3. Expiration, Eviction, and Cache Design

Redis keys can have TTLs set with `EXPIRE` or `SET key value EX seconds`. Expiration removes data after its freshness window, while an eviction policy decides which keys to remove when the configured memory limit is reached.

Choose a policy based on the workload: volatile-lru favors expiring keys that have not been used recently, while allkeys-lru can evict any key. A cache should always tolerate misses because evicted data is not durable application truth.