users. Vve see below
are related
how
he
and
may
may accomplish
R^(O) RgfO)
Opening an embedded
Our claLm in the
to be read.
is
that
opening an
system. The user
require to ensure
may
no interference from
this.
Read
Wg(0)
Modify
Figure 6-4: Imeracnvc Transactions
If
rwo users
arc concurrently accessing
an objea the following three scenarios are possible:
— no conflia
•
Two
users are concurrently reading the object
•
Two
users are concurrently modifying the objea
--
arises.
conflia potentially anscs.
-89-
Concun-cr.'J\
•
one use:
is
readme and
the
oihe:
me
narurc
n
modif>'Lnc the obie::
--
confiic; ro:er.::j^;> ar.ses
Since the pcten:;aJ for confiir: depcncs
mode
uansacaor.. the
or.
of the sim'jitaneo.:- LT.erac'.:\e
of access needs to be cxpiiciuy spiecified by the user.
Tne foDowmg
users are p>erforTrur.g uidependen; modificauons to the object. illustrate
that
kinds of wv\
Lhree
the
conflicts that
marA
case>
This means tha; rv.o
the cor.f.icts Lha: ar.se are vcrsior. confiicis. not senaiizabic conflic'Li.
examples
Ir.
may
arise
in
are three
interactne
transacaons.
Consider
a
TASK
objec; that has as one of
free text that allov.
field
IS
users
ma\ simaltaneousl>
though
s
users involved in the task to
edit the
to
PERSON
modifv the [phone
wnte about ihcu progress.
this
Two
corresponding objcCT form to note their evaluauon. Even
free text that reflects their evaluations.
a
The contents of
fields [progress repon].
accessing the same field, they arc installing independent pieces of
both, users are
Consider
its
object
VN'e refer to this as a
JOHN Two
n'^m'i^e:] field
users
may
and the other
to
version conflict.
simultaneously access the object, one
modify
the [salary] field.
We
also refer
to this as a versicr confuct.
Consider the
TASK
simultaneously. evaluation.
performed
We
object
again.
However one
Assume
user's
that
comments
rwo users arc
refer to this as a serializable conflia.
accessing the object
arc
dependent on the other user's
Access
to the
objea needs
to be
in serializable order.
User Interface
One opdon
may
is
to let the user specify access
access an objea either to
Read
when
or to Modify.
the object
form
is
Another option
opened.
is
to
Hence,
a user
have edit as one of
-90-
commands on
ihe bar
and clicking or the
far
ed;t
form
comjnand
is
ari
Opemnc
Lhe object fom-;
is
an indication of a read
indicat:on of a wnte.
Automatic Transactions
6.7.1.2
So
the object
we have
seen changes
in the
object space iniaaied and explicitly completed by an
interacuve user thjough Lhe use oi object fomis.
Users of Objea Lens can also create rule-based agents to automatically process information
A
content of objects
when
ru.e cor.ststi o: a descnptior.
and action specifies the consequence of the
the rule sho'uid firr
Description sp)ecuies
and an acuon.
The matching of
rjle.
a rule
and the execution of us action m.a\ be modeled by the transaction:
R(O^)
....
the
.AfO,
)
where .AfOj
form (move objea
I rules: or a
can either be
to folder,
copy object
senes of reads R(0;
actions of the form
summarizes how
{set
rules are
the predicate (description
is
>
)
to
pan
sunpie wnte
W(Oj)
to folder, delete
R-'O^j)
'
modeled
a
as transactions,
conflias
if
Tms means
rules are allowed to interleave.
preserve the serial order of the cxeoinorv^
We
that rule
--
objea from
folder) {Caieeory
W(Oj)
(Category 2
the case of
j*.
Figure 6-5
rules).
There would be a number of reads
of the rule consists of an
the value of the predicate.
RiO;
case of actions of
embedded objea.
dependent on the value of the object read. The consequence pan of a
upon
:
in the
followed by a write
(calculation)}
Tp^
if
TTic write in Tj^
rule
exccuuon may lead
is
conditional
to
senaluabie
Rules need to be executed atomically
to
consider rules and not rule sets to be the unit
of work.
Category Catrgory
Rules
1
1
specifically
viewed
rules
perform
their reads
on any kind of objea but jserform
their writes
on folders by adding or deleting links from folders. The write operation
as an
append operations, hence two different writes
without altering the resultant folder. This means that
ww
be resolved by merging the two modifications together.
may happxn
in
conflias of category
1
may
be
any order rules
may
-9i-
A'O
R.O whsr.
AO
Add
link
Dtlt.e link
Move obiec; to folder SET o.anaDle> TO (calcuboon where
object field of object
F(0;
icaicuiaooQ
0) r
F(0
Figure 6-5: Rule execution transactions R\^ conflicts cf ca!egc-^
cxecuaon of
a
1
rules
need
wnte operation of
mterleavmg
the execution of the
of the oLher
ma>
rw.
a rule
is
If obiec:
•
If object
much more
Recall that the
dependent on the value of the read.
lead tc conflicts and inconsistencies.
m pseudo
form:
TOM m Folder A then add link from Folder B to object = TOM not in Folder B then remove link from Folder A to object
interleave.
