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Redis lock recheck flag pattern

Skill kjuhwa/skills-hub/skills/backend/redis-lock-recheck-flag-pattern

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Install
npx -y skills add kjuhwa/skills-hub --skill redis-lock-recheck-flag-pattern

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Distributed Redis lock with a "recheck" flag so concurrent events arriving while the lock is held force the holder to run a second pass, instead of being silently dropped.

SKILL.md

1.9 KB, 383 tokens by cl100k_base, as published. Nobody here has run it

Redis Lock + Recheck Flag

Shape

lockKey    = prefix:<key>
recheckKey = recheck:<key>
instanceId = <hostname>:<uuid>

Acquire via SET lockKey instanceId NX EX ttl. If acquired, loop: clear recheck → run work → if recheck was re-set during work, loop again. On exit, release with a Lua script that only deletes if the current value matches instanceId (prevents stealing).

Steps

  1. Compute lockKey, recheckKey, instanceId.
  2. SET lockKey instanceId NX EX ttl.
  3. Acquire failed → SET recheckKey 1 and return. (Signals the holder: reprocess after done.)
  4. Acquire succeeded → enter process loop:
    1. DEL recheckKey (clear before work).
    2. Run business logic.
    3. EXISTS recheckKey? If yes, loop. Else break. (Cap with MAX_LOOPS to prevent starvation.)
  5. Release with Lua: 
    if redis.call('get', KEYS[1]) == ARGV[1] then return redis.call('del', KEYS[1]) end
return 0

Counter / Caveats

  • Choose TTL > worst-case processing time. If it expires mid-run, another instance grabs the lock — two workers.
  • DEL recheckKey must happen before work, not after, or events arriving during work are lost.
  • Cap the retry loop so a continuously-racing event stream can't pin one instance forever.
  • Use a UUID-per-invocation for instanceId, not a static hostname — otherwise two worker threads on the same host can't tell their locks apart.

What ships with it: 1 file

418 B alongside SKILL.md

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