Activity diagram is under the behaviour diagram categorisation, it describes how activities are coordinated to provide a service. Typically, an event needs to be achieved by some operations, particularly where the operation is intended to achieve a number of different things that require coordination, activity diagram can be used to represent such workflow.
An activity diagram can be used to model any type of process, from a high-level business workflow that involves different use cases, to the details of an individual use case, down to the specific details of an individual method.
Elements of an activity diagram
Activity
Activity is used to represent a set of actions, it should have a name begins with a verb and ends with a noun (e.g. Get Patient Information, Make Payment Arrangements). Different from action, activity can be further divided into asset of activities/actions.
Action
Action is a single task to be performed, and can’t not be decomposed.
Object node
Represent an object or data that is flow from one activity to the another (in the below example, the information of the patient is flowed to another activity). It is by its class name.
Control flow
Show the sequence of execution
Object flow
Show the flow of an object from one activity (action) to another activity (action).
Initial node
Portrays the beginning of a set of actions or activities.
Final flow node
Stop all control flows and object flows in an activity or action.
Final node
Stop all control flows and object flows in an activity or action.
Decision node
Represent a condition to ensure that the control flow or object flow only goes down one path. The outgoing edges should be denoted with conditions or guard expressions, the guard must be true to move the flow down to the next activity or action.
Merge node
Bring back together different decision paths that were created using a decision-node.
Fork node
Split behaviour into a set of parallel or concurrent flows of activities or actions.
Join node
Bring back together a set of parallel or concurrent flows of activity or action.
Swimlane
A way to group activities performed by the same actor. You should not have more than 5 swimlanes.
Example
Back to parent node: UML Diagrams
System_Design_and_ModellingUMLActivity_DiagramFunctional_Modelling