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One of the more common requirements for the modern IT department is to customize an commercial off-the-shelf, or COTS, software package to fulfil a complex business requirement.
However, in these situations it is rare for the COTS package to stand alone. Typically it will be a central "hub" for multiple actors to interact (eg your typical HR package) and/or will integrate with other business systems, such as a central authentication directory, a custom data entry point etc.
When developing these systems, it is vital that the organisation identify all integration points and plan for how these will be implemented. Depending on the degree of coupling in systems, it may be necessary for systems to share state or to otherwise be aware of the other's existence. In other words, planning for system-to-system interactions becomes just as important as human-to-system interactions.
Each integration point needs to be documented as a separate use case. These use cases should then be supplemented by a systems sequence diagram.
A systems sequence diagram is just like a regular sequence diagram, except that it documents interactions between actors and systems, not internal system interactions.
This is important because it provides a strong visualisation and timeline about what kinds of activities need to happen when. Although it is insufficient to be a systems spec in and of itself, it is invaluable in the planning phase to ensure that people involved in integration work consider any flow-on consequences.