Beyond SETNX: Implementing a Production-Grade Distributed Lock with Node.js and Redis Lua Scripts

Picture this: your restaurant booking platform is growing. You've scaled your Node.js API to four replicas behind a load balancer. Then Friday evening hits, and two guests—Charlie and Diana—both smash "Reserve Table 12" at the exact same millisecond. Both requests land on different instances. Both read the database: table 12 is free. Both write a booking row. Last writer wins. Charlie gets a confirmation email. Diana gets a confirmation email. Saturday night, both show up. The host is apologetic. The table is awkward. Your on-call engineer is not having a good weekend. That is a double-booking race condition, and it's the canonical distributed systems bug for any stateful, time-sensitive resource. Seats on a flight. Stock in a flash sale. A hotel room. A consulting slot. A database SELECT ... FOR UPDATE pessimistic lock would solve this on a single DB node—but only if every booking request hits that same node serially. The moment you have multiple Node.js processes, you need a lock tha