The
Initially the
objea
none of the
first
rule performs
TOM
is
in
folders.
Rj and
Wj
A
and Folder B: object
Figure 6-6 shows
how
which
and the second rule perform R^ and Wj.
Folder A.
are different, they arc both consistent with the rules.
the rules art
perfomied atomicaUy
is
Even though
may
Hence the order
in
not important as long as they are pcriowntd
aiomicaUy. Executions (2) and (3) dcpic; the uxong outcome conflicts of this type
TOM
the rules could
In executions (Ij and (4j, the rules are performed aiormcaUy in scnal order.
outcomes
vt-rite
=
either in both folder^ or
the
Hence
V\; illustrate this in the following
These two rules basically ensure consistency between Folder is
care.
c r^ies such that the read of one occurs before the
example. Consider the followmg two r^Ies
•
handled with
to be
if
interleaving
is
allowed.
be resolved by serializing the execution of the two rules.
Rw
.0}.
onec:
Obiec:>
fcrrr.
tLTiesiamr cf Lhe lates:
As
indi:a:ed
\r.
w.T.te
Cnap'.e:
T/0
Basi:
?.
alloued
are
timesLamps). then one of interactive transactions
me
fRfO
(RTM'O
of ihe iaies: read
ar.c
tne
\^T7>.1 O;.).
tc
ap^phcabie \n an environment where orie
p:o:tsc urr^
transactions
W(Oj;
--
ori!>
is
Recall that timestamping
represen:ar:ve of the obtec: exis:s
uhere transactions
umesiamp
also ha\e the
v, :i:
is
at
conf.ic:
is
deteaed
comparing
(b\
aborted, rolled back and redone.
In the case of
this implies that conflicts are detected ax
time fwhen user saves modifications,- The unte
modincations can no: be accepted
a
an opi.irnisuc approach
is
is
rcjeaed.
corrLTu: (save) time.
A
This
commi:
user only finds out that his is
highly undesirable in an
interacuve envu"onment.
Timestamping ma\ be used
execuuon. This however requires
to senaiize rules
Timestamping does not accommodate
re«xecuted v.hene%e: conflicts anst trearmen: of uvi conflicts in categorv
1
that rules be
for the special
rules.
To summar^e: 1.
2.
Timestamping
is
inadequate and inconvenient for Lnieractive transactions.
Tunestamping may be used for senakzing category 2 incurred in mainianing timestamps and reexecuting rules.
Another limi'^uons
to basic
Objea Lens would mostly
T/0
likely
is
its
assumpnon of one copy of an
suppon multiple copies of an
Overhead
rules.
object.
Distributed
object.
6.7.2^ Optimistic Concurrency Recall
thai
the
basic
idea
transaction, then validate
Certificauon, a
other
site that
wntc
it
of optimistic concurrency control
and commit
to an object
is
it
if
rejeaed
is
to
always execute a
the validation test is satisfied.
if
any
In Distributed
later read to the objcCT is validated ai
any
contains a representative of the object
In case of interactive transactions, this
means
that writes are rejected at
commit time
if
a
wme
simuJtaneous
read
environmen:.
Disr-.i^u-.ed csr.:::canor.
I:
rr.ay also
be used
Tnis scheme IT.
;c
resolve
occurs
This
resoi'-e
rr.a;.
ccnfjc-s bu;
'-..
agair.
:s
is
ncor.verjen:
and
v^-^
'-^
u-.terarvAe
ar.
ir.
conflict of caisgor>- 2 rjies.
ur.abie :c reat h-m corilicts of categors-
1
append-only cansaaions.
rales as
siot
cr
rr.a>
LT.proved
':>e
if
Lie grar.uianrv' of upxiate
is
rtduced from
objer. to a
ar.
jie object.
TO
D:f:e'er.ce ber'A-eer. 3ai::
Given our simple
ar.d Dis~.b-:'.ed
mterac::ve
ransactions
Op:urust:c Co.~.rcl are v-r. i-sr^n
condiuons
is
tr.e
database system.s
mar.\ v.
ccr^
'
Cerjjicanon
("Reads
or
Modifies).
Time stamping and
Tne optimistic connnl descr.bed above -.nder diesc
vanauon
to Basic T/0.
This observauon
here transactions consist of multiple reads and
is
\».Titcs
no: true in
of differing
obje~s.
From, the user control
only
makes
make
pom: a
of vie>* bOwi approaches look the same.
vtorkmg copy of
the objea,
one
in
Ho\J.ever, since cptimusnc
prmcipie can get access
the necessar- changes to resolve any conflicts.
to Lhai
copy and
This idea suggests versions as a
svTichronizatirn technique.
