ANSWER

A system use case pre-condition is a given state known to the system, that must always be true in order for use case execution to begin. For example, a use case for making online payments might have the following pre-condition: “The customer order is complete” (for a retail system) or “A course registration is submitted” (for an education or training system)

A pre-condition is valid only if it is true before the start of every possible flow within the use case. Something that is necessary for one flow but not another would not be a use case pre-condition.
What is a pre-condition not?

A pre-condition is not the action that invokes or triggers a use case. Using the online payment example, it would not be valid to include a pre-condition that ‘The Actor chooses to make an online payment’. Instead, initiating or invoking actions should be described at the beginning of each use case flow.

A pre-condition is not something that occurs outside the system (for a system use case). Receipt of a document is not a pre-condition for a system, unless of course the system is being designed to electronically receive that document. A pre-condition describes only something that is known to or detected by the system.

A pre-condition is not something that occurs inside the Actor’s head. A pre-condition should never be stated as ‘The Actor wishes to…’. I have not yet seen an IT system that is able to read minds – although if someone does create one, it will certainly make requirements elicitation much easier! This rule holds true for describing how a use case flow is initiated or invoked as well – never by an Actor’s wishes or intentions; only by an Actor’s actions.

Tips: Use present tense when writing pre-conditions (something “is”, rather than something “has been”). Word pre-conditions to describe the state that must be present, and avoid redundant terms like “--- exists”. The pre-condition describes a state that the system must be able to detect, which also means that it must inherently exist. In the online payment example, the focus is on the state (‘complete’) and the object (‘customer order’).

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Sandy Lambert
Business Architect
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