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## Chapter 24. Streams

A stream is the set of messages that form an HTTP request/response pair.

The term stream comes from HTTP/2. In Cowboy, it is also used when talking about HTTP/1.1 or HTTP/1.0. It should not be confused with streaming the request or response body.

All versions of HTTP allow clients to initiate streams. HTTP/2 is the only one also allowing servers, through its server push feature. Both client and server-initiated streams go through the same process in Cowboy.

## 24.1. Stream handlers

Stream handlers must implement five different callbacks. Four of them are directly related; one is special.

All callbacks receives the stream ID as first argument.

Most of them can return a list of commands to be executed by Cowboy. When callbacks are chained, it is possible to intercept and modify these commands. This can be useful for modifying responses for example.

The `init/3` callback is invoked when a new request comes in. It receives the Req object and the protocol options for this listener.

The `data/4` callback is invoked when data from the request body is received. It receives both this data and a flag indicating whether more is to be expected.

The `info/3` callback is invoked when an Erlang message is received for this stream. They will typically be messages sent by the request process.

Finally the `terminate/3` callback is invoked with the terminate reason for the stream. The return value is ignored. Note that as with all terminate callbacks in Erlang, there is no strong guarantee that it will be called.

The special callback `early_error/5` is called when an error occurs before the request headers were fully received and Cowboy is sending a response. It receives the partial Req object, the error reason, the protocol options and the response Cowboy will send. This response must be returned, possibly modified.

## 24.2. Built-in handlers

Cowboy comes with four handlers.

cowboy_stream_h is the default stream handler. It is the core of much of the functionality of Cowboy. All chains of stream handlers should call it last.

cowboy_compress_h will automatically compress responses when possible. It is not enabled by default. It is a good example for writing your own handlers that will modify responses.

cowboy_decompress_h will automatically decompress request bodies when possible. It is not enabled by default. It is a good example for writing your own handlers that will modify requests.

cowboy_metrics_h gathers metrics about a stream then passes them to a configurable function. It is not enabled by default.

cowboy_tracer_h can be used to conditionally trace streams based on the contents of the request or its origin. Trace events are passed to a configurable function. It is not enabled by default.

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