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Design Stack Overflow - Low-Level Design | OneNoughtOne

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Design Stack Overflow

Intermediate

Design an object-oriented system that models the core functionality of Stack Overflow — a Q&A platform where users can post questions, submit answers, comment, vote, and earn reputation.

The system should support tagging for categorisation, a reputation engine that rewards helpful contributions, and flexible search capabilities.

Core Use Cases

Post a question

Users can post questions with a title, body, and tags

Post an answer

Users can post answers to existing questions

Comment on questions/answers

Users can add comments to questions or answers

Vote on questions/answers

Users can upvote or downvote questions and answers

Accept an answer

Question author can mark one answer as accepted

Search questions

Search questions by keyword, tag, or user

Reputation system

Track and update user reputation based on community actions

Tag management

Categorise questions with tags for discoverability

Constraints

A user can vote only once on a given question or answer
Only the question author can accept an answer
Only one answer per question can be accepted at a time
Reputation changes: +10 for upvote on answer, +5 for upvote on question, −2 for downvote received
Questions must have a title, body, and at least one tag

Assumptions

Users are already authenticated (auth is out of scope)
No rich-text formatting — plain text bodies
Single-threaded environment (no concurrency control)
In-memory storage (no database persistence)

In Scope

Question and answer lifecycle
Voting and reputation system
Comment functionality
Tag-based categorisation
Search by keyword and tag

Out of Scope

User authentication and authorisation
Rich-text / Markdown rendering
Notifications and email alerts
Badges and privilege levels
Pagination and rate limiting

Approach Guide(Click to expand each section)

Before diving into code, clarify the use cases and edge cases. Understanding the problem deeply leads to better class design.

List Core Use Cases

Identify the primary actions users will perform. For a parking lot: park vehicle, exit vehicle, check availability. Each becomes a method.

Identify Actors

Who interacts with the system? Customers, admins, automated systems? Each actor type may need different interfaces.

Clarify Constraints

What are the limits? Max vehicles, supported vehicle types, payment methods. Constraints drive your data structures.

Ask About Edge Cases

What happens on overflow? Concurrent access? Payment failures? Thinking about edge cases reveals hidden complexity.

Follow-up Questions(Questions an interviewer might ask)

💡 Draw UML class diagrams, sequence diagrams, or any design visualizations. Submit from the Code tab.

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00:00:00

Design Stack Overflow

Intermediate

Design an object-oriented system that models the core functionality of Stack Overflow — a Q&A platform where users can post questions, submit answers, comment, vote, and earn reputation.

The system should support tagging for categorisation, a reputation engine that rewards helpful contributions, and flexible search capabilities.

Core Use Cases

Post a question

Users can post questions with a title, body, and tags

Post an answer

Users can post answers to existing questions

Comment on questions/answers

Users can add comments to questions or answers

Vote on questions/answers

Users can upvote or downvote questions and answers

Accept an answer

Question author can mark one answer as accepted

Search questions

Search questions by keyword, tag, or user

Reputation system

Track and update user reputation based on community actions

Tag management

Categorise questions with tags for discoverability

Constraints

A user can vote only once on a given question or answer
Only the question author can accept an answer
Only one answer per question can be accepted at a time
Reputation changes: +10 for upvote on answer, +5 for upvote on question, −2 for downvote received
Questions must have a title, body, and at least one tag

Assumptions

Users are already authenticated (auth is out of scope)
No rich-text formatting — plain text bodies
Single-threaded environment (no concurrency control)
In-memory storage (no database persistence)

In Scope

Question and answer lifecycle
Voting and reputation system
Comment functionality
Tag-based categorisation
Search by keyword and tag

Out of Scope

User authentication and authorisation
Rich-text / Markdown rendering
Notifications and email alerts
Badges and privilege levels
Pagination and rate limiting

Approach Guide(Click to expand each section)

Before diving into code, clarify the use cases and edge cases. Understanding the problem deeply leads to better class design.

List Core Use Cases

Identify the primary actions users will perform. For a parking lot: park vehicle, exit vehicle, check availability. Each becomes a method.

Identify Actors

Who interacts with the system? Customers, admins, automated systems? Each actor type may need different interfaces.

Clarify Constraints

What are the limits? Max vehicles, supported vehicle types, payment methods. Constraints drive your data structures.

Ask About Edge Cases

What happens on overflow? Concurrent access? Payment failures? Thinking about edge cases reveals hidden complexity.

Follow-up Questions(Questions an interviewer might ask)

💡 Draw UML class diagrams, sequence diagrams, or any design visualizations. Submit from the Code tab.

Loading canvas...

Design Stack Overflow

Core Use Cases

Constraints

Assumptions

In Scope

Out of Scope

Approach Guide(Click to expand each section)

Follow-up Questions(Questions an interviewer might ask)

Explore the full editorial.

Sign in to view submissions

Design Stack Overflow

Core Use Cases

Constraints

Assumptions

In Scope

Out of Scope

Approach Guide(Click to expand each section)

Follow-up Questions(Questions an interviewer might ask)

Explore the full editorial.

Sign in to view submissions

Design Stack Overflow

Core Use Cases

Constraints

Assumptions

In Scope

Out of Scope

Approach Guide(Click to expand each section)

Gather Requirements~3 min

Identify Classes & Entities~4 min

Define Relationships~3 min

Apply Design Patterns~4 min

Apply SOLID Principles~3 min

Code Organization~2 min

Follow-up Questions(Questions an interviewer might ask)

1How would you add a badge / achievement system?

2How would you add support for bounties on questions?

3How would you handle duplicate question detection?

4How would you make the voting system thread-safe?

Key Topics

Asked At

Explore the full editorial.

Sign in to view submissions

Design Stack Overflow

Core Use Cases

Constraints

Assumptions

In Scope

Out of Scope

Approach Guide(Click to expand each section)

Gather Requirements~3 min

Identify Classes & Entities~4 min

Define Relationships~3 min

Apply Design Patterns~4 min

Apply SOLID Principles~3 min

Code Organization~2 min

Follow-up Questions(Questions an interviewer might ask)

1How would you add a badge / achievement system?

2How would you add support for bounties on questions?

3How would you handle duplicate question detection?

4How would you make the voting system thread-safe?

Key Topics

Asked At

Explore the full editorial.

Sign in to view submissions