Use Case Diagram for Stack Overflow

Let’s build the Stack Overflow use case diagram and understand the relationship between its different components. First, we’ll define the different elements of our Stack Overflow system, followed by the complete use case diagram.

System

Our system is Stack Overflow.

Actors

Now, we’ll define the main actors of Stack Overflow.

Primary actors

• User: A registered user who can:

  • Create, edit, and flag questions and answers

  • Add bounties and tags

  • Upvote/downvote content

  • Accept answers

  • Add comments

  • Vote to close/delete questions and answers

• Moderator: A privileged user who can:

  • Close, delete, reopen, and restore questions

  • Delete answers

Secondary actors

• Guest: An unregistered visitor who can:

  • Search for and view questions and answers

  • Register an account

• Admin: A system administrator who can:

  • Block or unblock users

  • Award badges manually

• System: Responsible for:

  • Awarding badges based on rules

  • Sending notifications (on answer, comment, or vote)

Use cases

In this section, we’ll define the use cases for Stack Overflow. We have listed the use cases according to their interactions with a particular actor.

Note: Some use cases will occur multiple times because they are shared among different actors in the system.

User

• Add / Modify / Flag Question: Allows users to post new questions, update them, or report inappropriate ones.

• Add / Modify / Flag Answer: Enables users to contribute answers, edit content, or flag problematic responses.

• Add Comment: Let users add clarifying or supporting comments on questions and answers.

• Upvote a Comment: Empowers users to upvote a comment.

• Add Bounty: Users can offer reputation points to attract answers to their questions.

• Add Tag: Users can label questions with relevant topic tags for better discoverability.

• Vote to Close / Delete Question: Empowers users to initiate closure or deletion of low-quality or off-topic questions.

• Vote to Delete Answer: Empowers users to initiate deletion of low-quality answers.

• Upvote / Downvote to Question / Answer: Enables users to express approval or disapproval of questions and answers.

• Accept Answer: Let the authors mark the most helpful answer as accepted.

Guest

• Search / View Question: Allows non-logged-in users to browse and search for public questions and answers.

• Register Account: Enables guests to sign up and become a full-featured user.

Admin

• Block / Unblock User: Enables administrators to restrict or restore user access to the platform.

• Award Badge: Allows admins to assign special badges manually to recognize user contributions.

Moderator

• Close / Reopen / Delete / Restore Question: Grants moderators authority to manage question visibility and status.

• Delete Answer: Allows moderators to permanently remove inappropriate or low-quality answers.

System

• Send Notification: Automatically informs users of interactions like votes, comments, answers, or badges.

• Award Badge: Automatically grants achievement badges based on predefined contribution criteria.

• Determine Popular Tags: The system automatically determines the most popular tags.

Relationships

This section describes the relationships between and among actors and their use cases.

Generalization

• “Moderator” has a generalization relationship with “User” as the moderator can perform all the tasks a normal user can perform.

• “User” has a generalization relationship with “Guest” as the normal user can perform all those tasks that a guest user can perform.

Associations

The table below shows the association relationship between actors and their use cases.