Reddit DevOps / SRE Interview Questions

1.Tell me about a time you implemented a quick fix or workaround that wasn't the 'textbook' solution but got the job done. What was the situation and how did you decide to ship it?

2.Tell me about a time you had to optimize system performance for Reddit's scale. What bottlenecks did you identify and how did you prioritize which ones to fix first?

3.Tell me about a time when you had to scale infrastructure to handle a massive traffic spike. What was the spike, what broke first, and how did you handle it?

4.Describe a time you advocated for infrastructure changes that would give users more control or flexibility in how they interact with Reddit. What was your reasoning?

5.Describe a time when you had to make an infrastructure decision that would directly impact how communities or moderators use Reddit. How did you weigh the tradeoffs?

6.Walk me through a time you had to design monitoring or infrastructure to detect and respond to abuse or adversarial behavior. What patterns were you looking for?

7.You're managing Kafka clusters that handle Reddit's real-time comment and vote streams — millions of events per minute. One of your Kafka brokers starts falling behind and consumers are lagging. How do you diagnose and resolve this?

8.Reddit's recommendation service uses Cassandra to store user preference data that powers the home feed. You notice read latencies spiking during peak hours. Walk me through your troubleshooting approach.

9.Reddit's deploying a new microservice for processing reported content. The service needs to handle unpredictable traffic spikes when controversial posts hit r/all. How would you design the auto-scaling and resource management?

10.You're responsible for Reddit's GraphQL federation layer that serves data to web and mobile clients. A schema change in one service is causing timeouts across multiple federated queries. How do you investigate and resolve this?

11.Reddit's experimentation platform runs A/B tests on features like comment ranking algorithms. You need to design infrastructure to ensure test results aren't skewed by caching or CDN issues. How do you approach this?

12.Reddit's moving from Python monolith services to Go microservices. You need to design a migration strategy for the user authentication service that handles millions of login requests daily without downtime. Walk me through your approach.

13.Design a global CDN and edge caching layer for Reddit's mobile apps. The apps need to serve personalized home feeds to 500+ million monthly users, but we also need to respect community-specific content policies and moderator removals in real-time.

14.Reddit's Public Data API serves academic researchers and third-party developers. Design an API gateway that can handle 10,000 requests per second while enforcing different rate limits, data access policies, and privacy controls for each API consumer type.

15.Reddit's advertising platform needs to serve targeted ads to users browsing specific subreddits. Design a real-time bidding system that can handle 50,000 ad requests per second while ensuring ads are appropriate for each community's content policies.

16.Design the infrastructure for Reddit's machine learning recommendation system that powers home feed ranking. It needs to process user behavior signals in real-time, train models on petabytes of historical data, and serve personalized rankings for hundreds of millions of users without creating filter bubbles.

17.Design a distributed logging and observability system for Reddit's microservices architecture. You need to correlate traces across 200+ services while handling 10TB of logs daily, and the system must help engineers quickly identify which service is causing user-facing issues during incidents.

18.Design Reddit's deployment pipeline for rolling out changes to the comment processing service. This service handles millions of comments per hour across thousands of active subreddits, and a bug could break comment posting site-wide. You need zero-downtime deployments with the ability to quickly rollback.

19.Tell me about a time you had to convince other teams to adopt a new operational practice or tool. How did you approach getting buy-in?

20.Tell me about a time you mentored someone who was struggling with on-call or incident response. How did you help them improve?

21.Describe a time you had to manage an incident where the root cause was unclear and multiple teams were involved. How did you coordinate the response?

22.Walk me through a time you identified that your team's operational approach wasn't working and needed to change. How did you lead that transition?

23.Tell me about a time you had to make a judgment call about operational risk when you didn't have complete information. What factors did you consider?

24.Describe a time you had to influence a product or engineering decision based on operational constraints. How did you make your case?

25.Reddit's comment voting system experiences a mysterious 15% drop in vote engagement across all subreddits over 3 days. No code deploys, no incidents, CDN looks fine. Walk me through how you'd investigate this.

26.Estimate how many additional server instances Reddit would need if we decided to store all comment edit history permanently instead of just the current version. Walk through your calculation.

27.You notice that subreddit subscriber counts are updating inconsistently - some communities show real-time updates while others lag by hours. How do you debug this systematically?

28.Reddit's mobile app team wants to implement infinite scroll for subreddit feeds, but you're concerned about backend load. Estimate the infrastructure impact and propose how to make this feasible.

29.Design a system to automatically detect when Reddit's homepage algorithm might be creating filter bubbles that limit exposure to diverse communities. What metrics would you track and how would you measure success?

30.Reddit needs to estimate the infrastructure cost of expanding to 3 new non-English speaking markets. Each market has different mobile vs desktop usage patterns and varying comment-to-upvote ratios. Walk through your analysis.