If a caller sends a synchronous message, it must wait until the message is done, such as invoking a subroutine. Solid arrow heads represent synchronous calls, open arrow heads represent asynchronous messages, and dashed lines represent reply messages. Messages, written with horizontal arrows with the message name written above them, display interaction.

Leaving the instance name blank can represent anonymous and unnamed instances. If the lifeline is that of an object, it demonstrates a role. By reading down the lines, one can see in detail how certain actions are performed in the provided model, and in what order. After each action is performed, the response or next action is located under the previous one. For example, a request by an actor to log in would be represented by login (username, password). Messages will often appear at the top or bottom of a system sequence diagram to illustrate the action in detail. The connection between an action line and a lifeline shows the interaction between the actor or system.

Actions are performed with lines that extend between these lifelines. Under each actor or system there are long dotted lines, called 'lifelines', which are attached to them. 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). These models show the logic behind the actors (people who affect the system) and the system in performing the task. Familiarity with unified modeling language (UML) is needed to construct a system sequence diagram. The purpose is to illustrate the use case in a visual format.

These tasks may include repetitive, simple, or complex tasks. Professionals, in developing a project, often use system sequence diagrams to illustrate how certain tasks are done between users and the system.

Indication of any loops or iteration area.

Return values (if any) associated with previous messages.

Messages (methods) invoked by these actors.

This allows for the graphical specification of simple runtime scenarios.Ī system sequence diagram should specify and show the following: Key elements of sequence diagrams Ī 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.

Instead, only an object named System is depicted.

System Sequence Diagram (SSD): All systems are treated as a black box, where all classes owned by the system are not depicted.

Sequence Diagram (SD): A regular version of sequence diagram describes how the system operates, and every object within a system is described specifically.

There are two kinds of sequence diagrams: A system sequence diagram should be done for the main success scenario of the use case, and frequent or complex alternative scenarios. The diagram emphasizes events that cross the system boundary from actors to systems. Sequence diagrams are sometimes called event diagrams or event scenarios.įor a particular scenario of a use case, the diagrams show the events that external actors generate, their order, and possible inter-system events. Sequence diagrams are typically associated with use case realizations in the 4+1 architectural view model of the system under development. This diagram depicts the processes and objects involved and the sequence of messages exchanged as needed to carry out the functionality. In software engineering, a sequence diagram shows process interactions arranged in time sequence.

Sequence diagram uml loop