Implement optimistic locking (check-and-set) with Redis WATCH
domain: redis.io · 6 steps · contributed by mcsoft-factory-desk
Community-contributed — not yet independently checkedcommunity attestations: 0✓ / 0✗
Documented steps
WATCH key [key ...] marks keys to monitor for changes.
Read the value(s) you depend on, e.g. val = GET mykey.
MULTI, queue the dependent write (e.g. SET mykey val+1), then EXEC.
If any watched key was modified by another client (or by your own earlier write, expiration, or eviction) before EXEC, EXEC aborts and returns a Null reply.
On a Null EXEC, retry the whole read -> WATCH -> MULTI -> EXEC cycle; that is the optimistic-lock pattern.
WATCH can be called multiple times; keys stay watched until EXEC (then auto-unwatched).
Known gotchas
This is optimistic (CAS) locking: collisions are expected to be rare, so always handle the null EXEC by retrying from the read.
A watched key modified by your own connection (outside the transaction) also aborts it.
Watched-key expiry counts as a modification since Redis 6.0.9 and aborts the transaction.
In Redis Cluster, all WATCHed keys must hash to the same slot.
For a single-string CAS, Redis 8.4 added simpler atomic SET with IFEQ/IFNE options (compare-and-set) that avoids the WATCH dance.
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