1.4.4 Action Nodes in Activity Diagrams

Action Nodes Represent Work That Happens

A UML activity diagram models a process as a flow of actions.

An action node represents one step that performs work.

For example, a soccer registration process might contain actions such as:

Enter player name
Save registration
Display confirmation

Each phrase describes something that happens.

Write Actions as Clear Verbs

A useful action label begins with a verb.

Strong examples:

Enter jersey number
Assign player to team
Display roster

Weak examples:

Player
Roster
Team information

The weak examples name things.

They do not describe an action.

Keep One Main Action per Node

An action node should usually communicate one focused step.

Instead of:

Enter name, validate it, save the player, and display confirmation

separate the work:

This makes the flow easier to read.

Action Nodes Appear Between Flow Elements

A simple activity flow might be:

The action nodes describe the work between the start and end of the activity.

Actions Can Represent Method-Level Behavior

You already know that C# methods can perform actions.

An activity diagram can model the behavior at a higher level.

For example:

Set player availability

might later correspond to method logic that updates a Boolean field.

The activity diagram explains the process.

The C# source provides the executable implementation.

Action Nodes Do Not Show Object Structure

An activity node such as:

Assign player to team

does not replace a class diagram or object diagram.

Use:

Choose the diagram based on the question you are answering.

Use Enough Detail to Make the Process Understandable

Too little detail:

Process player

Too much detail:

Move the value from this exact variable into that exact memory location

A strong action label captures the meaningful step at the current level of abstraction.

Read the Diagram as a Sequence of Actions

When you see:

read it as:

If the wording does not form a clear process, revise the action labels or flow.