A UML diagram that depicts: ◦ interactions between objects. ◦ how the business currently works by showing how various business objects interact. Purpose: 1. Model flow of control 2. Identify synchronization 3. Illustrate typical scenarios
The focus is less on messages themselves and more on the order in which messages occur. The diagram conveys this information along 2 dimensions: ◦ the vertical dimension shows, top down, the time sequence of messages/calls as they occur, ◦ the horizontal dimension shows, left to right, the object instances that the messages are sent to.
Instance Name : Class Name ◦ Underlined specific instance (Bill, Fred) ◦ Not underlined kind of instance or role (buyer, seller)
Message =a line to the receiving object with: ◦ a solid arrowhead (if a synchronous call operation) ◦ a stick arrowhead (if an asynchronous signal) The message/method name is placed above the arrowed line. Return message = dotted line with an open arrowhead back to the originating lifeline (above the return value from the operation).
To model mutually exclusive choice between two or more message sequences (if then else) The word "alt" is placed inside the frame's namebox. Operands are separated by a dashed line.
To model a sequence that, given a certain condition, will occur; otherwise, the sequence does not occur (If then). The word “opt" is placed inside the frame's namebox.
To model a repetitive sequence. The word “loop" is placed inside the frame's namebox (add the minimum and maximum of iterations)
To model parallel processing activities. The word “par" is placed inside the frame's namebox. Break up the frame horizontal operands separated by a dashed line.