UML Slideshow: Sequence Diagrams

What are Sequence Diagrams?

• Sequence Diagrams are interaction diagrams that detail how operations are carried out

• Interaction diagrams model important runtime interactions between the parts that make up the system

• Interactions Diagrams

•  Sequence diagrams
• Interaction overview diagrams
• Timing diagrams
• Communication diagrams

What do Sequence Diagrams model?

• capture the interaction between objects in the context of a collaboration

• show object instances that play the roles defined in a collaboration

• show the order of the interaction visually by using the vertical axis of the diagram to represent time what messages are sent and when

• show elements as they interact over time, showing interactions or interaction instances

• do not show the structural relationships between objects

Sequence Diagrams

A sequence diagram or system sequence diagram (SSD) shows process interactions arranged in time sequence in the field of software engineering. It depicts the processes involved and the sequence of messages exchanged between the processes needed to carry out the functionality. Sequence diagrams are typically associated with use case realizations in the 4+1 architectural view model of the system under development. Sequence diagrams are sometimes called event diagrams or event scenarios.

For a particular scenario of a use case, the diagrams show the events that external actors generate, their order, and possible inter-system events. All systems are treated as a black box; the diagram places emphasis on events that cross the system boundary from actors to systems. A system sequence diagram should be done for the main success scenario of the use case, and frequent or complex alternative scenarios.

Key elements

A sequence diagram shows, as parallel vertical lines (lifelines), different processes or objects that live simultaneously, and, as horizontal arrows, the messages exchanged between them, in the order in which they occur. This allows the specification of simple runtime scenarios in a graphical manner.

A system sequence diagram should specify and show the following:

• External actors
• Messages (methods) invoked by these actors
• Return values (if any) associated with previous messages
• Indication of any loops or iteration area

Reading a system sequence diagram

Professionals, in developing a project, often use system sequence diagrams to illustrate how certain tasks are done between users and the system. These tasks may include repetitive, simple, or complex tasks. The purpose is to illustrate the use case in a visual format. In order to construct a system sequence diagram, you need to be familiar with the unified modeling language (UML). These models show the logic behind the actors (people who affect the system) and the system in performing the task. Reading a sequence diagram begins at the top with the actor(s) or the system(s) (which is located at the top of the page). Under each actor or system there are long dotted lines called lifelines, which are attached to them. Actions are performed with lines that extend between these lifelines. When an action line is connected to a lifeline it shows the interaction between the actor or system. Messages will often appear at the top or bottom of a system sequence diagram to illustrate the action in detail. For example, the actor could request to log in, this would be represented by login (username, password). After each action is performed, the response or next action is located under the previous one. As you read down the lines you will see in detail how certain actions are performed in the provided model, and in what order.