Explain key expiry and eviction policies in Redis.
Assesses fundamental understanding of Redis conventions, runtime behavior, and memory/performance considerations.
Hiring managers look for precision, avoidance of ambiguous jargon, and ability to explain trade-offs under real production conditions.
You set a time to live with EXPIRE, SETEX or PEXPIRE. Redis removes expired keys lazily on access and also runs a background sampling cycle that actively deletes expired keys. Expiry is per key and works with any data type.
Eviction happens when memory reaches maxmemory. Policies include:
- noeviction: reject writes, best when data must not be lost.
- allkeys-lru and allkeys-lfu: evict least recently or least frequently used keys across all keys, best for caches.
- volatile-lru, volatile-lfu and volatile-ttl: evict only keys with a TTL.
- allkeys-random and volatile-random: random eviction.
LRU in Redis is approximate, sampling a few keys rather than tracking perfect order. LFU tracks access frequency and suits skewed workloads. Choose allkeys-lru for a general cache and volatile variants if some keys must persist. Monitor evicted_keys and hit rate.
Candidate Response Strategy & Interview Tips
- Start with a concise one-sentence summary: Deliver a direct, confident answer first before expanding into nuances.
- Demonstrate real-world trade-offs: Discuss where this approach excels and when you would avoid it in production systems.
- Discuss complexity & edge cases: Proactively explain time/space complexity or boundary conditions (null values, scale limits).
- Prepare for interviewer follow-ups: Technical hiring panels frequently probe deeper into concurrency, backward compatibility, or alternative libraries.