These are shown in a special frame in the diagram and let you create complex alternative paths for the system. You can also model situations where one or more optional or alternative events take place. Each object has a lifeline that descends vertically and this lifeline will show an activation bar if the object is active. These messages are shown as arrows that can go both left and right. Objects are activated as messages travel back and forth on the horizontal axis. Sequence diagrams use a horizontal axis for the objects being modeled, with the vertical axis showing how time progresses from top to bottom. Sequence diagrams help a software engineer or developer visualize the order in which events occur and how they trigger activity within the system. While some of these diagrams, such as class diagrams or object diagrams, are designed to show the static structure of a system, a sequence diagram shows how the system behaves over time and how the different parts of the system interact during that time frame.Ī sequence diagram captures messaging between objects in a system and indicates which object is active at any particular point in the process of carrying out operations. Sequence diagrams are one of the 14 different types of Unified Modeling Language (UML) diagrams. What is the purpose of sequence diagrams?
