Use case diagrams

Use case diagrams document the interactions between your users and your system. This is typically done with text, but UML does support use cases.

Let's start by looking at use cases in text, and then review a UML diagram representing the same use cases. We will use an example of a grade book for an online education institution. 

The student logs on to the system and selects their class. The student then selects the assignment and uploads their document. Next, the student enters text and selects the submit button. 

The instructor logs on to the system and selects their class. The instructor then selects the assignment, and the student. They grade the assignment, enter a grade, and select the submit button.

These use cases are pretty basic and easy to understand in text. There are only a few constructs for the use case diagram in UML:

There are several visual components to the UML use case diagram:

• Actor: The stick figure is referred to as an actor. In our example, student and instructor were both actors. These are the users that use your system. Often, there are multiple user roles in a system.
• Relationship: The solid lines indicate which actors interact with which use case items.
• System: The overall system is represented by a rectangle. Actors are placed outside of the system and use case items are placed within the system.
• Use Case Item: Use case items are represented in ovals, as seen in our preceding Online Grade Book example. These are the components of your use case.

These visual components are further illustrated as follows: