LockForge

A small REST API for acquiring, releasing, extending, checking and listing named locks, backed by Redis. I built it to properly understand atomic operations. It's the kind of thing you need when two servers might process the same order at the same time.

A lock across machines has to be atomic (two servers can't both win), owner-safe (you can't release someone else's lock), and it can't stay stuck forever if the holder crashes.

Acquiring is a single SET key owner NX PX ttl, and since Redis runs commands one at a time only one caller can win. Release and extend have to check the owner and then act, so I wrote them as Lua scripts that run inside Redis and can't be interrupted halfway. The TTL handles crashed holders.

• acquireLock() is one SET … NX PX call. null means the lock is taken, and the API answers 409 with the current owner and retry_after_ms.
• releaseLock() runs a Lua script: GET == owner then DEL. That closes the gap where another process grabs the lock between my check and my delete.
• extendLock() does the same thing with PEXPIRE: only the owner can extend.
• listLocks() walks keys with SCAN … MATCH lock:* COUNT 100 instead of KEYS, which would block Redis.
• Release returns 403 for "not your lock" and 404 for "not locked". A GET after the script tells the two apart.
• The default TTL is 30s, and it runs next to redis:7-alpine in Docker Compose.

All five operations (acquire, release, check, extend, list) work. scripts/test-concurrent.sh fires 5 acquire requests at the same moment to check that only one gets a 200.