Reddit Senior Software Engineer Interview Questions

1.Tell me about a time you shipped something that wasn't your ideal solution but solved the immediate problem. What corners did you cut and why?

2.Describe a situation where you had to optimize for high throughput or handle a major traffic spike. What was your approach?

3.Walk me through a time you had to quickly scale a system that was hitting performance limits. What was breaking, and how did you approach the fix?

4.Walk me through a time you advocated for a feature that gave users more control or expression, even when it created complexity. How did you make the case?

5.Tell me about a feature decision where you had to consider how it would affect different user communities. What was the tradeoff and how did you navigate it?

6.Describe a system you've built that had to handle bad actors or abuse. What specific attack vectors did you design against?

7.We need to add a new GraphQL field that aggregates data from three different microservices. The frontend team wants it quickly, but this could create an N+1 query problem for popular posts. How would you design this?

8.Write a function that takes a Reddit comment thread and returns the top 3 most controversial replies. Define controversial as comments with high engagement but polarized voting patterns.

9.You're implementing a new comment ranking algorithm in our ML pipeline. The data science team gives you a Kafka stream of engagement events, but you notice some events are arriving out of order. How do you handle this?

10.A moderator reports that their community's automod rules stopped working after our recent deployment. The rules engine processes thousands of posts per minute across millions of subreddits. How would you debug this?

11.Design the backend API for Reddit's new real-time chat feature in subreddits. It needs to handle spikes during major events and integrate with our existing moderation systems. What are your key architectural decisions?

12.We want to migrate our user session storage from a single PostgreSQL cluster to a distributed cache layer. User sessions contain authentication tokens, preferences, and temporary state. Walk me through your migration strategy for 500 million monthly active users.

13.Design a distributed configuration service that can update feature flags and experiments across Reddit's microservice architecture in under 100ms. The system needs to work even when its own database is down.

14.Design a moderator dashboard that aggregates community health metrics from across 50+ different data sources and microservices. The dashboard needs to update in real-time and handle communities ranging from 100 to 50 million members.

15.Design the backend for Reddit's upcoming live event feature where millions of users can participate in real-time discussions during breaking news or AMAs. How do you handle the massive write volume while keeping comments relevant and accessible?

16.Reddit's ads team wants to launch community-sponsored content where subreddit communities can collectively fund promoted posts within their own community. Design the payment processing and escrow system for this feature.

17.Design a karma farming detection system for Reddit that can identify coordinated inauthentic behavior across millions of accounts in near real-time. How would you balance catching bad actors while minimizing false positives on legitimate users?

18.Design a system to auto-translate Reddit content into 20+ languages while preserving community context, slang, and cultural references. How do you handle the scale of billions of posts and comments while maintaining translation quality?

19.Tell me about a time you had to convince engineers across multiple teams to adopt a new practice or standard. How did you approach it?

20.Describe a time you mentored someone whose background or perspective was very different from yours. What did you learn from that experience?

21.Tell me about a time you had to make a tough engineering decision when stakeholders disagreed on priorities. How did you navigate the competing interests?

22.Describe a situation where you had to help a team through a significant technical change or migration. What was your role in driving adoption?

23.Walk me through a time you had to challenge a decision made by senior leadership or architects. How did you approach it and what was the outcome?

24.Tell me about a time you had to coordinate a complex technical initiative across teams when there was no clear owner or single point of accountability. How did you drive it forward?

25.Reddit's notification system sometimes sends users 5 notifications for the same comment thread within minutes. Users complain, but we can't reproduce it reliably. How would you debug this?

26.Estimate how many database queries Reddit's homepage generates for a logged-in user during peak hours. Walk me through your reasoning.

27.Reddit's mobile app crashes are up 15% this week, but our crash reporting shows no new error patterns. How would you investigate what's actually happening?

28.Reddit wants to estimate the revenue impact of reducing comment load times by 200ms across all mobile apps. What factors would you consider and how would you structure this analysis?

29.Reddit's search team wants to launch personalized subreddit recommendations, but early tests show it might reduce discovery of niche communities by 30%. How would you approach this tradeoff analysis?

30.You need to estimate the infrastructure cost of Reddit's anti-spam system processing 50 million posts and comments daily. What are the major cost drivers and how would you model this?