Redis

Expiration (Memory Leaks)

• Keys without TTL live forever—set expiry on every cache key: SET key value EX 3600
• Can't add TTL after SET without another command—use SETEX or SET ... EX
• EXPIRE resets on key update by default—SET removes TTL; use SET ... KEEPTTL (Redis 6+)
• Lazy expiration: expired keys removed on access—may consume memory until touched
• SCAN with large database: expired keys still show until cleanup cycle runs

Data Structures I Underuse

• Sorted sets for rate limiting: ZADD limits:{user} {now} {request_id} + ZREMRANGEBYSCORE for sliding window
• HyperLogLog for unique counts: PFADD visitors {ip} uses 12KB for billions of uniques
• Streams for queues: XADD, XREAD, XACK—better than LIST for reliable queues
• Hashes for objects: HSET user:1 name "Alice" email "a@b.com"—more memory efficient than JSON string

Atomicity Traps