Short note: Sequence diagram

A sequence diagram is a UML interaction diagram that shows how objects interact in a particular scenario of a use case, with the time order of messages emphasised. Time runs from top to bottom.

Notation

• Object/participant: rectangle at top, name:Class.
• Lifeline: vertical dashed line showing the object's existence over time.
• Activation bar (focus of control): thin rectangle on a lifeline while the object is executing.
• Message: horizontal arrow; solid with filled head = synchronous call, open head = asynchronous, dashed = return.
• Self message: arrow looping to the same lifeline.
• Object creation/destruction: a message to a new object box; an X at the end of a lifeline.
• Combined fragments: alt, opt, loop boxes for conditions and repetition.

 :Customer    :ATM         :Bank
     | insertCard |           |
     |----------->|           |
     |            | verify()  |
     |            |---------->|
     |            |<- - - - - |
     |<- - - - - -|  ok       |
     |            |           |

Uses

• Model the flow of one use case scenario in detail.
• Identify the methods needed in classes.
• Document the system behaviour and test cases.

A system sequence diagram is a special case that treats the whole system as one black box lifeline and shows the actor's events.