Architecting and designing > UML 2.0 Lite > UML models > UML Sequence diagram > Message Synchronization Type
  
Message Synchronization Type
A Message/Stimulus line can be notated by its type of synchronization. There are a number of types of synchronization that can be modeled.
To select the type of synchronization for a line
1 Select Edit (from drop-down list accessed through right-click, or Edit menu) to open the message's definition.
2 Click the Symbol tab, and then select the synchronization type from the drop down list.
Synchronization types
Simple
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Synchronous
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An operation begins only when the sending object has initiated the action and the receiving object is ready to accept it; both objects might wait indefinitely until they are both ready to proceed.
Asynchronous
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The sending object can send a message regardless of the ability of the receiving object to accept it. It is drawn with a half arrowhead (only one wing instead of two).
Time-out
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The same as synchronous, except that the sending object will wait a specified amount of time for the receiving object to receive it.
Balking
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The same as synchronous, except that the sending object will abandon the action immediately if the receiving object is not ready.
Procedure call, Procedure call return
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A procedure call is drawn as a full arrowhead. The return is shown as a dashed arrow. In procedural flow of control, the return arrow can be omitted (it is considered implicit). For non-procedural flow of control, including parallel processing and asynchronous messages, returns should be shown explicitly.
Wait
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When modeling a concurrent system a full arrowhead shows the yielding of a thread of control (wait semantics).
No-wait
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When modeling a concurrent system a half arrowhead shows the sending of a message without yielding control (no-wait semantics).
See also
UML Sequence diagram