Sequence Diagrams

Sequence diagrams

• A sequence diagram is an interaction diagram that emphasizes the time ordering of messages (operations)
• Details how operations are carried out -- what messages are sent and when.
• Organized according to time.
• Objects are placed across the X-axis on top of the diagram.
• Messages, represented by horizontal arrows are placed on the Y-axis.
• Beneath each object, there is a vertical dotted line representing its lifeline.
• A portion of the lifeline is replaced by a thin vertical rectangle called the activation bar. The activation bar represents the duration of execution of the message.
• Gives a clear visual cue to the flow of control over time.

Example

Exercise

• Student fills up registration form and submits it to registration manager.
• Registration manager adds the student to every course chosen by the student.
• Before adding, it is checked whether the course is still open.