Sequence diagram

Sequence diagrams are a graphical representation of the interactions between the actors and the system in chronological order in the Unified Modeling Language formulation:

  • Description of the order of interactions between the objects that make up the system.
  • Representation focusing on the sequence of interactions from a temporal point of view.

Sequence diagrams are interaction diagrams like collaboration diagrams. They are suitable for modeling dynamic aspects of real-time systems and complex scenarios involving few objects.

An interaction results in sending a message between objects. The sequence diagram shows the objects involved in the interaction; description of the interaction and interactions between stakeholders.


The object is composed of its role and / or the name of the instantiated class. The name is underlined to indicate that it is an instance. An object is always accompanied by its lifeline. It is represented by a vertical line below the object. It represents the period of time during the instantiated object exists.

To represent an object creation, a message points to the symbol of the object. The destruction of the object is represented by the end of its life line.


Messages and activities

Objects communicate by exchanging messages represented as arrows. The vertical dimension represents the flow of time. A message placed below another will be sent after the first one. Messages are often labeled by the name of the operation or signal invoked. The message may shown an activation of the objects, a label, be a sequential transmission or parallel transmission of n instances of the same message, and may own arguments.

A result return message is represented in dashed line. The sending of recursive messages is represented by a duplication of the activation band. An object can send itself a message.

Messages are mainly sent during an activity, and cause other activities. A period of activity is the time during which an object performs a direct or indirect action. It is represented by a vertical band along the line of life of the object.