Operation Contracts

1

• Lecture 6
• Operation Contracts

2

• Overview

• What is contract ?
• The guidelines for writing contracts for the
  system operations.
• Use Case realizations.
• Writing contracts leads to domain model updates.

3

• Contracts

• Contracts are documents that describe system
  behavior.
• Contracts may be defined for system operations.
• Operations that the system (as a black box)
  offers in its public interface to handle incoming
  system events.
• The entire set of system operations across all
  use cases, defines the public system interface.

4

• SSD for BuyItem use case

5

• System Operations and the System Interface

• In the UML the system as a whole can be
  represented as a class.
• Contracts are written for each system operation
  to describe its behavior.

6

• Guidelines for Contracts

7

• Use Case Realizations

• A use case realization describes how a use case
  is realized in terms of collaborating objects.
• UML interaction diagrams are used to illustrate
  use case realizations.
• Recall Process Sale from main scenario we
  identified a number of system events
  (operations).
• Each system event was then described by a
  contract.

8

• Contracts

• A contract is a document that describes what an
  operation commits to achieve.
• Emphasizing what will happen rather than how it
  will be achieved.
• Contracts can be expressed by pre- and post-
  conditions.

9

• Conceptual Model and Contracts

• A Conceptual Model is a visual representation of
  conceptual classes or real-world objects in a
  domain of interest.
• Contracts describe detailed system behavior in
  term of state changes to objects in the Domain
  Model, after a system operation has executed.

10

• Utilize Conceptual Model

• To make contracts you have to use the conceptual
  model.
• We added the conceptual model from previous
  slides to assist the comprehension of post
  conditions in the contracts.

11

• Domain Model

12
(No Transcript)
13

• Contract Sections

Operation Name Of operation, and
parameters. Cross References (optional) Use
cases this can occur within. Preconditions No
teworthy assumptions about the state of the
system or objects in the Domain
Model before execution of the
operation. Postconditions -The state of objects
in the Domain Model after completion of
the operation.
14

• Example AddLineItem Contract

• Contract CO2 AddLineItem
• Operation addLineItem (itemID ItemID,
  quantity integer)
• Cross References Use Cases Process Sale.
• Pre-conditions There is a sale underway.
• Post-conditions
• A SalesLineItem instance sli was created.
  (instance creation)
• sli was associated with the current Sale.
  (association formed)
• sli.quantity was set to quantity. (attribute
  modification)
• sli was associated with a ProductSpecification,
  based on itemID match. (association formed)

15

• Pre- Post- Conditions

• Preconditions are assumptions about the state of
  the system before execution of the operation.
• A Postcondition is an assumption that refers to
  the state of the system after completion of the
  operation.
• The postconditions are not actions to be
  performed during the operation.
• Describe changes in the state of the objects in
  the Domain Model (instances created, associations
  are being formed or broken, and attributes are
  changed).

16

• How to Make a Contract

• Identify system operations from the system
  sequence diagrams.
• For each system operation, construct a contract.
• Write the preconditions section.
• Write the postconditions section describing the
  state changes that occur to concepts in the
  conceptual model. Postconditon categories
• Instance creation and deletion.
• Attribute modification.
• Associations formed and broken.

17

• AddLineItem PostconditionsCreation and Deletion

• After the itemID and quantity of an item have
  been entered by the cashier, what new objects
  should have been created?
• A SaleLineItem instance sli was created.

18

• AddLineItem PostconditionsAttribute Modification

• After the itemID and quantity of an item have
  been entered by the cashier, what attributes of
  new or existing objects should have been
  modified?
• sli.quantity was set to quantity.

19
AddLineItem PostconditionsAssociations Formed
and Broken

• After the itemID and quantity of an item have
  been entered by the cashier, what associations of
  new or existing objects should have been formed
  or broken?
• sli was associated with the current Sale.
• sli was associated with a ProductSpecification,
  based on itemID match.

20

• Make New Sale Enumeration Example

• Assume a new sale is started, we will create a
  new object from the conceptual class Sale, with
  an ID5.
• If this sale started on the register number 3,
  then we have to create an association between the
  sale number 5 with the register number 3
  (associations are established by updating foreign
  keys).

21

• Writing Contracts leads to Domain Model Updates

• It is also common to discover the need to record
  new concepts, attributes or associations in the
  Domain Model.
• For example
• If in one of the postconditions, you wanted to
  create an instance to store some values in it,
  and it was not presented by any Concept in the
  domain model.
• Then you should go back and correct your domain
  model by adding this concept.

22

• The most common mistake in creating contract

• The most common problem in creating contracts is
  forgetting to include the forming of
  associations. Particularly, when new instances
  are created, it is very likely that associations
  to several objects need be established.
• Dont forget to include all the associations
  formed and broken.

23

• Example 1 End Sale Contract

Contract EndSale Operation EndSale
() Cross References Use Cases Process
Sale. Pre-conditions There is a sale
underway and all items have been
entered. Post-conditions Sale.isComplete
became true. (attribute modification)
24

• Example 2 Make Payment Contract

• Contract MakePayment
• Operation MakePayment (amount Number or
  Quantity)
• Cross References Use Cases Process Sale.
• Pre-conditions There is a sale underway
  and all items have been entered.
• Post-conditions
• A Payment instance p was created. (instance
  creation)
• p.amountTendered was set to amount. (attribute
  modification)
• p was associated with the current Sale.
  (association formed)
• The current Sale was associated with the Store
  to add it to the historical log of completed
  sales. (association formed)

25

• Example 3 Start Up Contract

• Contract StartUp
• Operation StartUp ()
• Cross References Use Cases Process Sale.
• Pre-conditions None.
• Post-conditions
• A Store, POST, ProductCatalog and
  ProductSpecifications have been created.
  (instance creation)
• ProductCatalog was associated with
  ProductSpecifications. (association formed)
• Store was associated with ProductCatalog.
  (association formed)
• Store was associated with POST. (association
  formed)
• POST was associated with ProductCatalog.
  (association formed)

26

• Summary

• Contracts describe detailed system behavior in
  terms of state changes to objects in the
  Conceptual model after a system operation.
• Contracts have sections of Operations, Cross
  references, Preconditions and Postconditions.
  Postconditions are the most important section.
• Postconditions describe changes in the state of
  objects in the Conceptual model.