40 Common Redis Interview Questions and Answers

This article compiles 40 frequently asked Redis interview questions, providing concise answers for candidates preparing for technical interviews.

1. What is Redis? Redis is an open‑source, BSD‑licensed, high‑performance key‑value store that supports persistence, rich data structures (strings, lists, sets, sorted sets, hashes), master‑slave replication, and more.

2. Redis data types include string, hash, list, set, and sorted set (zset). Advanced modules add HyperLogLog, Geo, Pub/Sub, BloomFilter, RedisSearch, Redis‑ML, etc.

3. Benefits of using Redis

Extremely fast (≈110 000 reads/s, 81 000 writes/s).

Rich data structures and atomic operations.

Supports transactions (MULTI/EXEC) and features like publish/subscribe, key expiration, and more.

4. Redis vs. Memcached Redis offers richer data types, higher speed, and persistence, whereas Memcached only stores simple strings in memory.

5. Persistence mechanisms

RDB : snapshot of the dataset saved to a dump file; fast startup, low overhead.

AOF : logs every write command; higher durability but larger files and slower recovery.

6. Performance tips include avoiding heavy RDB snapshots on the master, using AOF for critical data, and placing master and slaves in the same LAN.

7. Eviction policies (volatile‑lru, volatile‑ttl, volatile‑random, allkeys‑lru, allkeys‑random, no‑eviction) determine which keys are removed when maxmemory is reached.

8. Memory considerations advise using hashes to pack related fields, leveraging 32‑bit instances efficiently, and understanding limits (theoretical 2³² keys, billions of elements).

9. Common use cases include session caching, full‑page caching, message queues (list), leaderboards (sorted set), publish/subscribe, and distributed locks.

10. Redis clustering provides high availability (Sentinel) and horizontal scaling (Cluster) with 16 384 hash slots; master‑slave replication ensures fault tolerance.

11. Client libraries for Java include Redisson (recommended), Jedis, Lettuce, each with different feature sets.

12. Security commands to set and verify a password:

config set requirepass 123456

auth 123456

13. Practical tips such as using SCAN instead of KEYS in production, adding random jitter to bulk expiration times, and implementing delayed queues with sorted sets.

14. Distributed lock example uses SETNX followed by EXPIRE, or the atomic SET key value NX EX seconds command.

The article concludes with a reminder to share the content and join the community for further learning.