Sequence Diagram Notations

Sequence Diagram Notations

• A lifeline represents an individual participant in the interaction.
• An activation is a thin rectangle on a lifeline, representing the period during which an element is performing an operation.
• The top and bottom of the rectangle are aligned with the initiation and completion time, respectively.

Message Types

• Call message: represents an invocation of an operation of a target lifeline.
• Return message: represents the pass of information back to the caller of a corresponding former message.
• Self message: represents the invocation of a message of the same lifeline.
• Recursive message: represents the invocation of a message of the same lifeline, with the target pointing to an activation on top of the activation where the message was invoked from.
• Create message: represents the instantiation of a target lifeline.
• Destroy message: represents the request of destroying the lifecycle of a target lifeline.
• Duration message: shows the distance between two time instants for a message invocation.

Notes

• A note (comment) gives the ability to attach various remarks to elements.
• A comment carries no semantic force, but may contain information that is useful to a modeler.

Collaboration Diagrams

• Collaboration diagrams are illustrations of the relationships and interactions among software objects.
• They show the structural organization of objects and their interactions.
• Actors are instances that invoke the interaction in the diagram, with a name and a role.
• Links connect objects with actors and are depicted using a solid line between two elements.
• Messages between objects are shown as labeled arrows placed near a link.

Sequence vs. Collaboration Diagrams

• Sequence diagrams focus on the order of messages that flow between objects.
• Collaboration diagrams visualize the structural organization of objects and their interactions.

Collaboration

• A collaboration is a collection of named objects and actors with links connecting them.
• A collaboration defines a set of participants and relationships that are meaningful for a given set of purposes.
• Objects collaborate by communicating (passing messages) with one another in order to work together.

Steps for Creating Collaboration Diagrams

• Identify behavior whose realization and implementation is specified.
• Identify the structural elements (class roles, objects, subsystems) necessary to carry out the functionality of the collaboration.
• Decide on the context of interaction: system, subsystem, use case, and operation.
• Model structural relationships between those elements to produce a diagram showing the context of the interaction.
• Consider alternative scenarios that may be required.
• Draw instance-level collaboration diagrams, if required.
• Optionally draw a specification-level collaboration diagram to summarize the alternative scenarios in the instance-level sequence diagrams.

This quiz covers the basics of sequence diagram notations, including lifelines, activations, and call messages.