How to Read a Sequence Diagram

The elements for a sequence diagram are shown below and explained in the text that follows.

Example sequence diagram described in the text

Object Lifeline

Represented on a diagram by a rectangular box with a vertical dashed line descending beneath it. An object lifeline represents the existence of an object over a period of time.

Activation

Represented on a diagram by a thin rectangular box superimposed over the dashed line of an object lifeline. An activation represents the time during which an object is performing an operation.

Synchronous Message

Represented on a diagram by a solid line with a filled arrowhead. A synchronous message is one for which the sender must wait for a response before continuing with processing. Compare with "Asynchronous Message", below.

Creation Message

Represented on a diagram by the shifting down, relative to the originating object, of the rectangle and dashed line that represents the object to be created. A creation message is a message that leads to the creation of an object.

Stop Message

Represented on a diagram by showing the activation at the end of the message with a large cross through it. A stop message is a message that leads to the deletion of an object (or to the indication that an object is no longer needed).

Return Message

Represented on a diagram by a dashed line with a filled arrowhead. A return message is a message that returns from an object to which a message was previously sent. Return messages are valid only from synchronous messages, and are themselves synchronous.

Asynchronous Message

Represented on a diagram by a stepped line with a stick arrowhead. An asynchronous message is one for which the sender does not have to wait for a response before continuing with processing. Compare with "Synchronous Message", above.