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Sequence Diagram

These are a type of UML diagrams that show how objects in a system or classes within code
interact with each other. Especially these diagrams show interactions in the order they take
place, in a nutshell they show the sequence of events.
These diagrams are primarily used by Developers and Business professionals to understand
requirements for a new system or to document an existing process. These diagrams are
occasionally known as event diagrams or event scenarios.


  • It is used to capture dynamic behaviour of a system.
  • It is used describe the message flow of a system.
  • It is used to describe the structural arrangement of the objects.
  • It is used to describe the collaborate among objects.


  • They are good at describing performance but they do not define it.
  • They typically do not show all the iteration and control that is needed to give a
    computationally absolute description

How to draw a sequence diagram?
1st we must identify – Objects taking part in the interaction.
2nd Message flows among the objects
3rd The sequence in which the messages are flowing
4th Object organisation
The five main elements that make up a sequence diagram are activation boxes, actors,
lifelines, messages, and objects
Actor : indicates a role played by a user or any other system that work together with the
Objects:  UML Object symbol illustrates of a class or of classes.
Lifeline:  A lifeline, indicates an objects existence over time, it is symbolized by vertical
dashed lines.
Message: are arrows that symbolize communication between objects.

Type of arrow indicates types of messages:

  • Synchronous message: solid arrow with a solid head.
  • Asynchronous message: solid arrow with a stick head.
  • Create message: Dotted arrow with a stick head.
  • Delete message: solid arrow with a X.
  • Self message: U shaped arrow.
  • Return message: dashed arrow with stick head.

Activate Bar:  it is used to specify the time an object needs to complete the task it is illustrated
by a thin rectangle on the lifeline

Why use sequence diagrams? Why not code it?
1. A good sequence diagram is still above the level of the real code (not all code is
drawn on diagram)
2. Sequence diagrams are language-agnostic (can be implemented in many different
3. Non-coders can read and write sequence diagrams.
4. Easier to do sequence diagrams as a team.
5. Can see many objects/classes at a time on same page (visual bandwidth).

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