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umlsequence-diagram

Is nested alt in sequence diagram a good practice?

I have created a sequence diagram, and found it has multiple nested alt.

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Is this a good or bad practice?

If it is a bad practice, what should I do instead?

Solution

  • The good, the bad and the ugly

    It is difficult to give advice about good and bad practices without objective criteria, and these will depend on the purpose of the diagram:

    • If you use UML for some kind of visual programming, where a comprehensive diagram should show all the details of a specific interaction, nested alt could be a good practice if there is no alternative. Since different lifelines are driving independent alternatives (FusionAuth the outer alt, Occupations the inner one), the nesting represents the behavioral logic appropriately. But if a same lifeline would drive decision, a flattened alt could a be a more readable way, trading the more complex nesting against more but simpler branches.

    • If you use UML to communicate and to reason about a system, the diagrams should be easily understood: nesting would be a bad practice, since it adds a level of complexity.

    We fortunately have avoided the ugly: identical nested alt in several branches.

    Alternatives to nested alt

    The UML secret is to have more diagrams but smaller ones, each focusing on a single aspect. You can find this advice at the end of almost every chapter of Booch, Jacobson and Rumbaugh's book The UML User guide.

    Two strategies are applicable here:

    • Diagrams per scenario: the main success scenario would be one diagram, and the different failure scenarios would be the others. Super-easy to read.
    • Separation of concerns: Different concerns would be addressed in different diagrams you could for example separate the use of Occupations by its clients and the way Occupations does its business: separate Manager, Client, Occupations, and Occupations, FusionAuth and Database in two diagrams; You'd avoid nested alt, the inner one being in the second diagram and not necessarily relevant for the same audience.

    Remark: I'm not a big fan of visual programming. But if you are, the second strategy is fully compatible with it, with the advantage of preventing the ugly repeating of the same nested fragments in several places.