Redis Interview Questions

1. What is Redis and what makes it different from a traditional relational database?

2. What are the core data types in Redis and when do you use each?

3. How does Redis achieve its high performance as an in-memory store?

4. What is the difference between Redis persistence with RDB and AOF?

5. How does Redis handle key expiration and TTL?

6. What is the difference between Redis as a cache and as a primary datastore?

7. What are the common cache eviction policies in Redis (LRU, LFU, TTL)?

8. What is a Redis hash and how does it differ from storing a serialized object?

9. What are Redis sorted sets and what problems do they solve?

10. How does pub/sub messaging work in Redis?

11. What is Redis pipelining and how does it improve throughput?

12. What are Redis transactions and how do MULTI/EXEC work?

13. What is the difference between Redis replication and Redis Cluster?

14. How does Redis Sentinel provide high availability?

15. How does sharding work in Redis Cluster with hash slots?

16. What is a cache stampede (thundering herd) and how do you prevent it in Redis?

17. What is the difference between write-through, write-behind, and cache-aside patterns?

18. How do you implement a distributed lock with Redis?

19. What are Redis streams and how do they compare to pub/sub?

20. What is the difference between SET with NX and a plain SET in Redis?

21. How does Redis handle atomic operations like INCR?

22. What is a Redis bitmap and what use cases does it enable?

23. What is HyperLogLog in Redis and when would you use it?

24. How does Redis handle memory management and the maxmemory setting?

25. What is the difference between Redis SCAN and KEYS and why does it matter?

26. How do Lua scripts work in Redis and why are they atomic?

27. What is cache invalidation and what strategies work with Redis?

28. What are the trade-offs of using Redis for session storage?

29. How does Redis handle blocking operations like BLPOP?

30. What is the difference between Redis and Memcached?

31. A Redis instance shows intermittent latency spikes in production — how do you diagnose the cause?

32. Redis reports far more resident memory than stored data — how do you interpret and fix fragmentation?

33. How do you find and fix big keys and hot keys in a Redis deployment?

34. How does Redis client-side caching with CLIENT TRACKING work, and when is it worth it?

35. How do you secure a production Redis deployment beyond setting a password?

36. How does live resharding work in Redis Cluster, and what can go wrong during slot migration?

37. Why can a Redis failover lose acknowledged writes, and how do you reduce that risk?

38. What are Redis Functions, how do they differ from EVAL scripts, and what are the rules for writing them safely?

39. How would you upgrade or migrate a live Redis deployment without downtime or data loss?

40. How would you implement distributed rate limiting with Redis, and which algorithm would you choose?

41. Why can a Redis background save cause a memory spike and a latency pause, and how do you control it?

42. Can you build an event pipeline on Redis keyspace notifications, and what are their delivery guarantees?

43. Application latency to Redis is poor although the server reports fast commands — how do you tune the client?

44. How should a client library route commands in Redis Cluster, and what happens during MOVED, ASK and failover?

45. What happens between starting a Redis process and it serving traffic, and how do you make restarts safe?

46. Which Redis metrics would you alert on, and how do you choose thresholds without guessing?

47. A Lua script is running long and the whole instance is unresponsive — what is happening and what do you do?

48. A stream consumer crashes mid-processing — how do its pending messages get reclaimed and eventually retired?

49. Can a read from a Redis replica return a key that has already expired, and how do you reason about staleness?

50. Would you separate tenants or environments using Redis databases, key prefixes, or separate instances?

51. How do you test code that depends on Redis, including its failure behaviour?

52. Two Redis instances hold the same logical data but one uses far more memory — how do object encodings explain that?

53. Redis is described as single-threaded — what exactly is single-threaded, and what does that mean for how you design commands?

54. Which Redis commands are dangerous to run against a production instance, and how do you stop them being run?

55. What guarantees does SCAN actually give you, and how do you iterate a large keyspace safely?

56. What actually happens the moment a Redis instance reaches maxmemory?

57. What durability does an AOF actually give you, and how do the fsync policies and rewrites change that?

58. When is it safe to read from a Redis replica, and how do you deal with the staleness?

59. Why does a multi-key command fail with CROSSSLOT in Redis Cluster, and how do hash tags fix it?

60. Why is a naive SET NX lock unsafe, and what does a fencing token actually protect?

61. How do you choose between pipelining, MULTI/EXEC and a Lua script for a multi-command operation?