System Design Patterns: From Fundamentals to Real Systems

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Server-Sent Events

1. The Incident

Your food-delivery app has an order tracking page: driver location, order status, 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 are watching that page at the same time. Do the math: 200,000 people asking every 3 seconds is about 66,000 requests per second. Each request is a full round trip: TLS, an auth check, a database read, a response. And what does the answer say? 99 times out of 100: nothing changed

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