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Alternate action sequence

As explained in Software Design for Python Programmers

An alternate action sequence is an ordered set of interactions describing how a use case proceeds when the normal sequence cannot be followed.

An alternate action sequence is an ordered set of interactions that describes how a use case proceeds when something goes wrong during its primary action sequence. Document it from the triggering actor’s point of view. section 3.6.2

Place the sequence within a complete use case description:

  • Identify the triggering actor, preconditions, and trigger before describing the normal and alternate paths. section 3.6.2
  • Use the primary action sequence for the expected interaction, then specify the action steps required when that sequence cannot continue. section 3.6.2
  • Relate steps to other use cases where appropriate, and finish by recording postconditions and applicable nonfunctional requirements. section 3.6.2

Search Catalogue

A customer clicks Search after completing an Attributes form. The primary sequence begins with the application verifying the form and formulating a database query; an alternate action sequence records what should happen if something goes wrong during that flow. section 3.6.2

How Software Design for Python Programmers covers it

How deep it goes
Defines the alternate path within a structured use case but does not prescribe detailed implementation behavior.
Examples
Uses the Search Catalogue interaction, including a completed attributes form, a Search trigger, and database-query steps.
What it assumes
Assumes a defined use case with an actor, trigger, preconditions, and primary action sequence.
What you can do after
Practice documenting failure paths from the triggering actor’s perspective within a complete use case.
How it connects
Connects alternate action sequences with use cases, primary action sequences, preconditions, triggers, and postconditions.
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