Section 7.2. Time

7.2. Time

A sequence diagram describes the order in which the interactions take place, so time is an important factor. How time relates to a sequence diagram is shown in Figure 7-3.

Figure 7-3. Time runs down the page on a sequence diagram in keeping with the participant lifeline

Time on a sequence diagram starts at the top of the page, just beneath the topmost participant heading, and then progresses down the page. The order that interactions are placed down the page on a sequence diagram indicates the order in which those interactions will take place in time.

Time on a sequence diagram is all about ordering, not duration. Although the time at which an interaction occurs is indicated on a sequence diagram by where it is placed vertically on the diagram, how much of the vertical space the interaction takes up has nothing to do with the duration of time that the interaction will take. Sequence diagrams are first about the ordering of the interactions between participants; more detailed timing information is better shown on timing diagrams (see Chapter 9).