A sequence diagram shows, as parallel vertical
lines called lifelines, a lifeline represents an object and shows all its points
of interaction with other objects, different processes or objects that live
simultaneously, and, as horizontal arrows, the messages exchanged between them,
in the order in which they occur. This allows the specification of simple
runtime scenarios in a graphical manner.
The boxes across the top of the diagram represent
classifiers or their instances, can be use cases, objects, classes, or actors.
The long thin boxes on the lifeline are activation boxes, also called
method-invocation boxes, which indicate processing is being performed by the
target object/class to fulfill a message. Labeled arrow indicates the messages.
The benefits of sequence diagram are clearly depict the sequence of events, show
when objects are created and destroyed, are excellent at depicting concurrent
operations.
Sequence diagrams are useful for modeling -
complex interactions between components, use case elaborations, distributed and
web based systems, complex logic and for state machines. IT can be used to
document the dynamic view of the system design at various levels of abstraction.
Example: