Section 10.1. What Are Interactions?

10.1. What Are Interactions?

Interaction diagrams are defined by UML to emphasize the communication between objects, not the data manipulation associated with that communication. Interaction diagrams focus on specific messages between objects and how these messages come together to realize functionality. While composite structures show what objects fit together to fulfill a particular requirement, interaction diagrams show exactly how those objects will realize it.

Interaction diagrams are typically owned by elements in the system. For example, you may have an interaction diagram associated with a subsystem that shows how the subsystem realizes a service it offers on its public interface. The most common way of associating an interaction diagram with an element is to reference the interaction diagram in a note attached to the element.

You can show the details of an interaction using several different notations; however sequence diagrams are by far the most common. Other notations include interaction overviews, communication diagrams, timing diagrams, and interaction tables. Because sequence diagrams are used most frequently, each concept is introduced using that notation. The other notations are described in detail later in the chapter. The basic symbol for an interaction diagram is a rectangle with the keyword sd and the name of the interaction in a pentagon in the upper-left corner. Figure 10-1 shows an example sequence diagram. The various parts of this diagram are explained throughout the chapter.

Figure 10-1. A sample sequence diagram