Practice sequence diagram vocabulary: actors, lifelines, synchronous and asynchronous calls, return messages, and combined fragments like alt, loop, and opt.
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In a sequence diagram, what is a 'lifeline'?
A lifeline is the vertical dashed line that extends downward from each participant (actor or system) in a sequence diagram. It represents the participant's existence during the interaction. Activation boxes on the lifeline show when it's actively processing.
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What is the difference between a synchronous and asynchronous call in a sequence diagram?
Synchronous calls use a solid filled arrowhead — the caller waits for a response. Asynchronous calls use an open (stick) arrowhead — the caller sends the message and immediately continues without waiting for a response.
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What does the 'alt' combined fragment represent in a sequence diagram?
The 'alt' (alternative) combined fragment represents conditional branching. It's the UML equivalent of if/else — you divide the fragment into guards (conditions), and only the path whose guard is true executes. Example: [user authenticated] vs. [user not authenticated].
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When would you use a 'loop' combined fragment in a sequence diagram?
The 'loop' fragment shows repetition — the contained interaction repeats while its guard condition is true, or for a specified number of iterations. Example: 'loop [for each order item] — call inventory check service.'
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What does a 'return message' look like in a sequence diagram and what does it represent?
A return message is shown as a dashed arrow pointing back to the caller. It represents the response from a synchronous call — e.g., 'HTTP 200 OK with order data.' Return messages are optional but help make the flow explicit.