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.

Cowboy comes with two 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.