1. How do we convince people that in programming simplicity and clarity —in short: what mathematicians call "elegance"— are not a dispensable luxury, but a crucial matter that decides between success and failure?
- Dijkstra

2. System Sequence Diagrams

3. System Behavior What does the system do? System Sequence Diagram (SSD)
One technique to clarify input and output system events
Typically interaction between actor and system has form of a dialog – a request event followed by a response event

4. System Sequence Diagram
A picture that shows, for a particular scenario of a use case, the events that external actors generate, their order, and inter-system events
Systems are treated as black box (don’t care how)
Emphasis is on events/communication between external actors and system

5. Fig. 10.2

6. System Sequence Diagram
Notation
Iteration with box and continuation clause
Return values
Message with parameters
Naming events
Use action-direct object style
SSD is part of use case model
A visualization of the interactions in use case

7. Fig. 10.1

8. System Sequence Diagrams
Guideline: Draw an SSD for main success scenario of each use case, possibly for frequent or complex alternative scenarios
SSD answers question: What (external) events are coming into our system?
System software reacts to:
External events from actors
Timer events
Faults or exceptions (from external sources)

9. System Sequence Diagrams
System behavior – what system does – not how
Diagrams – UML has sequence diagrams, SSD is a particular form of a sequence diagram