Reddit Software Engineer Interview Questions

1.Tell me about a time your service started getting hammered with traffic — way beyond what you designed for. How did you figure out where the bottleneck was, and what did you actually do about it?

2.Tell me about a time you shipped something that was genuinely hacky — not the clean solution — because it was the right call given the constraints. What made you pull the trigger on it?

3.Walk me through a time you made an engineering decision — a design choice, a feature rollout, a data model — that you changed or walked back because of how it was affecting users or a community. What tipped you off?

4.Walk me through a time you pushed for a feature or product change that gave users more ability to express themselves or control how they showed up on the platform — especially if it wasn't the easy sell internally.

5.Tell me about a time you found or had to design around an abuse vector in a system — something where a bad actor could exploit normal functionality in a harmful way. How did you approach it?

6.Tell me about a distributed system you built or maintained where data consistency was actually hard — not just eventually consistent by default, but where getting it wrong had a real cost. How did you reason about the consistency model?

7.Say you're adding a new field to a GraphQL schema that's shared across Reddit's web and mobile clients. What do you think about before merging that change?

8.You have a Kafka consumer that processes moderation events — things like post removals and user reports. It suddenly falls hours behind. How do you triage it and what are your options for recovery?

9.Reddit's feed ranking runs ML models that score posts. A new model version ships and 48 hours later engagement is up but a handful of subreddits are seeing a dramatic drop in surfaced content. How do you investigate and what do you do?

10.You need to add rate limiting to Reddit's Public Data API — it serves both small developers and large ML training partners. How would you design it, and what tradeoffs would you call out?

11.We're seeing intermittent 500s from a Go microservice that sits behind our gRPC gateway. The errors spike for about 90 seconds, every few hours, then fully recover. Nothing correlates in the deploy log. Walk me through your investigation.

12.You're designing the backend for a new awards or reaction system on Reddit posts — users can give awards and see aggregate counts. It needs to handle high write volume during viral moments. How do you design the write and read paths?

13.Design a read-heavy comment count system for Reddit posts. A post hitting the front page might get thousands of comments in minutes — how do you keep the displayed count accurate-enough without hammering your database?

14.We want to build a simple 'recently viewed communities' feature — when a user visits their profile, they see the last 10 subreddits they browsed. How would you design the storage and retrieval for this?

15.Design the backend for Reddit's crosspost feature — a user reposts an existing post into a different subreddit. The original post's vote count and the crosspost's vote count need to stay independent, but the crosspost should display the original media. Walk me through how you'd model this.

16.Design a subreddit search feature — users can type a query and get a ranked list of subreddits to join. This needs to handle ~500 million subreddits existing but only a small fraction being active. How do you build the index and ranking?

17.Design Reddit's post scheduling feature — a moderator or user sets a post to go live at a specific future time. It sounds simple, but what breaks at Reddit's scale and how do you handle it?

18.Design a real-time moderator alert system — when AutoModerator flags a post in a large subreddit, the volunteer moderators need to see it within seconds on their mod queue dashboard. Some subreddits have 50+ active mods; a popular subreddit might generate thousands of flagged items per hour during a controversy. How do you build this?

19.Tell me about a time you disagreed with a technical direction your team was going in. How did you raise it, and what happened?

20.Tell me about a time you helped onboard or mentor a new teammate. What did you actually do, and what would you do differently now?

21.Describe a time you had to ship something under real time pressure — not artificial urgency, but genuine pressure — and quality suffered. How did you handle the aftermath?

22.Tell me about a time you had to coordinate a technical change across multiple teams — not just communicate it, but actually get alignment and unblock the work. What made it hard?

23.Tell me about a time you spotted a pattern of bugs or incidents in your codebase and decided to fix the root cause rather than the symptoms. How did you make the case for spending time on it?

24.Tell me about a time you had to change your technical opinion publicly — in a PR, a design doc, or a meeting — because someone else's argument was actually better. How did you handle it?

25.Estimate how many comments are posted on Reddit in a single day. Walk me through your assumptions.

26.A subreddit moderator files a bug report: their community's subscriber count displayed in the sidebar hasn't updated in two days. How do you figure out what's wrong?

27.Reddit's daily active user metric dropped 8% last Monday and recovered by Wednesday, with no deploys or incidents logged. How do you investigate it?

28.Estimate what percentage of Reddit's total server costs are attributable to serving images and video. What levers would you look at to reduce it?

29.Reddit is considering showing ads inside comment threads — not just between posts in the feed. Walk me through how you'd estimate whether this is a good idea, and what data you'd need before recommending it.

30.Reddit wants to reduce the time it takes for a new user to find their first relevant community — right now, too many new users churn before they subscribe to anything. How would you measure the problem and design an approach to fix it?