The UML use case diagram below shows that “Event Check-in” extend's “Validate License”

When modeling the corresponding sequence diagram for “Event Check-in,” what is the best practice for representing the extended use case?

Question

The UML use case diagram below shows that “Event Check-in” extend's “Validate License”

When modeling the corresponding sequence diagram for “Event Check-in,” what is the best practice for representing the extended use case?

BlackTom AI · Accepted Answer

A REF (interaction use) frame is the appropriate way to represent an extended use case in a sequence diagram because it explicitly reuses/refs the extended interaction and keeps the main scenario readable.

1) Relevant knowledge points and concepts
- UML use-case "extend" means an optional or conditional addition to the behavior of a base use case; the extension’s behavior is conceptually a separate interaction that can be inserted at extension points.
- In sequence diagrams, UML provides an “ref” (interaction use) frame to reference another interaction (sub-sequence) by name; this is the standard mechanism to show reuse or to pull in a separately modeled interaction.
- Combined fragments like “alt” are for alternative flows (mutually exclusive alternatives) within a single diagram, not for modular reuse of an extended use case.

2) Step‑by‑step reasoning
- Identify that Event Check-in is the extension: its behavior augments the Validate License flow.
- Map the base flow in the main sequence diagram (Validate License) or the primary scenario you are modeling.
- Where the extension should occur, insert a ref frame labeled with the name of the extended interaction (e.g., «ref» “Event Check-in”): this either points to an inline fragment or to a separate interaction definition.
- Optionally annotate the ref with a guard/condition if the extension is conditional, but the structural relationship is shown with ref, preserving modularity and reuse.

3) Intermediate modeling steps (conceptual)
- Draw main lifelines and messages for the base scenario.
- Add a frame marked ref at the extension point.
- Point the ref to the separate sequence representing the Event Check-in interaction (or include it inline).

4) Why the correct answer is correct
- REF explicitly expresses “use this separate interaction here” which matches the semantics of an extend relationship (a chunk of behavior that is applied/invoked at a defined point). It supports reuse and keeps clarity.

5) Why the other options are incorrect
- ALT condition: alt is for branching alternatives in one diagram, not for representing a separately defined, reusable extension.
- Create a completely separate sequence diagram: while possible, that alone doesn’t show the insertion point or reuse in the main diagram; REF both documents the reference and links to the separate interaction.
- “Frame to act as the ‘referee’”: vague and incorrect terminology — REF is an interaction use, not a controller of logic flow.

Conclusion: Represent the extended behavior using a REF frame — this matches the provided correct answer.

The UML use case diagram below shows that “Event Check-in” extend's “Validate License” When modeling the corresponding sequence diagram for “Event Check-in,” what is the best practice for representing the extended use case?单项选择题

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