UML timing diagram

A timing diagram plots how a participant's state changes over time. It is the UML equivalent of a hardware waveform, and it suits protocols where duration and ordering both matter.

Rare, and exactly right on the occasions it applies.

#When to draw one

• Protocols with timing constraints or timeouts.

• Explaining a race condition.

• Documenting how long a participant stays in each state.

#What is on the diagram

#A worked example

Two participants, their states, and a timeout:

The source, which you can paste into a new diagram and edit:

@startuml
robust "Client" as C
concise "Connection" as N

@0
C is Idle
N is Disconnected

@100
C is Connecting
N is Handshaking

@250
C is Connected
N is Established

@900
C is Waiting

@1400
C is Timeout
N is Disconnected

@1500
C is Idle

C@250 <-> C@900 : request in flight
C@900 <-> C@1400 : 500ms timeout window
@enduml

#A few things that catch people out

• Use real time units on the axis. A timing diagram with no units is a state diagram drawn sideways.

• Annotate the constraints; the durations are the reason to draw one.

• robust shows discrete states, concise shows a compact single line. Mix them deliberately.

#Related

Four types cover most of it.