• Its focus is on identifying interactions between objects over time.
• Its major benefit is helping the identification of the message exchanged between objects.

• Exchanging messages requires a sender and receiver.
• A receiver must have an interface in order to receive a message.
  • An interface is an operation signature on the class to which the receiving object belongs.
• The Sequence diagram helps us find and document new operations on-the Class diagram.

• The following figure models three objects: a customer actor, a theater system, and a venue, and three messages.
  • The theater system asks the venue for a set of events taking place in a specified date range.
  • The venue returns the set of events.
  • The theater system then sends the events to the customer actor (user interface) to be displayed.
  • The types of arrows describe the type of interaction.
  • The messages become operation signatures.
  • The returns help validate the interaction.

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Notations

• A sequence diagram shows an interaction, which represents the sequence of messages between instances of classes, components, subsystems, or actors.
• Time flows down the diagram, and it shows the flow of control from one participant to another.
• The diagram shows example instances and events, instead of classes and methods; more than one instance of the same type can appear in the diagram, and more than one occurrence of the same message can appear also.

1. Lifeline: A vertical line that represents the sequence of events that occur in a participant during an interaction, while time progresses down the line.
   • This participant can be an instance of a class, component, or actor.
2. Actor: A participant that is external to the system that you are developing.
   • You can make an actor symbol appear at the top of a lifeline by setting its Actor property.
3. Synchronous message: The sender waits for a response to a synchronous message before it continues.
   • The diagram shows both the call and the return.
   • Synchronous messages are used to represent ordinary function calls within a program, as well as other kinds of messages that behave in the same way.
4. Asynchronous message: A message that does not require a response before the sender continues.
   • An asynchronous message shows only a call from the sender.
   • Use to represent communication between separate threads or the creation of a new thread.
5. Execution occurrence: A vertical shaded rectangle that appears on a participant's lifeline and represents the period when the participant is executing an operation.
   • The execution begins where the participant receives a message.
   • If the initiating message was a synchronous message, the execution ends with a «return» arrow back to the sender.
6. Callback message: A message that returns back to a participant that is waiting for the return from an earlier call.
   • The resulting execution occurrence appears on top of the existing one.
7. Self message: A message from a participant to itself.
   • The resulting execution occurrence appears on top of the sending execution.