Sequence Diagram Messages

 A Sequence diagram's main purpose is to show the ordering, sending and receiving of messages among a family of objects. In a sequence diagram there are essentially three basic types of messages:

  1. Synchronous messages - Messages that the controlling object waits for the recipient to resolve and return status. These messages are notated by a solid line with an arrow head
  2. Asynchronous messages - Messages that are essentially fire and forget. The sender does not wait for resolution or status of the results of the message. These messages are represented by a solid with an half-arrow head.
  3. Return value - messages that are the response of another message. These can be either synchronous or asynchronous and are depicted by a dashed line with an appropriate arrowhead for synch or asynch.
Figure 1 - Message types