Redis key expiry and TTL: how to set and check expiration

Why expiry matters

Redis lives in RAM. You must control memory by expiring keys you no longer need — especially cached data and session tokens.

Setting expiry

# Set a key and expire it in 60 seconds
SET session:abc123 "user:42"
EXPIRE session:abc123 60

# Shorthand: set and expire in one command
SET session:abc123 "user:42" EX 60

# Expire at a specific Unix timestamp
EXPIREAT session:abc123 1735689600

Checking remaining TTL

TTL session:abc123   # → 47 (seconds remaining)
                     # → -1 (no expiry set)
                     # → -2 (key does not exist)

Removing expiry

PERSIST session:abc123   # removes expiry, key lives forever

How Redis deletes expired keys

Redis uses two strategies. Lazy expiry: a key is checked and deleted when you next access it. Active expiry: Redis periodically samples random keys with TTLs and deletes the expired ones. This means a key may survive a few milliseconds past its TTL under low access — design your application to tolerate this.

For caching, always set an EX value. A cache entry with no expiry is a memory leak.