System sequence diagrams (SSDs) illustrate input and output events between systems and actors. SSDs are drawn for main and alternative use case scenarios to show the events coming into a system and the system's response without detailing how it works internally. Defining a system's behavior through SSDs early in design allows investigation of what events a system must handle at a high level before proceeding to more detailed design.

1. System Sequence Diagrams
Fakultas Ilmu Komputer
Universitas Sriwijaya
2008

2. UP Artifact Influence

3. Example:
Main success scenario of a cash-only process sale scenario

4. What Are System Sequence
Diagrams?
• A system sequence diagram (SSD) is a
fast and easily created artifact that
illustrates input and output events
related to the systems under
discussion.
• Use cases describe how external
actors interact with the software
system we are interested in creating.
• During this interaction an actor
generates system events to a system,
usually requesting some system
operation to handle the event.
• Guideline: Draw an SSD for a main
success scenario of each use case,
and frequent or complex alternative
scenarios.

5. Why Draw an SSD? (1)
• An interesting and useful
question in software design
is this: What events are
coming in to our system?
Why? Because we have to
design the software to
handle these events (from
the mouse, keyboard,
another system, …) and
execute a response.

6. Why Draw an SSD? (2)
• Before proceeding to a
detailed design of how a
software application will
work, it is useful to
investigate and define its
behavior as a "black box."
• behavior is a description of
what a system does,
without explaining how it
does it

7. Relationship Between SSDs and
Use Cases?

8. How to Name System Events and
Operations