How I Built a Concurrency-Safe Reservation System

Short summary

A developer built a concurrency-safe movie reservation backend using FastAPI, PostgreSQL, Redis, and SQLAlchemy to solve the classic race condition where multiple users book the same seat simultaneously. The system uses Redis distributed locks with Lua scripts for atomic multi-seat locking, PostgreSQL unique constraints and transactions as a final consistency safeguard, and idempotency keys to handle duplicate requests. Load testing with Locust confirmed that out of 100 concurrent reservation attempts for the same seat, exactly one succeeded with zero duplicate bookings.

• •Redis Lua scripts provide atomic all-or-nothing multi-seat locking
• •PostgreSQL unique constraints and transactions serve as the final consistency layer
• •Locust load test: 100 concurrent users, 1 success, 0 duplicates

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