Building High-Throughput E-Commerce Platforms: Handling 10,000+ Requests per Second

1. What Happens When 50,000 Users Hit Checkout At Once

During high-profile flash sales or holiday promotions, database connection pools are overwhelmed in seconds. Standard web servers run out of threads, database CPU spikes to 100%, and frustrated customers are met with 504 Gateway Timeouts. Worse yet, two different buyers might purchase the exact same remaining inventory item simultaneously.

Surviving massive concurrency requires shifting from traditional synchronous CRUD operations to **High-Throughput Reactive Architecture**.

2. Solving the Overselling Race Condition

When 100 users attempt to purchase the last remaining iPhone in stock: - A naive `SELECT stock FROM products` followed by `UPDATE products SET stock = stock - 1` will oversell because 50 threads read `stock = 1` simultaneously before any write occurs. - **Atomic Database Decrements with Constraints:** Executing `UPDATE products SET stock = stock - 1 WHERE id = $1 AND stock > 0 RETURNING stock` guarantees only one transaction succeeds at the database engine level. - **Redis Distributed Locks (Redlock):** For ultra-high traffic, holding temporary inventory reservations in memory before committing to disk.