ANSWER

Yes, a use case can be used to document requirements for system-to-system interactions and exchanges of information between systems. External systems are treated as Actors, in the same way as human Actors. The standard use case structure of pre-conditions, post-conditions, basic flow, alternate flows and exception flows are all applicable when describing the requirements for interactions with an external system Actor as with a human Actor.

A key rule for this type of use case is that external systems must always be treated as ‘black-box’ entities. The use case cannot dictate requirements for external system processing beyond the bounds of its interaction with the in-scope software system. For example, requirements for an online retail sales system (“System X”) may include an interface to an external third-party payment system such as Paypal (“System Y”). The use case for this interface would describe what information shall be sent from System X to the external system Actor, what information shall be received by System X from the external system Actor, and what System X shall do in order to process the information exchanged. The use case cannot contain requirements for any processing that occurs within the external system (cannot describe data that System Y must collect, connect describe edits that System Y should perform, etc.).

Pre-conditions also must not describe anything that happens within the external system. A pre-condition can only be something that is known to or detected by the in-scope system.

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Sandy Lambert
Business Architect
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