6.7.2 J
Two-Phase Locking
Two-Phase Locking was described
earlier in chapter 3
inthe context of distributed daabase
Several 'wechmques (prjnary copy 2PL, centraiizad 2PL) were proposed for an
systems.
environment w.herc several representatives of an object
rw and
vw* conflicts by blocking read and
blocking
exist.
wnte operations
Two-phase locking resolves if
a write
is
occurring, or by
a write if a read is occurring.
In the context of interactive cransacnons this
other user
may
be able to modif>'
it
or read
ii.
means
thai if a user
is
modifying an object no
Recall thai interactive transactions
may
last a
-95-
long Dcncd of tune frorr.
couple of hours
(e.g.. a
seeing or moc::>uig
ar.
Tms means
i
obiec: lor such lor.:
that other users
penoCi of
cases v-here the modli^ca3on^ to an object are independent.
Locking works weL
cases of dependent modifications.
imposes unnecessa." srncmess n.'picaU) block reacers
li
a
m
if
it
ScnalLzabUir>-
unnecessary.
is
is
mod:f> operation
is
is
1
rules
vivi
unacceptable
for senaiizabie k-m
\r.
it.
corilicts be:
Two-phase locking
that readers be
conflicts.
ot biocicec
however necessar\
not consistent with the modifications currendy being
not required for category
is
being peii'ormcd. This again
would prefer
V>'c
is
too
strict in
allowed to read
made
an.
to the object.
Hence locking would be
Locking. ho\>.eve:, would be appropnaie to resolve n\ conflicts of category
and serialize the execuuon of categorv- 2
rules
It
the case of indep^enden: uvi conflicts
the context of interacnve operations.
object even
Tr^>
tLTie.
uoL^d
1
rules.
To summarize: 1.
2.
Locking
is
Locking
is
toe restrictive for most interactive transactions.
unnecessary for
many
executions of category
1
rules.
Writes do
not need to block other wntes because of the append nature of the writes. 3.
Locking
is
necessary to serialize execution of caiegorv 2 rules.
Discussion and Issues
Simila'-iT\
In
all
of Difrerem Locian^ Implemenianons
cases the Lens user has to wait
manner.
The
user
is
if
another user
notified once the lock
is
is
accessing the object in a conflicting
released.
The disadvantage of locking
is
disallowing access to an object for lengthy periods of time.
Differences
The
difference anses
availabilit\' in the
results in
ii
greater
i.-
the
improved access tune (because of
case where
"many copies
message overhead.
local access)
of the object" exist.
and increased
Replication, however,
Deadlock Free
Vv'iu".
our mods!
L-.:srac:i\e transacnor.s there
c:'
interacTive rrar.saaior.s arc simple objec'w.
This
is
(i.e..
no nsk of deadlock.
li
Thj>
'c)ecause
is
do no: involve readLng or modifv'ing more
one
thar.
noi n-ue for automatic nransacuons.
GrgrtuJi'-tr. 0^ Lockir.o
Tht level
res'alt in
viiil
more conom-ency than locking
m
Lens m-ay be hierarchical
depamnent objects that that
Locking
granularir.- of Lhe locks affect &it degre« of possible concurrency.
object.
it
we chose
i.e..
to.
model
ihe
If so. this
wiU
result
vieumg of embedded
This eliminates Lhe need for hierarchical locks.
embedded
objects.
Embedded
appropnate soiuuon would be If a user tnes to access a link
The request 6.7.2.4
is
may
student objea
a
level.
Objects
contain
a
in
granted
if
to
m
less
concurrency.
We
Locking an object
all
a
the
indicated earlier
when
they
will not lock
accessed.
are
maintain the granularity of the locking object, he
to
objects as simple non-nested transactions.
objects are locked
from an
Object
Link
This raises the question whether locking an object means locking
has links to
structure,
objea
ai the
the slot
at
may do
no conflicting locks already
so in two
exist
all
us
A more
object level.
ai the
modes CRead
&
Modify).
on the objea.
Version Control
This approach (used
in
representahves together.
CAD No
systems) resolves conflicts by merging various versions of
waits
oi"
restarts are necessary.
Version control
is
an extension
of optimistic concurrency control where the working copies are made accessible as new versions and validation and verificanon are performed by the uaer through merging.
system
known
A
B
and
to date
is
arc created.
The two versions need question
capable of providing automatic merging. Let us consider two users
concurrently accessing objea "x" in modify mode.
"xA" and "x£"
is
No
to be
At
this point
merged
into
how, when and who merges
versions of "x",
each version excludes the other users changes.
one
the
Two new
thai
would be
the current version of "x".
rwo versions together. The
possibilities are:
The
-9"-
•
The
tw.0 versior:;-
more
•
'•s.ds.
