Sequence Diagram vs. Activity Diagram: What Is the Difference?

In software development, activity diagrams and sequence diagrams represent actions and relationships within a software system. To understand these two types of diagrams, we should first walk through UML diagrams on the whole.

Traditionally, UML, or Universal Modeling Language, is used in software diagrams to represent a system’s data, processes, and relationships. 

But why is it so important? 

UML diagrams can be used to visualize projects before they begin and document them after they have been completed. However, their ultimate goal is to allow teams to visualize how projects work and can be used in other fields besides software engineering. 

You can use it for sequence and activity diagrams and other types of UML diagrams like class diagrams, use case diagrams, communication diagrams, object diagrams, etc.

Sequence Diagram Definition

A sequence diagram is a subset of an interaction diagram which is a class of template created in UML.

Sequence diagrams are interaction diagrams that show how a set of objects interacts with one another and in what order. These templates are often used by software developers and business professionals to document an existing process, and they describe how a group of objects works together. For this reason, sequence diagrams are occasionally called event diagrams or event scenarios.

What is the Importance of a Sequence Diagram?

Software developers use sequence UML diagrams to document a system’s requirements and design in detail. Sequence diagrams are so useful because they demonstrate the interaction logic between objects in the system in the order in which they occur.

Activity Diagram Definition

An activity diagram represents business and software processes as a progression of actions performed by people, components, or computers. 

Activity diagrams are used to explain business processes and use cases, as well as to describe and document the implementation of system processes.

Activity diagrams represent sequential, multi-step work processes, and they enable the visualization of even the most complex workflows. Within the activity diagram, sequential and peripheral workflows are characterized by control and object flows.

What is the Importance of an Activity Diagram?

An activity diagram helps programmers understand the flow of programs on a high level and identify constraints and circumstances that cause certain events. 

A flow chart becomes an activity diagram if complex decisions have to be made.

Sequences & Activity Diagram Differences

Here are some significant differences between the activity diagram and sequence diagram.

Definition

The activity diagram is a graphical representation of a workflow of related stepwise activities and actions, supporting choice, iteration, and concurrency. 

On the other hand, the sequence diagram displays object interactions arranged in a time sequence. 

Without any doubt, this is the main difference between the activity diagram and sequence diagram.

Main Focus

A sequence diagram depicts the flow of data from one object to another by showing the picture representation of the data. It is time-ordered, indicating that the exact interactions between objects are displayed step by step. 

In contrast, an activity diagram shows the workflow of a system by the pictorial representation of the message flow between activities.

Type

As the message sequence and order are modeled in sequence diagrams, they are classified as dynamic modeling diagrams. 

On the other hand, Activity diagrams represent process flows captured in the system. So they are not classified as dynamic modeling diagrams.

Use Case

As mentioned already, sequence diagrams describe the behavior of several items within a single use case with the implementation of all possible logical conditions and flows. 

Activity diagrams explain the general sequence of actions for a number of objects in several use cases.

Activity and sequence diagrams are two different types of behavior diagrams. Activity diagrams show control flowing from one activity to another, particularly when trying to understand the logic of conditional structures, loops, and concurrency. On the other hand, sequence diagrams represent the sequence of messages flowing from one object to another and how messages/events are exchanged, and in what time order.

Conclusion

If you’ve been looking to create a behavioral UML diagram and are thinking about the different options, hopefully, this guide helps clear up some of the differences between activity diagrams and sequence diagrams. If you want to learn more about the different advantages of UML diagrams, check out our post on Fresco.

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