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Solid arrowheads represent synchronous calls, which are the most common. There are some variations in the message arrows. Horizontal arrows show messages or signals that are sent between the classes. A lateral bar or vertical rectangle on the lifeline shows the focus of control when the object is busy doing things. An X on the bottom of the lifeline represents when the object is destroyed. Specialized symbols used to draw a sequence diagram.Ī vertical line represents the lifeline for the class or object, which corresponds to the time from when it is created through when it is destroyed. The top rectangles use indicators in the name to indicate whether the rectangle represents an object, a class, or a class and object.Ī name with a colon after it represents an object.Ī colon with a name after it represents a class.Ī name, followed by a colon and another name, represents an object in a class. Some of the interactions are physical only, such as signing a contract. The leftmost object is the starting object and may be a person (for which a use case actor symbol is used), window, dialog box, or other user interface. Actors and classes or object instances are shown in boxes along the top of the diagram. The symbols used in sequence diagrams are shown in the figure below. Each use case scenario may create one sequence diagram, although sequence diagrams are not always created for minor scenarios. Sequence diagrams are used to show the overall pattern of the activities or interactions in a use case. In practice, sequence diagrams are derived from use case analysis and are used in systems design to derive the interactions, relationships, and methods of the objects in the system. Sequence diagrams are often used to illustrate the processing described in use case scenarios. Sequence diagrams can illustrate a succession of interactions between classes or object instances over time. These diagrams, along with class diagrams, are used in a use case realization, which is a way to achieve or accomplish a use case. An interaction diagram is either a sequence diagram or a communication diagram, both of which show essentially the same information.