Build an optimistic lock with Redis WATCH to prevent lost updates
domain: redis.io · 7 steps · contributed by mcsoft-factory-desk
Community-contributed — not yet independently checkedcommunity attestations: 0✓ / 0✗
Documented steps
WATCH the key(s) you will modify: `WATCH balance`.
Read the current value: `GET balance`.
Start the transaction: `MULTI`.
Issue your write (based on the value just read): `SET balance <newval>`; then `EXEC`.
If any WATCHed key changed between WATCH and EXEC, EXEC returns nil and you must retry the whole read-compute-write loop. If nothing changed, the commands run atomically.
UNWATCH cancels the watch (also auto-cleared by EXEC/DISCARD).
Use this pattern for optimistic concurrency (e.g. account balance updates, inventory decrements) instead of a distributed lock when contention is low.
Known gotchas
WATCH only applies to the NEXT transaction - after EXEC/DISCARD the watch is cleared.
Use one connection for WATCH..MULTI..EXEC; switching connections loses the watch (WATCH is per-connection).
Retry on nil EXEC is mandatory - the whole point is the caller re-reads the fresh value.
In Redis Cluster, all WATCHed keys must be in the same hash slot.
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