Home Computer Science Object-Oriented Analysis and Design for Information Systems Modeling with UML, OCL, and IFML
By definition, preconditions are facts considered true before a use case even starts. Preconditions must not be confused with exceptions: exceptions may be detected only after the use case has started. Exceptions are detected during a use case because most of the time it is not possible to verify if the conditions were true or not before the use case begins. For example, it is not possible to assure that the customer has enough credit to pay for the order before starting the Order books use case, because the value of the order cannot be known at that time; therefore this is an exception. However, it is possible to admit that only a book that was sold and delivered can be returned. Thus, the sale and delivery of the book is a precondition for its return.
As preconditions are accepted as true facts before the use case starts, they will not be checked during the use case. Although there can be other interpretations in literature, these kind of preconditions do not generate exceptions. If they are false it should be impossible to start the use case (otherwise they are exceptions, not preconditions).
Success post conditions establish the results of a use case, that is, what will be true after the use case is executed. For example, the Order books use case may have as a post-condition the following result: “the order was registered by the system.”
Sometimes, without the presence of the client, the team cannot decide about some issue that may depend on company policies. For example, may the customer pay the order in installments? Are there special offers for customers that buy more than certain quantities?
If the client is not immediately available, these doubts must be recorded in the use case section “open issues” in order to be solved as soon as possible.
At the end of the analysis activities of one iteration it is expected that all open issues have been resolved and incorporated into the use case description.
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