Sequence diagrams are used to visualize and explore the interaction between users, screens and
object instances within the system. They provide an ordered sequential map of messages passing
between objects over time.
Frequently these diagrams are placed under Use Cases or Components in the model to illustrate a
scenario, or common set of steps followed in response to an event that generates an outcome. The
model includes what initiates activity in the system, what processing and changes occur internally
and what outputs are generated. Often, the object instances are represented using special
stereotyped icons - icons exist for boundary objects, controllers and persistent entities.
The sequence diagram is typically represented by a horizontally deployed set of actors and object
instances, each having a vertical life span bar. Messages (usually method calls but may also
represent messages passed using message queuing services and other events) are drawn from
one object to another with an arrow indicating the direction of flow.
Lifelines are drawn vertically and are intended to show the span of control over time as well as
when control ends. Time can be seen to pass from the top to the bottom of the diagram.
In the following example, the actor will access the system via the Invoice screen. He/She will
access the customer table to retrieve the customer details necessary for the invoice. The details
pertaining to the invoice will then be entered.
The system will then automatically calculate the totals and VAT. The user will then save the invoice
after which the system will automatically print it.
