SEQUENCE DIAGRAMS
The sequence diagram is a dynamic view that shows the symbols of UML objects/classifiers, and messages passed along. It is a graphical description of the operations of a system based on chronology and it represents the life of UML objects over a definite period of time. It displays an interaction as a two-dimensional chart. The vertical dimension is the time axis, and the horizontal dimension shows the UML objects roles that represent individual objects in the collaboration.
A sequence diagram represents behavior in terms of interactions. It complements the class diagram that represents the static structure of the system. It specifies the behavior of classes, interfaces, and the possible use of their operations. It also describes the scenario of a specific use case – its functionality.
If an actor is the invoker of the interaction shown on the sequence diagram, it is usually represented by the first (farthest left) lifeline in the sequence diagram. If you have several actors in the diagram, you should try to position them to the farthest left or to the farthest right lifelines because actors are, by definition, external to the system.
An object lifeline is the dashed vertical line under the UML object symbol. Time always proceeds down the page. The object lifeline indicates the period during which a UML object exists. If the UML object is created or destroyed during the period of time shown on the diagram, then its lifeline starts or stops at the corresponding point.
A message is a communication between objects that conveys information with the expectation to invoke an operation. The receipt of a message should normally have an outcome. A message has a sender, a receiver, and an action. The sender is the UML object that sends the message. The receiver is the UML object that receives the message. The action is executed to transmit the information. A message is shown as a horizontal solid arrow from the lifeline of one object to the lifeline of another. The arrow is labeled with the name of the message.
When you create a message, you can choose to create an execution specification at the same time. Drawing an execution specification can be particularly useful if you want to describe a procedural flow. The execution specification represents the period of time during which a UML object is performing an action and is shown as a thin rectangle.
A self message is a message sent from a UML object to itself, the sender and the receiver is the same UML object.
An example sequence diagram for “Verify Account Balance” use case
An example sequence diagram for “Make hotel reservation” use case
An example sequence diagram for “Place New Order” use case
Systems Analysis
Karolina Muszyńska