Tnt luo
need
be merged
v.
her. a user
anempis
versio.-i are rr.erzed
Trus occur?
v.
ihe \3s\ use: to
b;.
contro;
is
'
and sees
iha:
it
comm::.
provides no mechajiisrr. to resolve senaiizable
Noucc
approach the disuncuon between
thai in this
object instance reprcsenianve and object instance version
made
i '
her changes made b> one user are dependent on changes made b\
another user uorking concurrently.
is
tc read
ct.^ c^rrer.' \ersior. rreser.'.
The disadvancace of version corJiias.
lo
zx-p'.'.zw \i a lor.r
\ersior. historv- of an object
is
becomes vague. The distmciion
going to be mamtained.
6.7.2.5
Proposed Synchronization Technique: Hybrid of Locking and Versioning
So
ue have
far
opamisnc concurrency control
elirrunatec tL-nestampLne and
as
suitable
synchronizanor. techjuques for mteraaive transactions in Distributed Object Lens. Locking is
suitable to avoid senaiizable conflicts but
is
too harsh and
Versior. cor.CTo! has the opposite problem.
conflicts.
does not guard
agains:
senaiizable
It is
We
conflicts
technique best suitable for Distributed Object Lens
inconvemem
for version
suitable for version conflicts but
propose that the synchronization is
one
combines both version
that
control and locking.
Our hybrid synchronizauon technique uses version exploits
explicit
locking
to
resolve
serializable
control to resolve version conflicts and conflicts
in
the
case
modifications in interactive transactions, category 2 rule execution and
category
1
the system
rules.
The locking scheme described
whenever
a read or write
is
earlier
of dependent rv,'
used implicit locking enforced by
jjerformed by a user or a rule transaction.
locking allows the user to lock an object, so that crucial modifications to be
objea with no interference from other the
same object would be informed of
conflicts of
users.
its
Another user trying
Explicit
made
to concurrently
to the
modify
sutus and would have to wait for the lock to be
-9S-
Readers however will not be blocked and will be allowed
released.
comrmrted version uiih an indication
that
it
is
to read
the latest
being modified.
Interactive Transactions
A
user
is
also provided v-iih
amr-de
laiSsez-faire
new
is
user
may
adopted here and concurrent users are aiiowed
is
own workspace and
versions arc broadcast to
notified of the
use:
create a
all sites
incoming version
(e.g.
new
his
latest
version.
modificanon^.
ic
version of the objea. At commit
have a rcpresentauve of Lhe object.
that
A
by shading the objea form). The concurrent
made
then decide that irrespective of other changes
make
A
option of modifv'ing the objecrt without locking u.
i.version controlj
modifN' an object lt their
time the
±s
The incoming version
is
Only comrruned modificanons
he
to Lhe object
still
wants
to
ignored and user's version becomes the
are
seen.
uncomrr^ned concurrent modificanons
The user may
objea. Our adaptadon of version control
relies
The
user
is
made aware
of
also view earlier versions of the
on the user
version conflicts; the
to resolve
system just detects jxtiennaJ version corilicts and notifies the user of
their existence.
Modif\ -Mod:f\ Conf.ic:
At time
user
I
A
opens the form of object
representatives of "x" arc notified. t-t-1.
He
is
notified that "x"
is
being
updawd
B
is
the other user's t+2.
modif>
work (assuming both
notified of the arrival of the
sent to
new
All sites carrying
At
this point
commits
are authorized users).
all sites
version.
A
as the
both
A
User
«fe
his
in
modify mcxie
AU
but decides to go on.
the responsibility of the user that
The new version V,^2 ^
mode.
also modifying object "x".
aware of the existence of another modifier. It is
m
User B opjcns the form of objea "x"
represcntanves of "x" are notified that
others' existence.
"x"
sites carrying
A
is
then
made
B know about each
changes
B commits
his
last to
changes
most ctirrem version of objea
might choose
ai
to ignore this version or
merge at
"x".
time
A
is
compare
-99-
chances
N',.-
l'
commr.!- her chances objec: ihe
a
late-^:
1;
.
i>
and merge
h:s z'r.zr.z^>
t:
a:
\~: crea;j-.c
broadcasted tc
z
ir.tr
appropnatei)
rv,c
new version
Tnis
^',.-j
is
.
Ir.
now
eiine: case user
A
the lates; \ersior. of
sues una: conain representanves of "x" and installed as
ai!
version
1'2
Mcsify
V
V i*2
Figure 6-". Resoivnc Modif>-Modif> ConfiiCT using
t*3
Hybnd Implcmentauon
Read-Mod:'^ Conflicr Referring
to fig~jre 6-", an>'
Reads ben>.eer tune
read.
tha: objer.
x" '
Vj^-.
A
So
we have
far
is
B
A
being modified.
a:
A may
will result in version "V^ being
\\ being read but with an indication
in
user reading object
x" bcrwecn times t*2 and t-3 reads
'
time t-4 reads version
if
t
a modifv'
Vj_^-.
is
done during
a read.
