Server-Sent Events

1. The Incident

Your food-delivery app has an order tracking page. It shows the driver location, the order status, and the ETA. It works by polling: every 3 seconds, the page calls GET /orders/{id}/status and asks, "anything new?" Simple, reliable, shipped.

Then you grow. On a rainy Friday night, 200,000 customers watch that page at the same time. Multiply it out: 200,000 people asking every 3 seconds is about 66,000 requests per second. Each request is a full round trip: TLS (the encrypted connection setup), an auth check, a database read, a response.