Project Detected: Node.js Express API using PostgreSQL, Prisma, and multiple application instances --- Race Condition Found: - The order service reads stock and decrements it in separate queries - Invariant at risk: Available product stock must never become negative or satisfy more orders than available units --- Concurrency Boundary: - Shared resource: Product inventory row - Competing actors: Simultaneous checkout requests across application instances --- Repair Applied: - Replaced the separate stock check and decrement with a conditional atomic update requiring stock greater than or equal to the requested quantity - Added a transaction covering inventory reservation and order creation - Added a database uniqueness guard for checkout idempotency keys --- Files Modified: - src/services/orderService.ts - src/repositories/inventoryRepository.ts - prisma/schema.prisma - tests/concurrency/inventoryRace.test.ts - tests/fixtures/products.ts --- Commands Executed: - npx prisma validate - npm run typecheck - npm test -- tests/concurrency/inventoryRace.test.ts - npm run build --- Validation Result: Schema validation, type checking, the targeted concurrency test, and the production build completed successfully against an isolated PostgreSQL database. --- Repeatability Test: - Concurrent operations: 20 requests for 1 remaining unit - Successful operations: 1 - Safely rejected or deduplicated: 19 - Final invariant: Stock remained at 0, one order was created, and no duplicate reservation was recorded --- Remaining Risks: - Payment capture occurs after inventory reservation and still requires expiration handling for abandoned reservations --- Important Notes: The repair relies on PostgreSQL atomic updates and a database constraint, so it remains effective across multiple application instances. No production orders or payment providers were accessed.