Consider the
Figure 6-8.
in
reads object
User
A
not addressed what happens
'
x' ai time
begins to modif> object
"x".
and t*2 will also result
:
use- reading objec: 'x'
overlap dcpiaed
User
reads that occur before time
x". '
t.
Version V( apjpears
User
A
is
in the object form.
notified of the
fan
At time
thai another user
is
i-l user
modifying
choose to terminate his read untU the new version becomes available or
continue the read. view, of object "x".
Once
the
modify
is
committed
at
t+2, user
A
is
prompted
to
to refresh his
100-
•^et:
1*2
i-l
^^
V
V
t R»'rt«h
Figure 6-8: Resolving
a
Read-Modif> CoaLiCt
Hybr.d LT.piemenracon
usir.g the
Automatic Transactions R-^e rar.sa:r:or.s need
".o
acqj-s
the reaci a.-c uT.tes.
Ir.
mtcracuve cansacuons we had
ihe appropr.ate -c:id ar.t modiy_^ locks bercr; pertcrr^ir.g
users are always allowed :o read
ne most
Consider
Locjong ensures
categon. Z rules.
firs:
No
senalizable crds:
neec for 'ead locks because
r.o
rec«n: corruTurted version of a object.
rv-c r^Is rar.sac::or.5
ma>
iha: the
ho'.d
r^e?
are perfcrrned a:orrjcal".>
ir.
ccr/.icung locks on Lhe same objec:.
Confliaing locks are modii>-modi^' and read-modifv-. Category
1
conf:iring.
rjles are created slighLy differently.
Two
category
1
rules
OrJy read-modify locks
would be allowed
append-orJy 'Ante. The resulting versions ntay simply
'oc
to
are
considered
sLmul'LaneousIy pjcrform uheir
merged.
Deadlock
In the case of interactive ransactions
were
sL-nple
(i.e.,
an ass'umpuon • If
m
objea =
we
disregarded deadlock because the transacaons
only co'uld wnte the suigle object thai was read). the case of automatic transactions.
We
this rule.
The
make such
Consider the rule
task then set taskmanager. count to taskmanager. count
and two agents corxurrently applying
can not
first
+
1
agent Tl. reads object
reads and writes the taskrr^tager objea Oj. The second agent
tl,
reads objea
O
and then
O2 and
Lhen
-101-
reac anc u-r/.e
'jie
Figure 6-9 iUustrates^
obiec! O.
i3s'r:rr.2s\2cer
2
pcssibJe deadlock tha:
m2> anse
Tl
WfOp
TI
R'O
RfO,)
t:
R.0-'
R''C..w,o,i
-
TZ
ReadJocki'O'
ReadJock
Grar.-.e::
Granjec
Tl -Readjock(0,i
T:
Gramed
Grarued
Tl
-
Wnielock(0,)
Denied
T2
-
-
0--i
ReidJcx±(0,j
WntciockfO,^
Derued
Deadlock
Figure 6-9; Rule Execution Deadlock
We in
co'jJd
choose
an ordered
prevcn: deadlocks
tc
mame:.
is
such cases by insisting that rules obtain al locks
Lock orderjig avoids deadlocks and enforces
a
senal
order
execuaon.
Another
possibiiin.-
is
deadlocks using timeouts.
tc detect
halted for a specific period, Lhis rules
execuuon need
to be rolled
is
If
execution of rules have been
an indication of a deadlock.
If a
deadlock
by another rule or a user). Tt\z system should permit the user
bcrween
interactive read
and
a rule write to
ii
has been locked
in case of an overlap
view the changes made by
the rules as soon as
they arc committed. Similarly for an overlap of an interactive modify with a rule
modifier should be able
To summarize, •
to
access the changes performed by the rule write
the user will be
Read-Modify Conflict
-
exposed
A
reader
is
he %^ishes
always allowed to read. TTie most current
A
is
modifs' as soon as
knovvTi to the svstcm.
is
if
\*Tite, the
to the following in case of concurrent access.
version of the objca it
detected,
back and reexecuted.
In executing rules the latest committed version should be read unless (either
is
returned.
reader
is
made aware of
a concurrent
to.
-ic:-
•
Modify -.Modii>' Conflic: cnce
he commits
merge
to
rr.ocry :>€Zjs.-
accounted for
•
Locking This
ir.
his
m
second modifier
a
the respons:bU:r. cf each modif.e: v^hsr.
:s
I;
made aware of
is
modificanons wi^h any version received SLice
Tr.-.i
r-arar.'.ees
'Jie
latest version
'-".i:
al
•c.e\ir.".
changes comm;r.td so
to
:ar are
prevent modify -modiiy- conflicts.
cases where the modifier feels his changes are
an object w-X no: block readers
h-^s
ob-tc.
r:' tr.e
Locking may oe expjc:t-\ done
-
done
IS
Tne modine:
kr.o'AT. :c 's\t s' blinking the refresh
command on
the object
form-
Adding
links to an
opened folder
(i.e..
an append transacnonj
may
be done automaucaUy by
n
the system.
