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Unified Modeling Language

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Unified Modeling Language

A sequence diagram is a Unified Modeling Language (UML) diagram that provides a
view of the chronological sequence of messages between instances (objects or
classifier roles) that work together in an interaction or interaction instance. A
sequence diagram consists of a group of instances (represented by lifelines) and the
messages that they exchange during the interaction. You line up instances
participating in the interaction in any order from left to right, and then you position
the messages that they exchange in sequential order from top to bottom. Activations
sometimes appear on the lifelines.

A sequence diagram belongs to an interaction in a collaboration or an interaction
instance in a collaboration instance.

Model Elements and Relationships in Sequence Diagrams

The UML sequence diagrams produced by the UML/SD Viewer illustrate program
interactions with an emphasis on the chronological order of messages.

Activations

An activation (also known as a focus of control) is a notation that can appear on a
lifeline to indicate the time during which an instance (an actor instance, object, or
classifier role) is active. An active instance is performing an action, such as executing
an operation or a subordinate operation. The top of the activation represents the time
at which the activation begins, and the bottom represents the time at which the
activation ends.

For example, in a sequence diagram for a "Place Online Order" interaction, there are
lifelines for a ":Cart" object and ":Order" object. An "updateTotal" message points
from the ":Order" object to the ":Cart" object. Each lifeline has an activation to indicate
how long it is active because of the "updateTotal" message.

Shape

An activation appears as a thin rectangle on a lifeline. You can stack activations to
indicate nested stack frames in a calling sequence.