# 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.

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## When to draw one

* Protocols with timing constraints or timeouts.
* Explaining a race condition.
* Documenting how long a participant stays in each state.

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## What is on the diagram

| **Element**  | **Meaning**                                         |
| ------------ | --------------------------------------------------- |
| Lifeline     | One participant, with its states stacked vertically |
| State change | A step in the line at the moment it changes         |
| Time axis    | Horizontal, with real units                         |
| Constraint   | A duration annotation between two points            |

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## A worked example

Two participants, their states, and a timeout:

![UML timing diagram](https://help.gocapable.com/images/att1417379849.svg)

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
```

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## 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.

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## Related

[Class diagramTypes, fields and the relationships between them.](https://help.gocapable.com/diagrams/class-diagram.html)

[Sequence diagramWho calls whom, in what order.](https://help.gocapable.com/diagrams/sequence-diagram.html)

[UML diagramsBack to the UML overview.](https://help.gocapable.com/diagrams/uml-diagrams.html)

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_Four types cover most of it._