Redis Key Expiration

A TTL marks a key for automatic deletion. Expiry is how a cache stays bounded without an eviction policy having to do the work.

SET shop:session:abc "…" EX 3600        # set with expiry, atomically
EXPIRE shop:cart:9f3a 1800              # add expiry to an existing key
TTL shop:cart:9f3a                      # seconds remaining, -1 no TTL, -2 no key
PERSIST shop:cart:9f3a                  # remove the expiry

How expiry actually happens

Two mechanisms, and neither is instantaneous:

1. Lazy expiration. When a key is accessed, Redis checks whether it has expired and deletes it then.
2. Active expiration. A background cycle samples keys with TTLs and removes expired ones, repeating while it finds a high proportion expired.

The consequence: an expired key that nobody accesses continues to occupy memory until the active cycle happens to sample it. Memory does not drop the instant a large batch of keys expires. On replicas, expired keys are not deleted independently — the primary sends an explicit DEL, so a replica may briefly hold keys that are logically gone.

Operations that remove the TTL

Note that TTLs apply to the whole key. There is no per-field expiry in a hash in core Redis — a common wrong assumption when modelling objects as hashes.

Choosing TTLs

• Always set one on cache entries. A cache without expiry is a memory leak with good latency.
• Add jitter. Thousands of keys created together with an identical TTL expire together, causing a synchronised miss storm against the origin. See Cache Stampede.

ttl = base_ttl + random.randint(0, base_ttl // 10)

• Match the TTL to tolerable staleness, not to convenience. If a stale value for ten minutes is acceptable, ten minutes is the answer.

Monitoring

INFO stats        # expired_keys, evicted_keys
INFO keyspace     # keys and keys-with-expiry per database

db0:keys=1450000,expires=1200000 tells you 250,000 keys have no TTL. If that number grows, keys are being created without expiry or having it cleared — worth investigating before memory does it for you.

Keyspace notifications can publish expiry events, which is occasionally useful:

notify-keyspace-events Ex