subsystems

• Subsystems are another form of abstraction, using collaboration graphs and subsytem cards
• collaborations represent the flow of information in a system statically along all possible paths of flow
• collaboration graphs include: classes and their inheritances, contracts and collaborations
• class hierarchies are represented as nested boxes

  
• contracts are represented by numbers in circles and collaborations are lines from one class (client) edge to another (server)

  
• groups of classes (subsystems) can collaborate to support a set of contracts to provide a clearly defined unit of functionality, despite having complex interrelationships inside
• collaborations can come from the outside, where they are redefined to be a collaboration the whole subsytem, which is then delegated to an appropriate class

  
• subsystems help simplify a design near the end of the process, or to identify the major subsystems and how to split them early on
• subsystems only exist as a design entity - ie there is no direct implementable equivalent, though java interfaces can be used fore the subsystem to interact with the outside world
• subsystem cards are like CRCs but show the tasks and the classes carrying them out instead

  
• subsystems can be identified by: classes that heavily depend on each other, frequently used collaborations, a web of many collaborations among a certain group of classes
• full graph example

  
• full card example

  
• when contracts appear to be almost identical, they could be sharing a responisibility (eg. 3 and 5)

  
• subsystem eg.

  
• edit subsystem UML equivalent

  
• can also 'zoom out' with subsystems, adding more abstraction

  
• to simplify collaboration patterns: minimise collaborations with other classes and subsystems, minimise number of delegations, minimise number of contracts supported per subsystem
• subsystem interface - receives all the inputs/contracts and then redistributes them. gives better encapsulation

  
• too many contracts can = too much intelligence, should break down into internal subsystems