Abstraction Patterns in Model-Based Engineering - ModProd

Abstraction Patterns in Model-Based Engineering Bran Selic

Malina Software Corp., Canada Zeligsoft (2009) Ltd., Canada Simula Research Labs, Norway University of Toronto, Canada Carleton University, Canada

[email protected] © Copyright Malina Software

What is ―Abstraction‖?  A human defence mechanism for coping with overwhelming complexity  Often, it is our only mechanism for dealing with complexity

SYSTEM

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© Copyright Malina Software

Abstraction: A Fundamental Engineering Skill  The ability to see the forest (i.e., the system as a whole) despite the trees (i.e., technological detail)

 It seems to be less common than people might expect  ...particularly among software people  Many programmers but few ―architects‖ • Agile development and ―big design‖ perceived as mutually exclusive • E.g., M. Fowler: ―Is design dead?‖

 Requires an interest in and a concern for the system and its purpose (users)

 Can abstraction be taught? 3


Background: Definitions  ABSTRACTION: A process by which ―higher‖

concepts are derived from the usage and classification of literal (―real‖ or ―concrete‖) concepts, first principles, and/or other abstractions [Wikipedia]

 ABSTRACTION (computer science): a mechanism

and practice to reduce and factor out details so that one can focus on a few concepts [Wikipedia]

 Two approaches:  Relaxionist: By weakening of constraints  Reductionist: By removing ―irrelevant‖ detail • Relevance is a function of viewpoint (set of concerns)

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For Our Purposes...  ABSTRACTION : Selective reduction of information

of a system which preserves its salient properties relative to a given set of concerns  Refinement is the inverse process

In engineering: abstraction  modeling

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Engineering Models  ENGINEERING MODEL: A selective representation

of some system that captures accurately and concisely all of its essential properties of interest for a given set of concerns • We don’t see everything at once

• What we do see is adjusted to human understanding

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Why Do Engineers Build Models?  To understand  …the interesting characteristics of an existing or desired (complex) system and its environment

 To predict  …the interesting characteristics of the system by analysing its model(s)

 To communicate  …their understanding and design intent (to others and to oneself!)

 To specify  ...the implementation of the system (models as blueprints)

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The Trouble with Models  ―The devil is in the details‖  Leaving out something crucial  Underestimating its relevance  Accidental oversight

 Inaccurate and untrustworthy models

Exacerbated by the common practice of NOT documenting the abstraction process 8


Characteristics of Useful Engineering Models  Purposeful: 

Constructed to address a specific set of concerns/audience

 Abstract 

Emphasize important aspects while removing irrelevant ones

 Understandable 

Expressed in a form that is readily understood by observers

 Accurate 

Faithfully represents the modeled system

 Predictive 

Can be used to answer questions about the modeled system

 Cost effective 

Should be much cheaper and faster to construct than actual system

To be useful, engineering models must satisfy at least these characteristics! 9


What About Software Modeling? PH reached X Current PH

start Control PH

Monitor PH

stop Raise PH

Input valve control

―…bubbles and arrows, as opposed to programs, …never crash‖ -- B. Meyer

―UML: The Positive Spin‖

American Programmer, 1997 10


The Prevalent Attitude to Modeling  We don’t do modeling here...it’s a waste of time  But...

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Modern Model-Based Software Engineering  Models can be refined continuously until the application is fully specified  the model becomes the system that

it was modeling!

void generate_data() {for (int i=0; i