Sequence diagrams describe interactions among classes in terms of an exchange of messages over time.
Basic Sequence Diagram Symbols and Notations
Class roles
Class roles describe the way an object will behave in context. Use the UML object symbol to illustrate class roles, but don't list object attributes.
Activation
Activation boxes represent the time an object needs to complete a task.
Messages
Messages are arrows that represent communication between objects. Use half-arrowed lines to represent asynchronous messages. Asynchronous messages are sent from an object that will not wait for a response from the receiver before continuing its tasks.
Various message types for Sequence and Collaboration diagrams
Lifelines
Lifelines are vertical dashed lines that indicate the object's presence over time.
Destroying Objects
Objects can be terminated early using an arrow labeled "<< destroy >>" that points to an X.
Loops
A repetition or loop within a sequence diagram is depicted as a rectangle. Place the condition for exiting the loop at the bottom left corner in square brackets [ ].
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