Synchronous and return messages

How to read it

UML 2.5.1 draws a message as a line from the sender's lifeline to the receiver's, running across the page or down it. The line and its arrowhead tell the kind of message: a synchronous call has a filled arrowhead; an asynchronous one has an open arrowhead; a reply has a dashed line with either an open or a filled head. The label is the operation's name with its arguments, such as isEligible(student), and a reply can show the value returned. Scott Ambler, in The Object Primer, explains the difference: in a synchronous call the sender waits for the result before going on, and in an asynchronous one it doesn't. Messages are read from the top of the diagram down, in the order they happen.

Also searched as: return message, sequence diagram arrows, synchronous and return messages symbol, uml synchronous and return messages.

Every message arrow

The forms are those of UML 2.5.1, clause 17.4.4. A message that ends in a large X on the receiving Lifeline deletes that object.

Replies left out

Reply arrows are often left off. Ambler says return values are optional and that he omits them when it's obvious what is returned, and the University of Washington's slides describe a dashed arrow back as indicating a return. A call with no dashed arrow after it doesn't mean nothing came back.

Look-alikes

A creation message and a reply are both dashed; the creation message ends at the head of a new lifeline, lower than the others. Ambler draws an asynchronous message with a partial (half) arrowhead, where UML 2.5.1 specifies an open one. Outside sequence diagrams, a dashed line with an open arrowhead between classes is a Dependency.