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10. Subsystems

31/10/22

Subsystem​

  • Inital design - classes, responsibilities, collaborations
  • Second stage - analyse hierarchies, identify contracts
  • New goals: Simplify patterns of communication, identify subsystems, design good interface to subsystems and classes

Collaboration Graphs​

Represent the flow of information in system during execution. Collaboration graph includes:

  • Classes and their hierarchies
  • Contracts
  • Collaborations

Subsystems - Groups of classes that collaborate among themselves to support a set of contracts From outside - The classes work closely together to provide a clearly defined unit of functionality From inside - Classes can have some complex interrelationships of collaborations

Superclass vs Subsystem​

  • Superclass groups a set of classes with identical responsibilities
  • Subsystem is a group of classes with different responsibilities that can work together to fulfil a greater responsibilities

Contracts​

Subsystems also support contracts.

  • Collaboration from a class outside the subsystem to a class inside subsystem is redefined to be a collaboration with subsystem
  • The contract used for collaboration becomes a subsystem contract. This delegates contract to the class that supports it

Subsystems from Top​

Help simplify a design near the end of design

  • Used early in design:
    • Identify its major subsystems
    • Treat them as classes
    • Decompose subsystems into other subsystems
  • Used to start out a class:
    • Too complex or has too many responsibilities
    • Gets subdivided into more classes that combine into subsystem

Subsystems are Conceptual​

Only conceptual units

  • They don't exist at execution time but their classes do
  • Don't directly fulfil responsibilities

Identifying Subsystems​

Look for strongly coupled classes Coupling is a measure of how much they depend on each other

Coupling​

  • Frequently used collaborations
  • A web of many collaborations among a set of classes that are connected to reset of the graph with fewer collaborations

Simplifying Interactions​

Should help to simplify class interactions

  • Complex interactions are hard to maintain, lead to spaghetti code, and are generally to be avoided
  • Area of complexity in graph should be focus of effort
  • Proposed change should simplify graph