K ATHOLIEKE U NIVERSITEIT L EUVEN D EPT. OF C OMPUTER S CIENCE ESC Europe 1999 report 1 System Design: Architectures and Archetypes ( Shlaer-Mellor Method)

KATHOLIEKEUNIVERSITEITLEUVEN

DEPT. OFCOMPUTERSCIENCE

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System Design: Architectures and Archetypes(Shlaer-Mellor Method)

by Steve Mellor



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In general Speaker

Stephen J. Mellor (Project Technology inc.) worked at CERN (accelerator control systems) now Senior Vice President of Project Technology

Presentation (very) interesting detailed theory but few examples



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Presentation Overview

Architectural Styles Shlaer-Mellor Method

Subject matters Executable domain models Translation

Software Architecture Executable domain models Translation Specifying the architecture System construction



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Architectural Styles






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Architectural Styles

Architectural style is the particular approach to building a software architecture

Most common architectures in real-time and embedded systems: Monitor and Control Systems Transporter Systems Transactions Systems



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Monitor and Control Systems

Definition: collection of interrelated control loops (control loop: reads in values, computes desired value, compares, writes values)

example: chemical plant, car, airplane, ... characteristics:

often hard deadlines data represents `real world` (or part of it) data must be up to date significant computation on data



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Transporter Systems

definition: systems that move data from one place to another

examples: telephony, telemetry, ... characteristics:

generally little computation on data itself high throughput requirements generally reponse time requirements management of buffers have persistent application data describing routing



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Transaction Systems

definition: systems that maintain a picture of a real or hypothetical world, accepting requests to query or update that picture

examples: on-line banking, reservation systems, ... characteristics:

considerable persistent application data variable response times significant throughput requirements



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Hybrids

Of course ... ... most systems are hybrid



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Selecting an Architecture Christopher Alexander:

form and context

In terms of software: the form (= software architecture) is determined by its context (=properties of the `environment`)



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Context of a System The External World

rate and volume of events in the external worldnormal ratesburst rates

its natural periodicities how frequently data values change



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Context of a System Non-Localized Requirements

continuous operation? fault tolerance/recovery personnel and equipment safety

Business Constraints number/location of processors upward compatability hardware/software platforms



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Context of a System The context of a system also shows performance

requirements detect the `High Spots`

Identify critical threads Identify worst case bursts Identify the required processing for each ...

= quantitative requirements



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Shlaer-Mellor Method






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Shlaer-Mellor Method

Shlaer-Mellor method is based on separation of subject matters

I/O, User Interaction, Application domain, Software Architecture

specification of (executable) model for each subject matter translation of these models into code



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Subject Matters


I/O, User Interaction, Application domain, Software Architecture




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Subject Matters

different categories of subject matters (domains) application domain (visible to end user) services domain (more general domains) software architecture domain (data, control, structures and

time) implementation domain (OS and programming languages)

separate teams can work on separate domains



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Subject Matters

example: chemical plant

Chemical Plant domain

User Interface

I/OAlarm Service

Software Architecture

Unix C++

end user domain

services domain

s.a. domain

impl. domain



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Model Specification


I/O, User Interaction, Domain1, Software Architecture specification of (executable) model for each subject matter translation of these models into code



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Model Specification Special focus on

end user domain:specification of an executable model

software architecture domainconsists out of an execution engineand a set of archetypes

both domains should be strictly separated



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Model Specification

public class A{...}

Translation

code

Application Model

Architecture- execution engine- set of archetypes

Phone

On

Off

Hook

Hook



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Translation into Code Shlaer-Mellor method is based on

separation of subject matters I/O, User Interaction, Application domain, Software Architecture




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Translation into code

The translation is done with archetypes specify how model concepts are translated to code

In fact we get some form of a model compiler a `standard` compiler generates assembler from source code a `model` compiler generates source code from a model (for

example in UML)



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Software Architecture






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The Software Architecture

Software Architecture is the abstract organization of the software

Proclaims and enforces system-wide rules for the organization of data control structures time



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Data

The software architect prescribes storage schemes tables or arrays trees, linked lists, ... ...

And how to access them direct access by name or by pointer? indirect access (encapsulation)?



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Control

The software architect prescribes control what causes a task to execute? what causes a task to relinquish control? what`s the next function to execute in a taks? how to coordinate multiple tasks accessing common data to

ensure consistency?

Strongly dependent on organization of data



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Structure

The software architect prescribes how to package code and data (= the application) in: tasks functions shared data areas classes

and the allocation criteria for allocating parts of the application to these structures



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Time

The software architect prescribes how to provide time-related services: absolute time relative time



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Executable domain models






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Executable Domain Models Notation chosen: UML Purpose: define an executable model for each subject

matter Three kinds of models

Essential models: capture the complete scope and behavior of the system

Derived models: additional views of the essential models Auxiliary Models: augment the essential models



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Executable Domain Models

Essential models: Package Diagram Class Diagram State Diagram Processing Spec

Example used: Chemical plant Recipes: how can a substance be made Batch: an amount of identical substances to make Temperature Ramp: status for one substance in a batch



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Three steps

Create a class diagram Create a state diagram for each class Associate actions to states



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Class Diagram

Example: chemical plant



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Lifecycles



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Actions

Specifiy the logic for each state`s action

Creating

Do Temp. Ramp(Batch ID,End Time, End Temp)

Start Controlling (Ramp ID)

Creating

Entry/BatchID, EndTime, EndTemp

>> TempRamp;CurrentTime > Self.StartTime;Self -> [R4] CookingTank.ActualTemp

> Self.StartTemp;Signal Start Controlling(Ramp ID);



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An Executable Model



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Capturing the Models

How to capture the executable model? use a model repository

The structure of this repository is defined by a metamodel



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Metamodel



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Metamodel



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Metamodel



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Translation






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Translation

To generate code traverse the repository (= MetaModel Database) and output text using archetypes



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Archetype Language



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Archetype Language

Generation of code text not preceded by `.` is copied as is text preceded by `.` are commands

select for ifelse ...



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Archetype Language



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Archetype Language



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Archetype Language



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Archetype Language



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Archetype Language



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Archetype Language



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Archetype Language



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Archetype Language



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Specifying the architecture






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Specifying the Architecture

To specify the architecture build a model of its conceptual entities

This can also be done in UML



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Specifying the Architecture



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Example Architecture Remember our example: Chemical Plant Possible architecture

two tasks one of which is periodic and of higher priority one bit per instance to indicate presence in periodic state duplicated data for the control loop copied over by the periodic task when required by it



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Example Architecture



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System construction






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System Construction

Recap: at this point we have a populated instance database for the application describing the

system to be built archetypes for objects in the OOA (Object Oriented Architecture)

of the architecture

What`s next: Producing the executable code



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System Construction



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System Construction



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System Construction



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System Construction



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System Construction

An architecture is an OOA-model compiler It translates a system specified in OOA into the target

programming language incorporating decisions made by the architect about data control structures time

Architectures, like programming languagecompilers, can be bought



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Conclusion



Software Architecture Executable domain models Translation Specifying the architecture System construction Conclusion



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Conclusion Benefits of the Shlaer-Mellor Method:

ensures that the architectural design decisions are made by the architect

offers the possibility to alter performance without affecting the system behavior (=application)

adaptable to changes in context enforces uniformity across different development teams facilitates integration

K ATHOLIEKE U NIVERSITEIT L EUVEN D EPT. OF C OMPUTER S CIENCE ESC Europe 1999 report 1 System Design: Architectures and Archetypes ( Shlaer-Mellor Method)

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