What is Sequence Diagram

What is Sequence Diagram?

9-11 minutes

UML Sequence Diagrams are interaction diagrams that detail how operations are carried out. They capture the interaction between objects in the context of a collaboration. Sequence Diagrams are time focus and they show the order of the interaction visually by using the vertical axis of the diagram to represent time what messages are sent and when.

Sequence Diagrams captures:

• the interaction that takes place in a collaboration that either realizes a use case or an operation (instance diagrams or generic diagrams)
• high-level interactions between user of the system and the system, between the system and other systems, or between subsystems (sometimes known as system sequence diagrams)

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Purpose of Sequence Diagram

• Model high-level interaction between active objects in a system
• Model the interaction between object instances within a collaboration that realizes a use case
• Model the interaction between objects within a collaboration that realizes an operation
• Either model generic interactions (showing all possible paths through the interaction) or specific instances of a interaction (showing just one path through the interaction)

Sequence Diagrams at a Glance

Sequence Diagrams show elements as they interact over time and they are organized according to object (horizontally) and time (vertically):

Object Dimension

• The horizontal axis shows the elements that are involved in the interaction
• Conventionally, the objects involved in the operation are listed from left to right according to when they take part in the message sequence. However, the elements on the horizontal axis may appear in any order

Time Dimension

The vertical axis represents time proceedings (or progressing) down the page.

Note that:

Time in a sequence diagram is all a about ordering, not duration. The vertical space in an interaction diagram is not relevant for the duration of the interaction.

Sequence Diagram Example: Hotel System

Sequence Diagram is an interaction diagram that details how operations are carried out — what messages are sent and when. Sequence diagrams are organized according to time.

The time progresses as you go down the page. The objects involved in the operation are listed from left to right according to when they take part in the message sequence.

Below is a sequence diagram for making a hotel reservation. The object initiating the sequence of messages is a Reservation window.