# Activity diagram

An activity diagram is UML's flowchart, with two additions that earn their keep: forks and joins for parallel work, and partitions for who does what.

It suits processes with genuine concurrency, which a plain flowchart handles badly.

---

## When to draw one

* Processes with parallel steps that must all complete.
* Algorithms and workflows with real branching.
* Documentation where the swimlane and the flow both matter.

---

## What is on the diagram

| **Element**    | **Meaning**                                     |
| -------------- | ----------------------------------------------- |
| Start and stop | Filled circle and bullseye                      |
| Action         | A rounded box: ":do something;"                 |
| Decision       | if/then/else, with guards in brackets           |
| Fork and join  | Parallel paths that all run and all must finish |
| Partition      | A swimlane, written as a named block            |

---

## A worked example

A release process with parallel checks:

![Activity diagram](https://help.gocapable.com/images/att1416331660.svg)

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

```
@startuml
|Engineering|
start
:Merge to main;
:Build artefact;
fork
  :Run unit tests;
fork again
  :Run integration tests;
fork again
  :Security scan;
end fork
if (All checks passed?) then (yes)
  |Release manager|
  :Deploy to staging;
  :Smoke test;
  if (Approved?) then (yes)
    :Deploy to production;
  else (no)
    :Roll back;
    stop
  endif
else (no)
  :Notify author;
  stop
endif
|Engineering|
:Monitor for 30 minutes;
stop
@enduml
```

---

## A few things that catch people out

* A fork must be matched by a join. Parallel paths that never rejoin are a modelling error.
* Guards belong in brackets on the branches, so the reader knows which path applies.
* Partitions are the swimlanes. Use them when ownership matters, and leave them out when it does not.

---

## Related

[State machine diagramWhat states exist, and what moves between them.](https://help.gocapable.com/diagrams/state-machine-diagram.html)

[Component diagramComponents, interfaces and what plugs into what.](https://help.gocapable.com/diagrams/component-diagram.html)

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

---

_Four types cover most of it._