UML Sequence Diagram

Interaction⌘

• An interaction is a unit of behavior that focuses on the visible exchange of information between participants (limited in time).
• 4 interaction diagrams in UML 2.0:
  • Sequence diagrams - sequence in which messages are exchanged
  • Communication diagrams - relationship between the participants
  • Timing diagrams - state changes of the participants relative to the time
  • Interaction overview diagrams - order of interactions in an activity-like notation

Sequence Diagram⌘

• Participants are represented by a rectangle and a dashed line (lifeline)
• Messages are represented by arrows between lifelines
• Time runs from top to bottom
• The entire diagram represents one interaction

Connectable Element⌘

Connectable Element

• a set of instances
• can be considered as objects of the specified type
• lifelines (participants) represent connectable elements

Execution Specification (Execution Occurrence)⌘

• Specification of the execution of a unit of behavior within the lifeline.
• Sending and receiving messages determine start and end of execution specification.

Message⌘

Message Types⌘

• Synchronous message (filled arrowhead):
  • caller waits until called behavior terminates
  • reply message is represented by dashed line with either an open or filled arrow head
• Asynchronous message (open arrowhead):
  • caller doesn't wait but continues after call, no reply
• Found message (open arrowhead originating from a filled circle):
  • receiver known, sender not known
• Lost message (open arrowhead pointing to a filled circle)
  • sender known, receiver not known
• Create message (dashed line with open arrowhead pointing to header of lifeline):
  • new lifeline is created at this point in the diagram
• Delete message (open arrowhead):
  • object destruction, stop (destruction of an object is represented by a cross on the lifeline)

Event Occurrences⌘

• send event is denoted by an exclamation mark (!p)
• receive event is denoted by question mark (?p)
• A valid sequence for the interaction: < !p, ?p, !q, ?q >
• It's not the only valid sequence!

Event Occurrences valid trace⌘

Event Occurrences Rule⌘

• Send event before receive event
• The order of events along one lifeline is relevant.
• The position of one event relative to an event on another lifeline is insignificant.