UML Activity Diagrams: A Complete Guide

Understanding activities and processes is a crucial part of improving them. Visual representation of an activity is often the easiest way to clarify it. However, creating a visual representation is itself a complicated task. With a UML activity diagram, you can use a consistent and well-understood method to represent workflows visually.

UML activity diagrams are a key tool in understanding how to accomplish a task and how a business operates. Once you know how everything works, an activity diagram may also provide insight into how to do it better.

As a result, creating an activity diagram is an important skill for any business person or anyone interested in improving how they do things.

Introduction to UML activity diagrams

An activity diagram is a visual representation of actions, constraints, requirements, and other factors that play into accomplishing tasks. You can think of them as a detailed flowchart, revealing each step and decision point in a process. Mapping activities in this way can reveal new information, help identify inefficiencies, and provide other important benefits.

What is an activity in UML?

Universal modeling language (UML) is a standardized method of creating visual representations showing the relationships between different elements, whether structural or behavioral in nature.

An activity diagram represents the flow of what happens in the system rather than a static state. It visualizes the dynamic behavior of a system, showing the flow from one activity to another.

Each component of an activity diagram is called an element. An activity is the highest-level element in the diagram. It’s not unusual for an activity diagram to focus on the constituent parts of just one activity. In that case, the diagram as a whole would represent the activity.

In a broader business and legal sense, an activity can mean anything a company does to earn a profit or deliver some benefit. However, in the case of an activity diagram, the definition can be more fluid. It may be a component of a business process, an entire process, or some other set of dynamic relationships you’re interested in modeling.

When to use a UML activity diagram

A UML activity diagram is used whenever you need to represent the dynamic flow of relationships that make up a business activity. It can be a physical process, with elements representing workers, managers, and systems, or customers and systems. As it was developed to represent software functionality, it also represents the more abstract relationships of a piece of software.

You can use a UML activity diagram to:

• Track the logic of an algorithm, such as a function.

• Represent the steps in a UML use case.

• Model a business process or workflow.

• Describe the flow of actions for any activity.

They’re helpful in the planning stage to clarify an expected workflow. An activity diagram is also useful during operations or once a project is complete to help you understand an existing process or even improve it.

Key components of an activity diagram

Several components make up activity diagrams, defined in the universal modeling language.

• Action: An action represents a single step within an activity. Actions are often represented by rectangles with rounded corners.

• Start node: The starting point or triggering event for the activity, represented by a solid black circle.

• End node: The final step in the activity, when the dynamic flow ends or is picked up by another activity. This step is represented by a solid black circle within a larger white circle.

• Control node: An element that coordinates the flow between other nodes. The most common example may be a decision node, which chooses between outgoing flows depending on set conditions. It’s represented by a diamond with one input flow and two or more output flows.

• Control flows: Also known as control edges. This represents the directed flow of control from one element to another, with a solid line.

• Object flows: Also known as object edges. This represents the directed flow of objects from one element to another, with a dotted line.

• Activity partition: A column or row that’s used to show areas of responsibility for different actors. It may be more commonly known as a swimlane.

Note that this is only a small selection of the components you can use in an activity diagram. UML offers a complete ‘language’ for representing activities, containing a full vocabulary of different diagram elements.

Similar types of UML diagrams

While activity diagrams may be one of the most common uses of UML, they can represent various concepts. Other common behavioral UML diagrams are the use-case diagram and the sequence diagram.

While similar to activity diagrams, they’re used for different purposes and are visually distinct.

Activity diagram vs. use case diagram

An activity diagram describes a flow of system-to-system activity but, depending on its design, may not address users or actors at all. On the other hand, a use case diagram is concerned with users and how they interact with a system.

The classic use case diagram example may detail how users interact with an app. For example, a diagram for a shopping app might depict users’ options, including searching for new clothes, selecting the clothes they want, paying for them, selecting shipping options, etc.

Sequence diagram vs. activity diagram

An activity diagram shows the process flow of messages from system activity to activity and contains conditional flows, loops, and concurrent activities.

On the other hand, a sequence diagram shows the order, sequenced by time, of messages flowing from one object to another. As the name implies, the focus is on the sequence over time.

Creating a UML activity diagram

Familiarity with the universal modeling language is important for using it. As with any language, the more fluent you become, the more information you can convey.

However, as you’ll see in our examples below, you can get started with just a few basic symbols. It’s important to understand the activity you’re modeling.

Ideally, you would sit at your desk and consult a straightforward activity summary listing each element and relationship. However, an activity diagram is often used to help spot gaps in understanding an activity. Collecting relevant data is, therefore, a step that can occur at any point in this process.

Miro offers a UML Activity Diagram Template to make life a little easier, offering all the components you’ll need to represent an activity. Whether using this template or starting from scratch, follow these steps to craft your diagram:

1. Identify elements and actions

You may not have a full understanding of an activity before you start. After all, you wouldn't need the activity diagram if you did. However, you often know how an activity is performed, the actions involved, and the resources required.

You know the activity's starting state, triggering event, and endpoint, if nothing else. It's possible to identify other elements by using that information. It may not be vital to note every detail in your diagram, as the goal is to represent the flow of an activity so it can be understood. Too much detail may add confusion rather than dispelling it.

2. Determine the relationship between elements

Start mapping the relationships between actions, decision points, actors, and other elements in the diagram. Note that UML will use different components to represent different types of relationships, so try to find the component that best represents the relationship.

For example, a decision node is used when there’s a condition that determines how activity flows among several options. A fork, on the other hand, represents a relationship where activity flows along two paths at the same time.

An important aspect of creating an activity diagram is identifying decision nodes, guards, and other constraints or requirements. These elements often represent key aspects of the activity, such as user inputs, resource constraints, deadlines, or completed actions.

3. Organize diagram elements

Once you have all the important elements and relationships, you can create the diagram. Miro makes it easy to find the correct symbol in UML to represent each element and relationship.

Checking your work is always a good idea. Once your diagram is complete, trace the flow from the start node to each end state. Doing so guarantees that the activity diagram represents the activity you’re interested in.

It may take a couple of tries to represent relationships in a useful way while also representing the flow of activity. The more information you add, the more complicated it is to illustrate the flow.