The Websocket protocol

This chapter explains what Websocket is and why it is a vital component of soft realtime Web applications.


Websocket is an extension to HTTP that emulates plain TCP connections between the client, typically a Web browser, and the server. It uses the HTTP Upgrade mechanism to establish the connection.

Websocket connections are fully asynchronous, unlike HTTP/1.1 (synchronous) and HTTP/2 (asynchronous, but the server can only initiate streams in response to requests). With Websocket, the client and the server can both send frames at any time without any restriction. It is closer to TCP than any of the HTTP protocols.

Websocket is an IETF standard. Cowboy supports the standard and all drafts that were previously implemented by browsers, excluding the initial flawed draft sometimes known as "version 0".

Websocket vs HTTP/2

For a few years Websocket was the only way to have a bidirectional asynchronous connection with the server. This changed when HTTP/2 was introduced. While HTTP/2 requires the client to first perform a request before the server can push data, this is only a minor restriction as the client can do so just as it connects.

Websocket was designed as a kind-of-TCP channel to a server. It only defines the framing and connection management and lets the developer implement a protocol on top of it. For example you could implement IRC over Websocket and use a Javascript IRC client to speak to the server.

HTTP/2 on the other hand is just an improvement over the HTTP/1.1 connection and request/response mechanism. It has the same semantics as HTTP/1.1.

If all you need is to access an HTTP API, then HTTP/2 should be your first choice. On the other hand, if what you need is a different protocol, then you can use Websocket to implement it.


Cowboy implements Websocket as a protocol upgrade. Once the upgrade is performed from the init/2 callback, Cowboy switches to Websocket. Please consult the next chapter for more information on initiating and handling Websocket connections.

The implementation of Websocket in Cowboy is validated using the Autobahn test suite, which is an extensive suite of tests covering all aspects of the protocol. Cowboy passes the suite with 100% success, including all optional tests.

Cowboy’s Websocket implementation also includes the permessage-deflate and x-webkit-deflate-frame compression extensions.

Cowboy will automatically use compression when the compress option is returned from the init/2 function.

Cowboy 2.0 User Guide


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