Sequence Diagram

Advantages of Sequence Diagrams

Some of the advantages of sequence diagrams are as follows:

• Sequence diagrams are easier to generate and maintain.
   
• It helps to represent the message flow between the objects in an interaction.
   
• It can be used to explore any high-level system or real-life application.
   
• It allows for both forward engineering and reverse engineering.
   
• It represents the details of a UML use case.
   

Also read about  V Model in Software Engineering

Notations in Sequence Diagram

In this section, we will see the different notations used in sequence diagrams.

Lifeline

An individual participant in a sequence diagram is called a lifeline, and it is placed at the top in a sequence diagram. 

The symbol is shown below:

Actor

An actor represents the role of an entity while interacting with the subject. It may not always be a physical entity and can play several roles. 

The symbol is shown below:

Activation

It represents the time needed by an object to complete the task an is denoted by a thin rectangle whose top and bottom is associated with the start and end times respectively.

Messages

Messages are the interactions between the objects in the system and they are in sequential order. The messages are represented by arrows.
The following are the different types of messages present in a sequence diagram:

• Call message
  It represents an invocation of the operation of the target lifeline.
  
  
   
• Return Message
  It represents the flow of messages from the receiver of the corresponding caller process.
  
   
• Self Message
  It represents the invocation of the message of the same lifeline.
  
• Recursive Message
  It represents a self message which is invoked for recursive calls.
  
• Destroy Message
  It represents the request of destroying the lifecycle of the target lifeline.
  
• Duration Message
  It is useful for modelling a real-time system to represent the passage of time a message.
  

FAQs

1. What is the purpose of the sequence diagram?
   A sequence diagram depicts the sequence of messages passed between the objects and the control structures between objects. 
    
2. Can a sequence diagram have two actors?
   Yes, a sequence diagram can have multiple actors.
    
3. What is a lifeline in sequence diagram?
   A lifeline represents an individual participant in a sequence diagram. A lifeline will usually have a rectangle containing its object name.
    
4. What does a dotted line mean in a sequence diagram?
   Dotted lines represent the reply messages in a sequence diagram.
    
5. How do you represent asynchronous calls in a sequence diagram?
   Open arrowheads are used to represent asynchronous messages in a sequence diagram.

Key Takeaways

In this article, we have extensively discussed sequence diagrams in UML.

We hope that this blog has helped you enhance your knowledge regarding sequence diagrams and if you would like to learn more about object-oriented software design, check out our articles on Software engineering, Component diagram, Activity diagram, Characteristics of good software. 

Check out this problem - Shortest Common Supersequence.

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