Implementation
A
disiributed
Objca Lens enviror-ment wuh mulaple
An objea manager each
site
at
each
site
representatives of objects
concrols access to objects
ai thai site.
maintains Lhe following informanon for each object:
•
Locked
•
Be^ng Modified, by
or Unlocked.
whom
and
at
what
site.
is
assumed.
The objea mana.ger
ai
-IC?-
•
Being Read. b>
and
v.norr.
\^ha: sue.
a:
• Times:a.T.r of the late?; ^ersio^. 'uher.
in
modif\ mode,
ioc^ed. a:^e^-
is
d=la\ed untJ lock
is
the object
:?
venfie?.
:he object
is
is
to date as tc
who
Once
modifier.
nev^ versions
commit
the
The
manager
sites will
beer. rece:\ec
installs
broadcast to
is
remove
'user"
at
If the obiect is
it
is
manager then broadcasts
rum
will in
all
managers
verifies that
the user
is
list
is
is
is
control
at
implememauon by adding
Lnformc^
The object
at
commit
time.
Receiving
The new version is
being locally
made.
if
the
objea
presented to the requester.
automatic
objea "x"
If the
"x".
is
If
is
being modified and
an object manager
being read then the user
most 10 objea forms opened
for
umesiamp generited
of modifiers for objea "x".
The informauon maintained by each objea manager since a user will have
no
and interacnvely merge the rwo together.
manager checks
modif>'ing an objca that
up
of a neu
notif>' local users
objca manager
to other
are kept
sues that contain representatives of objca "x".
'sue" from the
Tne
being modified.
new version has been received,
explicitly
received \e-sion
shaded form of the object
Concurrency
by another user
his changes, the
the latest version of object
a note of the
else
They
If
Object Mar.ager
"x'\ so thai other object
as a nev. versio.-! uith a global
t^i..
Similarly for a read, the object
someone
If a
object
Object Manager.
If not, Lhe
reieaseerformance that centra] site
paradigm.
still
we decided
the
or.
car.
i.
crainonai
proposed hybnd scheme of locking anc the obiec; exits Ln
some
state or
Ln fact. Lhe
equivalent with respect to our proposed concurrencv
is
disnbuted option with pnmarv' copy update.
the
tc
if
amestampmg). The concurrency scheme mos:
more than one cop\ of
that
"centralized with caching" option control
valid only
Tnis leaves us with the centralized with caching opuon.
one time.
a:
.
first clarr: is
Objen Lens was
suuabie for Disr:bj:ed versions.
because of us SLTiple: concurrency conro! scheme and
arn-2r::'>e
is
Despite the unproved
be attained r> caching, the bottJeneck imposed on the system b\
Tne
exists.
centralized cached approach implies a central rule resolution
Ail roles are resolved
m
Note, however that e%en
t-ne
the
at
the central site, and the objects that
match
centralized case, pnvate objects are located
are
reromed.
at
the local
worksatation. hence any rules that consider shared as well as private objects need to be applied
at
the lozzl
wcrkstanon
for the personal objects
and
distributed approach
the due to
ai
the central site for shared
objects.
Much
of the comp!ex!t%
scheme and com.piex
in Lhe
rule resolution
somewhat
concurrency scheme
is
distnbuted approach.
The
scheme.
the
same
rule resolution
a large
overhead
As mentioned for the
complex concurrency
earlier
performance of the
centralized with caching
paradigm on the other hand
objects are distributed over the system.
problem but introduce
is
is
and the
complex because
Fully redundant replication will eliminate this to
propagate updates and a large increase
in the
amount of memorv- space needed.
We
have eliminated concurrency control as a key consideration in deciding between the
"central with caching" arc rule resolution
and the "distributed approach". The key considerations
and contenuon.
Rule resolution
may
that
remain
be simplified in the distributed
-lOS-
approach
memon.
if
we
cor.s:der ihe
space to store
To summanze.
i".e
a:
fuJJ>'
redundant case.
each v.orks:ai:on
2I'.
The
fully
redundant case requires more
objects used whether personal or shared.
k?> issues m. c.sz\d-^s berv-eer.
tr.e
isrr.buted and cenralized approach
are:
Csr.~2us.td
Disinbited Arrroach
cac:
'A :-"!
BorJeneck
No
Simple Rule Resolution
Complex Rale Resoluuon Simplified in FuUy Redundant Case
contention
a:
central sue
Tradeoff between and Space and Lncrtased Locking and L'?ca;e Prorasatior.
Overheads
Creation of Shared Objects
We
proposed two ways of making an object shared. The ongmaior can:
insen a reference to
1.
