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What is the difference between the EXPIRE, TTL, and PERSIST commands in Redis? What eviction policies are available in Redis for handling the situation when memory usage reaches its configured maximum limit? 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 does Redis actually implement key expiration internally, using both a passive and an active approach?
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How would you diagnose and resolve a situation where important data is being unexpectedly evicted from Redis due to memory pressure?

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Answer

You would check the current maxmemory setting and eviction policy, review which keys are consuming the most memory, determine whether you actually need more available memory or a different eviction policy, and consider whether some data should be marked without an expiration and moved to a separate Redis instance using noeviction to guarantee it is never evicted.
CONFIG GET maxmemory-policy
INFO memory
MEMORY USAGE somekey
Real-world example A team discovers important configuration data was being evicted from a shared Redis cache under memory pressure, so they move that critical data to a separate Redis instance configured with noeviction to guarantee it is never accidentally removed.

Common follow-ups: Should critical, non expendable data be kept in the same Redis instance as cache data at all?;How do you calculate how much memory your critical data actually needs?

Redis Memory Optimization;Redis Monitoring & Observability
All 7 Expiration & Eviction questions