Redis Interview Questions
Q: What are the eviction policies in Redis?
A: noeviction (error on full), allkeys-lru (evict least recently used), volatile-lru (LRU on keys with TTL), allkeys-lfu (least frequently used), volatile-ttl (shortest TTL first). allkeys-lru is the most common for caching.
Q: What is the difference between RDB and AOF?
A: RDB = point-in-time snapshots (compact, fast recovery, possible data loss). AOF = logs every write (more durable, max 1s loss with default everysec policy — configurable via appendfsync; always = no loss, no = up to ~30s loss; larger files). Can use both together. RDB for backups, AOF for durability.
Q: How does Redis Sentinel work? A: Sentinel monitors Redis master/replica instances. If master is down, Sentinel promotes a replica to master and updates clients. Provides: monitoring, notification, automatic failover. Requires at least 3 Sentinels for quorum.
Q: What is a Redis pipeline? A: Batching multiple commands into a single network round-trip. Instead of send→wait→send→wait, send all commands at once, receive all responses. Reduces network latency significantly (10x-100x for many small commands).
Q: How do you use Redis as a message broker? A: (1) Pub/Sub for real-time fanout (no persistence, fire-and-forget), (2) Lists for reliable queues (LPUSH + BRPOP), (3) Streams for durable messaging with consumer groups (XADD + XREADGROUP). Use Streams for production; Pub/Sub for real-time notifications.