Chapter 7. Modeling Ordered Interactions: Sequence Diagrams

Use cases allow your model to describe what your system must be able to do; classes allow your model to describe the different types of parts that make up your system's structure. There's one large piece that's missing from this jigsaw; with use cases and classes alone, you can't yet model how your system is actually going to its job. This is where interaction diagrams , and specifically sequence diagrams, come into play.

Sequence diagrams are an important member of the group known as interaction diagrams. Interaction diagrams model important runtime interactions between the parts that make up your system and form part of the logical view of your model, shown in Figure 7-1.

Figure 7-1. The Logical View of your model contains the abstract descriptions of your system's parts, including the interactions between those parts

Sequence diagrams are not alone in this group; they work alongside communication diagrams (see Chapter 8) and timing diagrams (see Chapter 9) to help you accurately model how the parts that make up your system interact.

Sequence diagrams are the most popular of the three interaction diagram types. This could be because they show the right sorts of information or simply because they tend to make sense to people new to UML.

Sequence diagrams are all about capturing the order of interactions between parts of your system. Using a sequence diagram, you can describe which interactions will be triggered when a particular use case is executed and in what order those interactions will occur. Sequence diagrams show plenty of other information about an interaction, but their forté is the simple and effective way in which they communicate the order of events within an interaction.