Server-Sent Events

1. The Incident

Your food-delivery application tracks driver location, order status, and ETA. The page uses polling by calling GET /orders/{id}/status every 3 seconds to ask whether anything changed. The design is simple, reliable, and already in production.

On a rainy Friday evening, 200,000 customers open that page at once. At one request every 3 seconds, they produce about 66,000 requests per second. Each request performs a complete network round trip, including TLS setup, authentication, a database read, and a response.

However, 99 of every 100 responses report no change