What is Activity Diagram? Definition, Examples and Activity Diagram in UML

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What is Activity Diagram?

An activity diagram, a dynamic and integral facet of the Unified Modeling Language (UML) is defined as a sophisticated visual representation in software engineering and diverse domains. It excels in illustrating the seamless flow of activities, actions, and processes within intricate systems, business workflows, or any dynamic process.

Unlike conventional flow charts, this advanced diagram not only models the progression from one activity to another but also intricately captures decision points, control flows, and the nuanced interplay of elements. Widely embraced for its clarity and versatility, activity diagrams empower stakeholders to collaboratively comprehend and refine processes, making them indispensable tools for communication between business and development realms.

In essence, they provide a holistic insight into the dynamic orchestration of tasks, making complex systems and workflows accessible through intuitive visualizations.

Key elements and concepts of an activity diagram include:

• Activities: These are the specific actions or tasks within the system or process that are being modeled. Activities are represented by rounded rectangles in the diagram.

• Transitions: These are arrows that connect activities, indicating the order in which they are executed. Transitions show the flow of control from one activity to another.

• Decisions: Diamond-shaped symbols represent decision points where the flow of control can take multiple paths based on conditions or choices. Each path from a decision point is labeled with a condition.

• Forks and Joins: Forks and joins are used to represent parallel activities. A fork symbol splits the flow into multiple concurrent paths, and a join symbol brings these paths back together.

• Initial and Final Nodes: An initial node is represented by a small filled circle and indicates the starting point of the diagram. A final node is represented by a larger circle with a border and marks the end of the activity.

Activity diagrams are versatile and can be used to model various processes, from business workflows to software algorithms. They are particularly helpful for understanding and communicating the dynamic behavior of a system or process. Activity diagrams can be used in combination with other UML diagrams, such as use case diagrams, class diagrams, and sequence diagrams, to provide a comprehensive view of a system’s behavior and structure.

Example for Activity Diagram

Let’s explore some practical examples of activity diagrams.

Example 1: Online Purchase Process

Imagine you’re designing an activity diagram to model the online purchase process in an e-commerce platform.

• The start represents the beginning of the process.

• The initial activity is Browse Products, where the user explores available items.

• Upon selecting a product, the system proceeds to Add to Cart.

• At this point, a decision is made based on product availability. If the product is in stock, the user can continue by selecting Checkout; otherwise, they may need to choose another product.

• Checkout includes multiple sub-activities like entering shipping information and payment details. These are shown in a parallel manner, indicating that they can be completed concurrently.

After all required information is provided, the system processes the payment and concludes the process with the Order Confirmation.

Example 2: Software Development Process

Let’s consider a software development process as another example. In this scenario, we’re using an activity diagram to model the development of a feature in an Agile software project:

• The process begins with a user story in the Backlog.

• The team discusses and estimates the story in Sprint Planning.

• Once planned, the development and testing activities can occur concurrently.

• If testing fails (represented by the decision point), the story goes back to the Development phase for revisions.

• If testing passes, the story is marked as Done, and the process concludes.

Example 3: Restaurant Ordering System

For our final example, let’s explore a restaurant ordering system:

• Customers start by entering the restaurant and are seated (Seat Customer).

• They can choose to view the menu or ask the server for recommendations, indicating parallel activities.

• After deciding what to order, they place their order with the server.

• The kitchen prepares the food, and when it’s ready, the server delivers it to the customer.

• After eating, customers have the option to pay the bill, give feedback, or order dessert.

• The process ends when the customer leaves the restaurant.

Learn more: What is UML Diagram?

Activity Diagram in UML

An activity diagram in UML (Unified Modeling Language) is a type of diagram used to represent the dynamic aspects of a system, business process, or workflow. It focuses on modeling the flow of activities, actions, and decisions within the system or process. Activity diagrams are particularly useful for understanding the order of execution, parallelism, and decision points in a process. Here’s a breakdown of the key elements and concepts you’ll encounter in an activity diagram:

• Activity: An activity is a specific task or action within the system or process. Activities are represented by rounded rectangles. They can be as simple as a single action or as complex as a sub-process. For example, an activity might be “Log in” in a user authentication system.

• Control Flow: Control flow arrows, also known as transitions, connect activities to illustrate the order in which they are executed. These arrows show the direction of control from one activity to another. For example, you might have a control flow from “Log in” to “Dashboard” indicating the sequence of actions.

• Decision Node: A diamond-shaped symbol is used to represent a decision point in the process. It typically has multiple outgoing control flows, each labeled with a condition. Based on these conditions, the process follows one of the outgoing control flows. For instance, a decision node could be used to determine if a user is an admin or a regular user.

• Fork Node: A fork node is represented by a bar symbol and is used to indicate parallel processing. It splits the flow into multiple concurrent paths. For instance, in an online shopping process, a fork node could be used to show that a customer can add items to their cart and browse for more products in parallel.

• Join Node: A join node is also represented by a bar symbol and is used to bring multiple parallel flows back together into a single flow. It indicates the synchronization of concurrent activities. In the online shopping example, a join node might be used to signify that once the customer has added items to their cart and browsed, they must proceed to the checkout process.

• Initial Node: This is a small solid circle that marks the starting point of the activity diagram, representing the initiation of the process.

• Final Node: A larger circle with a border represents the end of the activity diagram, indicating the termination of the process.

Activity diagrams can be used in various domains, including software engineering, business process modeling, and system design. They help stakeholders and developers understand the flow of activities, which is essential for designing efficient and effective systems and processes. When combined with other UML diagrams, such as use case diagrams, class diagrams, and sequence diagrams, activity diagrams provide a comprehensive view of a system’s behavior and structure, aiding in the analysis, design, and communication of complex systems.

Learn more: What is a Fishbone Diagram?