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How do you set an expiration time on a Redis key so it automatically gets deleted after a certain period? What is the difference between the EXPIRE, TTL, and PERSIST commands in Redis? What is the difference between the LRU and LFU eviction algorithms, and when would you choose one over the other? How do you monitor how many keys are currently being evicted from Redis due to memory pressure, and why is this an important metric to track? How would you diagnose and resolve a situation where important data is being unexpectedly evicted from Redis due to memory pressure? How does Redis actually implement key expiration internally, using both a passive and an active approach?
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What eviction policies are available in Redis for handling the situation when memory usage reaches its configured maximum limit?

Intermediate

Answer

Redis offers several eviction policies including noeviction, which rejects new writes once memory is full, allkeys-lru and volatile-lru, which remove the least recently used keys, allkeys-lfu and volatile-lfu, which remove the least frequently used keys, and random based policies, letting you choose the behavior that best fits whether your data is primarily used as a cache or as a persistent store.
CONFIG SET maxmemory-policy allkeys-lru
CONFIG SET maxmemory 2gb
Real-world example A caching layer for a busy website configures the allkeys-lru eviction policy, ensuring that when memory fills up, Redis automatically removes the least recently accessed cached items to make room for new ones.

Common follow-ups: What is the difference between the allkeys and volatile variants of these eviction policies?;What happens if you use noeviction and memory becomes completely full?

Redis Memory Optimization;Redis Architecture & Installation
All 7 Expiration & Eviction questions