Operation Contracts

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Chapter 11

• Operation Contracts

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Operation Contracts

• A tool of requirements analysis (OOA) that
  describes in great detail the changes required by
  a system operation.
• Analysis of what must happen, not how.
• Stated in terms of postconditions
• Observations about the domain model objects that
  are true when the operation has finished.
• Describe state changes
• Instance creation and deletion.
• Attribute change of value.
• Associations formed and broken.

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System operations handle input system events
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Sections of a Contract

• Operation
• Name of operation and parameters.
• Cross References
• Use cases this operation can occur within.
• Preconditions
• Noteworthy assumptions about state of system
  before execution of operation.
• Postconditions
• The state of objects in the Domain Model after
  completion of the operation.

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An Example Contract

• Operation enterItem( itemID ItemID, quantity
  integer )
• Cross References Use Cases Process Sale
• Preconditions There is a sale underway.
• Postconditions
• A SalesLineItem instance sli was created.
• sli was associated with the current Sale.
• sli.quantity became quantity.
• sli was associated with a ProductDescription,
  based on itemID match.

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When to use Contracts

• High-complexity domains.
• For system operations that are complex and
  perhaps subtle in their results.
• Dont clutter up a use case with fine-grained
  details.
• DONT write contracts if developers can easily
  infer what to do from the use case.