Design Reddit
PremiumProduct architectureEasy
- Voting system
- Hot ranking
- Nested comments
Quick answer
This product architecture interview question asks you to design Reddit. Aim for a design that meets its requirements at the stated scale, then explain its main components, data flow and trade-offs. Interviewers want to see how you scope the problem, size it and defend each choice.
The problem
Design Reddit, a community discussion site where users post into subreddits, vote posts and comments up or down, and talk in nested comment threads. The hard parts are ranking posts by votes and age as votes pour in, and loading comment trees with tens of thousands of replies quickly.
What must the system do?
- Post: A user creates a text or link post in a community; it appears under New at once.
- Vote: Upvote or downvote any post or comment, change or remove the vote; one vote per user per item.
How well must it do it?
- Fast pages: A community listing, and the first 200 comments of a thread, load in under 300 ms at p99.
- Fresh enough: Votes show in scores within a few seconds, and Hot order refreshes within a minute.
What scale should you design for?
Use these numbers in your estimates.
The scale numbers unlock with this questionUnlock
Key concepts
- Voting system
- Hot ranking
- Nested comments
- Sorted sets
- Read heavy caching
- Redis
How you’ll practise
- Requirements
- Core entities
- API design
- Client and server flow
- Scale and edge cases
- Wrap-up
Lala, personalized to you, gives a hint when you ask and reviews your design when you are ready.
What do interviewers look for?
In any product architecture design round.
- Clarifies the requirements before drawing anything
- Uses a few numbers that actually change the design
- Draws a clear path from the client to storage
- Picks storage and caching with reasons, not by habit
- Explains what breaks first under load and how to fix it
- States trade-offs openly instead of claiming a perfect design
Questions people ask about this interview question
- Which design type is this question?
- It is a product architecture question. You design Reddit and explain your choices the way you would in an interview.
- Is this question asked at entry level?
- It is marked Easy, so it suits engineers who are new to product architecture interviews. It is also a good warm-up before harder questions.
- What should I cover first?
- Start with the requirements: what the system must do, how well it must do it and which numbers change the design. Agree on scope with the interviewer before you design anything.
- Can I practise it for free?
- It is a premium question. Sign up and unlock it once for 10 credits; it then stays unlocked. When you finish, Lala, your personalized AI mentor, can review your design for 30 credits.
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Updated · Written by the SevyDevy team