2. insert a link to
We
concluded
esscnual to
it
it
in a
in a
shared object.
Lha: Distributed
initiate
Mail Message.
Object Lens -ttdtd.
sharing of a parent objea.
objeas.
Protection of Shared Objects
The two options we examined were;
1.
Capability -Based System
2.
Access Control
List
Svstem
to
supper. boLh methods.
T^e second was
Tne
first
was
narural for sharing of child
-lOt-
The proposed scheme
Hsbnc
•
A
is.
Capab^;r\ -based anc Access Cona-o! Lisi
c:
capability -based protecxior. mecharusrr.
Object Lens
is
a uzt.t: -or.sr.-.tz s>s:err.
ids) to objects before they
objea unique
object
id
Capability -based
An
with
SN'stetr.
access control
has
pitfalls
its
list s\sierr.
the sense that users
can access them.
relevant
the
ir.
the namraJ choice for Distributed
is
It
indicating
fields
uould mean
a
We
to use access control lists in a restrictive
propose
based
s}.-stem.
ir.
TTie overhead incurred here size of the list
read,
write
would be too long
the unique
would be
Tlie rescr,cti\e
list
objea
would be
manner
2.
Basic Timestamping
3.
Distributed Certificauon
4.
Version Control
access.
in
maintaining
lists for
to
if
suppon if
the capabilir.--
they have the correct
they are not restnaed by the
list.
permissive case mainly because of the
usemame
Concurrency Control and Update Propagation
Simple Two-Phase Locking
and
easier to maintain.
specifv auLhorj^d users' attributes.
1.
id
less thar. the
creator by indicating auLTonzed users b\
The four options examined were:
and delete
to maintain.
T>J3 hybrid s>stem allows users access tc objects
access rights as indicated
have links (unique
thus reasonable to augment the
tremendous overhead
A
Lis:
to
Lens.
(uncontroUcd access, rcvocarion of access).
each object.
permissive
is
need
Objea
The
list is
specified by the
or by formulating a set of rules that
-110-
The proposed scheme
•
Users
Hybr.w
o:'
lock^-.: ar.d '.ersior. ;or.ro!
LT Distributed
Object Lcr.s L-/^rarj'.
Two-Phise Locking object.
is;
Tr^s mear^
resolves corilicts by
tr.a:
the
Basic Timestamping scores
The user
:s
notif.ed that hjs
vi,orse
s;r.:e
work mignt Such
to
tr.e
at
long pcnocs
most one user
a: a
o: time.
time
to
wait (perhaps for a long tme).
use:
is
res'ul: in a
i:rir.endl_-.ess
modify an Locking
is
not aware of other concurren: users. coriiict
and thus rejcr.ed
would no: be
v.
hen he
is
tolerated by an Object
user.
Distributed Certification scores badly the
aiiowmg
second user his
ready to commit the changes.
Lens
ei> ed;: objects for
Timestampmg approach.
It
is
a:
as well.
com-m::
The user
tme
faces the
same urir.endimess
as
that a user's mocjifications are rejected
because of a coniiict.
Version control's to
make then
mam
artractiveness
is its
laissez-farr att:njde.
modificaiior^s and hence creaie
versions can be then merged to create the
new
new
Concurrent users are
free
accessible versions of Lhe objea.
These
scheme however
fails to
-jrrent object. This
resolve senaiizable corilicts imodificauons that are dependent on a concurrent users'
work).
We
hence proposed a hybrid scheme
version control.
Explicit locking
is
thai com.bines locking (explicit), notificaaon
provided for
serializab'.e contlicts.
responsibility to assess his changes as vital and hence lock the
is
It
objea making
it
and
the user's
inaccessible
-into cLneri
1:'
oner:
tr,e
aireac\ locked, iher. oLner users
No:j"ica::or.
to the object are yr.il
environrr.en:
is
Trj> m^cificauor.
tc de:£rrr.:r.t ar.v
u ams
rr.:zh:
it.z:
ha\e
Lhia:
changes made
lead to informal communicatior. berwecn ihe users
At comrru; time, the user
beer. r^:f.\td. since the user
7.2
The Proposed Distributed Object Lens
We
choose
tc
trusiLng
users of o*-hcr concurren: users in Lhe laissez-faire
possibilirv- of cor_f.icis.
any pending \ersions
w. ai:.
began
his
is
prompted
to
merge
work.
descr.^e uie disrributed approach to implementing Distributed Object Lens
because of Lhe genera:;:> cf the solution. caching' approach. Tne system
»•
I -»
I '.
e
"
c
'
Zitz: re-;e'
1
is
depicted
A in
speciaj case of this
is
Figure 7-1.
werkfttnor
r
the 'centralized wiLh
.
