System Sequence Diagram (SSD)
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What it is
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a diagram showing, for one particular use case scenario, the events that external actors generate, their order, and any system-internal events — treating the system as a single black box
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Why draw one
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systems must react to external actor events, timer events, and faults/exceptions; identifying these system events is a key part of analyzing system behavior
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Notation guidance
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name system events at the level of intent, not physical input device — e.g.
enterItem, notscan— staying noncommittal about whether the input comes from a laser scanner, keyboard, or voice
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Derivation
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one SSD is derived per use case scenario, using UML sequence diagram notation
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