A sequence diagram is known as interaction diagram which details that details how functionalities are carried out. It captures interaction among entities by collaboration. These are time focused diagrams and depict the interaction order visually through the vertical axis of the diagram.
Basic notations of sequence diagram
- Participants or class roles: Class roles summarise the way an entity behaves in context. UML object symbol is used for illustrating the class role.
- Activity or execution occurrence: Activation box represents time taken by an entity to complete a task. When an entity is busy in execution process or is waiting for a response message, then the thin grey rectangle is positioned vertically on the lifeline.
- Messages: These are the arrows which represent interactions among objects. Half arrowed lines are used to present asynchronous messages. Asynchronous entities are transmitted from the object which will not wait for a reply from receive prior its task continuation.
- Lifeline: Lifeline is a vertical dashed line which indicates the presence of an object over time. Lifeline notation having an actor element symbol is involved when specific sequence diagram is governed by a use case. Lifeline having boundary elements demonstrates software elements and system boundary within a system.
- Loops: A loop or repetition in a sequence diagram is illustrated as a rectangle.
Various types of messages in sequence diagrams
- Synchronous message: Synchronous message needs a reply prior the interaction continues. It is drawn with the help of line having solid arrow head pointing from one entity to other.
- Asynchronous message: Asynchronous message doesn’t require any form of response for continuing an interaction. It is drawn through an arrow linking two lifelines. The arrowhead is open, and there is no return message illustrated.
- Response or return message: A reply message is drawn using dotted line along with open arrowhead that points back to an original
- Self-message: This is a message that is transmitted by an object to itself generally with the help of U-shaped arrow that points back to it. It demonstrates specific communication occurring between lifelines of interaction. This form of message presents the message invocation of the common lifeline.
- Create message: This message is created by the new object. It is depicted through the dashed line as well as open arrowhead that is pointing to rectangle presenting the object developed.
- Delete message: It is a message which destroys the object. This is a message which presents the request for deleting lifecycle of target lifeline.
- Lost message: This is a message that is sent to the unknown recipient. This can be shown by an arrow moving from lifeline to endpoint.
- Call message: This message specifies a specific communication among lifelines of interaction. Call message is a form of a message that presents invocation of the functionality of target lifeline.
- Duration message: Duration message defines the distance between two instants of time for invocation of the message.
How to draw sequence diagrams
Sequence diagram depicts the flow of event or scenario in a unique use case. The flow of message of a sequence diagram is grounded on the narrative of specific use case. Prior you start drawing a sequence diagram and decide the interactions to be involved in it. A use case diagram is to be drawn and give the comprehensive summary of what a specific use case performs. Prior drawing a sequence diagram, it is essential to determine the actors or objects that are to be included while developing a new account for the user. When objects have been identified, it is essential to describe the functionality of use case. This description helps to figure out an interaction that occurs between objects when use case executes.
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