10.6 Representing Time on a Sequence Diagram
In a sequence diagram, time progresses vertically down the diagram and, as stated earlier, occurrences on a lifeline are correspondingly ordered in time. In addition, the send occurrence and receive occurrence for a single message are also ordered in time. However, particularly in distributed systems, a message may be overtaken by a subsequent message sent from the same lifeline; that is, the first message may arrive after receipt of the second message. Sequence diagrams allow this kind of situation to be drawn using a downwards-slanting arrow between two lifelines, as shown in Figure 10.9.