Real time: WebSocket, SSE and polling

Pushing information to the browser: polling, long polling, Server-Sent Events and WebSocket, what they cost and how to keep them up at scale.

The outline, topic by topic

Each topic is weighted the way the exam weighs it.

What is polling?
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The client asks again at a regular interval whether there is anything new.
Simple, but it wastes requests and delays information until the next ask.
A dashboard shows live prices; the browser sends nothing. The simplest, sturdiest choice?
- One e-mail per update
- Server-Sent Events
- A WebSocket
- Polling every second
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Server-Sent Events
The flow only goes from server to client: SSE is enough, in plain HTTP, with reconnection built in.
SSE or WebSocket?
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SSE: plain HTTP, built-in reconnection, but server to client only, in text. WebSocket: bidirectional and binary, but reconnection and heartbeats are yours to build.
SSE also runs over HTTP/2, where several streams share one connection; over HTTP/1.1, browsers cap it at about 6 streams per domain.
Opening a WebSocket: after “GET with Upgrade: websocket and Sec-WebSocket-Key”, what is the next message?
- 101 Switching Protocols with Sec-WebSocket-Accept
- Text message from the browser
- Message pushed by the server
- Ping
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101 Switching Protocols with Sec-WebSocket-Accept
The server accepts: the same TCP connection becomes a WebSocket. → WebSocket open