A message defines a particular communication between Lifelines of an Interaction.The top and the bottom of the of the rectangle are aligned with the initiation and the completion time respectively.A thin rectangle on a lifeline) represents the period during which an element is performing an operation.A lifeline represents an individual participant in the Interaction.A person may play the role of several different actors and, conversely, a given actor may be played by multiple different person.An actor does not necessarily represent a specific physical entity but merely a particular role of some entity.represent roles played by human users, external hardware, or other subjects.

external to the subject (i.e., in the sense that an instance of an actor is not a part of the instance of its corresponding subject).

a type of role played by an entity that interacts with the subject (e.g., by exchanging signals and data).

Note That: Class and object diagrams are static model views. The object initiating the sequence of messages is a Reservation window. The objects involved in the operation are listed from left to right according to when they take part in the message sequence.īelow is a sequence diagram for making a hotel reservation. The time progresses as you go down the page. Sequence diagrams are organized according to time. Sequence Diagram is an interaction diagram that details how operations are carried out - what messages are sent and when. The vertical space in an interaction diagram is not relevant for the duration of the interaction. Time in a sequence diagram is all a about ordering, not duration.

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

However, the elements on the horizontal axis may appear in any order

Conventionally, the objects involved in the operation are listed from left to right according to when they take part in the message sequence.

The horizontal axis shows the elements that are involved in the interaction.

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

Either model generic interactions (showing all possible paths through the interaction) or specific instances of a interaction (showing just one path through the interaction).

Model the interaction between objects within a collaboration that realizes an operation.

Model the interaction between object instances within a collaboration that realizes a use case.Model high-level interaction between active objects in a system.

System sequence diagram online