scheme can be derived according to a classi cation of web pages and their links. 1 Introduction ... another web page. For example, one would not create an own.
Design of World Wide Web Information Systems K. Lenz, A. Oberweis Lehrstuhl fur Wirtschaftsinformatik II, Universitat Frankfurt/Main, D-60054 Frankfurt/Main, Germany
Abstract:
Public global networks as for example the internet with the world wide web represent more and more a suitable platform for distributed information systems. Yet, the lack of methods for conceptual modelling (as known for the database design) often leads to quick and dirty implementations with uncontrollable data, data redundancies and data inconsistencies. In this paper, we present a method for the conceptual modelling of information systems within the world wide web. Starting from an extended Entity Relationship model, a page link scheme can be derived according to a classi cation of web pages and their links.
1 Introduction With its growing acceptance and use public global networks represent more and more a suitable platform for distributed information systems. An example is the internet and the world wide web (WWW), the globally distributed public hypermedia information system on the internet. Speci c problems and methods for the design of distributed information systems on the internet may be easily transferred to other online services such as CompuServe or America Online. In the WWW multimedia information can be retrieved directly by specifying a unique address, the URL (uniform ressource locator), via search engines, by navigation, that is following certain pointers between chunks of information, or by speci cation, that is navigation following a given hierarchy. The information consumer therefore can individually determine his method of accessing the information he searches for and of handling the information
ooding. The information supplier on the other hand may support quick and easy information retrieval by providing information in a structured way and in the right context to the information consumer. But facing the mass of information, the information supplier may want to present in the web, he has to develop the information system very carefully, paying attention to all phases of the development process. Nowadays, the implementation phase is already supported by various tools as for example HTML-editors, database interfaces or tools for visualizing the structure of the information system. Until now, methods for the design of information systems only exist for speci c problems (Isakowitz et al. (1995), Garzotto et al. (1996), Goll (1996)). In practice, the lack of comprehensive methods for conceptual modelling still leads to quick and dirty implementations with dicult information management, inconsistencies and uncontrollable redundancies.
In this paper, we present a method for the conceptual modelling of a web information system using an extended Entity Relationship model. For that purpose we extend existing methods for database design for the design of web information systems. In the next section we present the page link model which allows the modelling of information and its interrelated pointers. The derivation of a page link scheme for a simple university example will be described in section 3. Finally a brief outlook on future work is given.
2 Page Link Model For the conceptual modelling of web information systems, we propose a graphical modelling language similar to the Entity Relationship model for the database design. It allows to model aspects of the real world independently from implementation aspects. The language consists of two modelling constructs: chunks of information, the web pages, and their interrelated pointers, the links. Because web information systems are based on hypertext techniques, the page link model has to combine net like structures of the linked multimedia information and the hierarchical structure of supplementary navigational help ('go one page up', 'go to homepage' etc.). In the following, we describe the components of the page link model in detail. 2.1
Web Pages
In order to reduce the number of real world objects to be modelled and thereby to increase readability of the page link scheme, all web pages of the same type, with identical structure and comparable information content form a class of web pages: the page type. In the scheme, the page type is represented by a rectangle with the page type name in it (see Figure 1 (a)). The speci c web pages are instances of the page type. name1 name (a)
name2 (b)
name grouping criteria
(c)
Figure 1: Graphical representation of page types Ergonomical aspects play an important role for the design of web information systems so that some ergonomical considerations may or even should be anticipated during the phase of conceptual modelling: If, for example, information that is to be presented on one web page is too small (in the
sense of disproportionate high retrieval time and costs), it should be integrated into another web page. For example, one would not create an own address web page for sta members whose addresses are not very detailed, but include the address on their home page. The page link model therefore allows aggregation of page types (here 'sta ' and 'address'). The graphical representation is shown in Figure 1 (b). For the same reason, it may be useful to combine all instances of a page type on one single web page and thus create information lists (for example a list of all sta members) which can be grouped by arbitrary criteria. The corresponding graphical representation, two overlapping rectangles, can be seen in Figure 1 (c). The grouping criteria is only indicated when the list has been grouped. 2.2
Links
In analogy to web page classi cation, links between web pages of the same page type with comparable navigation characteristics are collected to one link type. Links with comparable navigation characteristics, i.e. especially links of the same link type, must belong to the same class of links. Links can be classi ed with respect to whether they are a part of a speci c link pattern, their semantics, their validity for special groups of users, whether they can be (locally) administrated, and other criterias. In this paper, we concentrate on the rst two classi cation criterias. The rst classi cation is the most relevant for the page link model. The classes and their graphical representation in a page link scheme are shown in Figure 2. An index link between two page types means that starting from a web page of the rst page type, we can navigate to an index page and there have bidirectional links to several web pages of the second page type. In contrast, the guided tour link allows to visit several web pages of the second page type one after another with an extra web page as starting and ending point. The index guided tour link combines both navigation structures (Isakowitz et al. (1995)). unidirectional bidirectional link link
index link
guided tour link
Figure 2: Classi cation of links
index guided tour link
Due to their di erent semantics, we distinguish content links and structural links. Data, other applications or meta-information can be accessed from the context via content links whereas structural links serve as supplementary navigation possibilities. Structural links are graphically represented by dashed lines.