-ii:-
Lidicatirig the o-Jne: iocatior.s of rep:essn:a:;ves of shared objects stored Io:a]I>.
needed
to er.sure
'j-.a;
prora£a;ed b^ the
5::e
-pdaies are propagated to those sites
uhere
the jrdate occurs
u^
• Trar.slatior. table
the
ids
L'-.temai
r.eu
m case
or
:s
versions) aie
Distr.b'jted jrda'.e rro'oatatior.
case of version cono-oi anc pnmar>- copy iocking and upxdaie
Tne Object Mar.age:
Updates
Tn:s
is
used
ir.
iocKng.
have the foliowir.g duaes:
of the system wide ids generated by the objea manager and Lndicating the physical location
of the
objea
in
memory
generated by the local object ser.er.
•
A
list
of
all
objects currently being
•
A
lis:
of
all
locked objects.
•
A
list
of an> received versions of objects bei-.g viewed.
• .A
•
Tne
capabilirv- to
reduce access
the local
modes of an
w orkstauon.
Tnese need
to
'This ciuJog could be
^Agaic such
object given the system, wide
id.
Maintain restrictive access control
of change
at
catalog of local snared objects and their ouher sites-
objea
•
viewed
be replicated
mainumed
a hst could be
ai
each
centrally for
lists
site
all
maiDiained centrally
for shared objects
where
objcc:
to
a
copy
at
the local site.
o: the object exists".
maaagen
mminuze on
space used aod updaii propa^anoo
is
case
-115-
7.3 Direction of
Future \\ork
Vv'e f:e'. Lha:
quest for
ir.
o-oi
ar.
obje:; sharing
light or. the relevant fearures that influence the
scheme
for
Object Lcr.s v-e have shed some
design of a concurrency concoi scneme
ir.
an object-onenied h>-penext system that incorporates both user-driven long transactions and
Our scheme
shor. lived automati: rransactionv
differs
allows for rephca5 and versions of an object to exist.
We
most recent version of an object.
the
potenuaJ conf.ict and lease versions together
system using a
It
is
that
it
also pwrmits a user to always viev.
it
enough
is
to
warn
the user of a
to explore the possibilities of
automauc merging by
the
of rules.
sp)ecific set
character.s::c
It
earlier in that
user to resolve the conflia by merging relevant
the
to
wonhwhile
is
Initiating sharing using mail
This
it
propose
from those proposed
messages
is
another feature not
common
in
hypenext systems
of our systerr. because of us integration with an clecn-omc
system.
Protecting
object.'^
commuTury. sharing
is
\^'e
is
feel
ar.
issue
that thus
has not been seriously addressed in the h\-penext
that
is
important in a distributed environment where object
possible.
Out examination of object sharing collaboranve
applications
requirements
for
some
which
in
Objea Lens have
transactions
less
reveals a class of infomiarion shanng and stringent
concurrency
and objects they support
and
We
consistency
identified
transactions as long interactive transactions thai involve editing object forms.
users in such applications need not see consistent copies of the
aware of the existence of other versions. Moreover, a user's
objea
role has
those
Multiple
as long as they are
expanded
to include
also
suppon shon
automatic transactions thai rcquu-e to see consistent copies of the objea.
Concurrency
explicit
merging of different versions.
These applications need
to
.
conn"ol cor.trr!
IS
much more
rcr
one
s~j-.geri:
c!
set
a less
to
case
this
~ar.5ac;ions
concurrency conroi scnemes
Tne bcnents of
m
or
I:
another,
accomjnodate
not sufncient to provide concurrency but
rather
ix)th ry-pcs
Oiir
svstem
must
oro\'.de
of transacnons.
stringent consistency and conc,irrency requirements Lndicate that
work applicaiicns should
future cooperat'.' e
is
aliovi.
uorkgroups
to
choose berue«n snc:
concTirrency conrol provided by lockir.g and more flexible concurrency control provided
by version merging.
This work has
m
prcsenor.g
recommending schemes
for object
out m.an> of the implementation details.
left
the design space for D;str.b-ted Object
Lens and
m
It
was successful
creation, object protecuon. concurrency concrol and ujxiaie propagation.
Tne mie
these schemes
is
Lens prototype
Depending on
the choice of a pla'Jorm for Object Lens a
foUowmg •
l-.
tr.e
nee-ds to be
Objea
actual implem.entation c: the EXsrnbuted Object
performed
l".
the
foUowmg
area
test
more detailed analysis of
d-urj~.g the
of
Lhe
implementation phase:
Deletion.
• Distributed vs. Prur.ary
outlLied a disnbuted
copy update
opuon whuch
li the
case of version control.
We
have
co'uld suTiply be specialized to Lhe prjr.ary
copy option. •
Rule resolution.
•
Recovery-.
•
Version management.
^
The pjower of coopcrarive work applicauons shanng.
We
hope
thai Lhe first prototype of
imponance of scmifonnal systems information.
is
only realized with the availability of object
Disributed Object Lens would
to acbueve sharing of scmistnictured
illustrate the
knowledge and
-115-
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?.b.
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