3 Derivation of a Page Link Scheme The basis for a page link scheme is an extended Entity Relationship diagram (Schlageter and Stucky (1983)). The Entity Relationship model is widely accepted for the conceptual modelling of database systems. But it is also very suitable for the derivation of the page link scheme, because relationships between objects of the real world indicate a link between the information concerning those objects. However additionally, a hierarchical order has to be found for the page link scheme to support structural navigation and for example the le management. Finally, the information system can be implemented according to the page link scheme. school 1 belongs to n 1
1 department
has n staff
offers n courses 1
guest
function
responsible for
professor
teaching personnel
n
Figure 3: Simple Entity Relationship diagram for a department The Entity Relationship diagram of Figure 3 shows an extract of the university world: a department (belonging to a school) wants to present its sta and courses in a web information system. The sta of the department consists of the teaching personnel, the professors who belong to the teaching
personnel (but have to be modelled explicitely, because they may have additional attributes or relationships compared to the teaching personnel), and nally the guests (specialization in the ER-scheme). The department o ers several courses for which the teaching personnel is responsible (one-to-many relationships of the ER-scheme). The further explanation of the single steps of deriving a page link scheme will be based on this example. 3.1
Step 1: Page Types
First, we create a corresponding page type for each entity type of the ERscheme. For all page types, we then have to consider if ergonomical aspects imply some modi cations. For each entity of the type 'sta ', only few information would be presented on a corresponding web page: The more detailed information can be retrieved on the web page which corresponds to the entity of one of the specialized entity types 'guest', 'professor' or 'teaching personnel'. Here, a list page for all sta members is very useful and, in addition, we group all list entries by function. Thus, we create a page type 'sta ' with the graphical representation of Figure 1 (c) and a single instance, the list page. In contrast, it makes no sense to have such a list page for the courses. Too much information on one page can be confusing for the information consumer. The graphical representation of all other page types therefore corresponds to that in Figure 1 (a). Now, the page link scheme has to be completed by the links. 3.2
Step 2: Content Links
We derive the content links from the relationships (including the specialization) of the ER-scheme. For one-to-one relationships or specializations, we create a bidirectional link between the corresponding page types, for one-tomany relationships a unidirectional link and an index link. Moreover, the index page of the index link can be an own web page as shown in Figure 4 (a) or an anchor on a web page of the page type from which the link starts (Figure 4 (b)). For many-to-many relationships, we consequently create two index links in both directions. In order to complete the page link scheme by structural links, we then have to derive a hierarchical structure from the ER-scheme. 3.3
Step 3: Derivation of a Hierarchy
In the beginning, we mark those entity types which do not directly belong to our application and whose corresponding web pages are not under our control ('school' and 'guest' in our example). They will not be part of the hierarchy. Afterwards, we determine a primary entity type (the 'home page')
teaching personal
teaching personnel
course
course
respons. for respons. for (a)
(b)
Figure 4: One-to-many relationship 'teaching personnel is responsible for courses' whose corresponding page type is going to be the root of the hierarchy tree. As long as there are still entity types left whose corresponding page types are not yet nodes of the tree, we continue searching subordinated entity types for all leafs. An entity type E is called subordinated to an entity type E if there exists a one-to-one or a one-to-many relationship from E to E or if E is a specialization of E . For example, the entity type 'sta ' is subordinated to 'department' and 'professor' is subordinated to 'sta ', being a specialization of it. The page type of the subordinated entity type becomes the child of that page type. 0
0
0
department staff professor
course
teaching personnel
Figure 5: Hierarchy for the university example This procedure does not lead to a unique hierarchy tree. For example, an entity type can be located in the same depth of the tree, but in di erent branches. As we have no sort of ordering of the relationship types of the entity relationship model (which could help to resolve the ambiguities), it is left to the developer to decide for one speci c hierarchy. The hierarchy in Figure 5 can now be used to create structural links and as a unique ordering to store the web pages during the implementation phase. 3.4
Step 4: Structural Links
The page link scheme is being completed by structural links if no corresponding content link of the same class already exists. However, with regard to
the design of the user interface, the modelling of the link as both content link and structural link can be reasonable. For example, a link can have two anchors on a web page, one within the context (for the content link) and one as a navigation button in the page header (for the structural link). The structural links are derived from the page type hierarchy as follows: for each page type, unidirectional links to the page types corresponding to the root and the parent node must be created. Optionally, we can create a unidirectional link to all page types which are brother nodes (in our example between 'sta ' and 'course'). Further, we can create an index link from a page type to itself (for example if the page for a member of the teaching personnel contains a list of the other teaching personnel). The complete page link scheme for our university example is shown in Figure 6. The structural links are represented by the arrows with dashed lines. Furthermore a link (between 'school' and 'department') is shown whose anchor is on a page of an external application and therefore can not be (locally) administrated. department
school
offers
staff course function
professor guest
teach. personnel
resp. for
resp. for
other tp
other tp
content link structural link link without administration possibility
Figure 6: Page link scheme for the diagram of Figure 3
4 Future Work In this paper we have described some data modelling concepts for the design of world wide web based information systems. The concepts are integrated into INCOME/STAR (Oberweis et al. (1994)) which is a collection of methods and tools for the cooperative development of distributed information systems. In the future, some aspects of the page link model must be re ned. The page link model always must be adapted to new standards or extended standards for the world wide web like for example frames and layers in HTML. Furthermore the modelling of user interfaces has to be discussed in more detail: e.g., the use of templates or the representation of di erent link classes. References
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