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This document explains in as much details as possible the list of planned changes and work to be done on the Cowboy server. It is intended to be exhaustive but some elements might still be missing.

2.0 (R17 and R18)

The main features that will be added to Cowboy 2.0 are support for HTTP/2.0 and Websocket permessage deflate compression.

A complete analysis of the httpbis set of specifications will be performed and extensive tests will be written to ensure maximum compatibility.

A number of backward incompatible changes are planned. These changes are individually small, but together should result in a large improvement in usability.

New cowboy_req function

A convenience function will be added to more easily process HTML forms that were sent using POST. These forms have two main ways of being submitted: as a form urlencoded format, or as multipart. The latter case may also be used to submit files, therefore we also need a way to handle this. We also need to take into account that we are reading from the socket and can't take as much a shortcut as with match_qs/2.

The function will work similarly to other body reading functions, in that it may require more than one call to obtain everything. In this case there would be two return values: the ok return with the map filled with key/value pairs, ending the body reading; and the file return that informs the caller that a file has been provided and the caller must handle it. If the caller calls the function again without doing anything, the part is just skipped, like what part/1 is doing. If a file is the last input from the form then a subsequent call will return an ok return with an empty map.

The interface would look as follow:

match_body(Fields, Req) -> match_body(Fields, Req, [])
match_body(Fields, Req, Opts)
	-> {ok, Map, Req}
	| {file, FieldName, Filename, CType, CTransferEncoding, Map, Req}
	when Req::req()

It would be up to the caller to decide what to do with the maps returned. Fields are in order so the map returned may be empty if the form starts with a file, or may only contain the values before the file input if this one is in the middle of the form. It is of course possible to merge all maps returned into one though that should not be needed very often.

It is also possible to switch from this function to only multipart functions if the function returns a file tuple, as this function is a higher level interface that simply calls the multipart functions when the request body is in multipart format.


The interface of the onresponse hook will change. There has been a number of issues and added complexity with the current interface that warrant fixing. The main problem is that the hook may be used to change the reply, by calling the reply function again, forcing us to be careful not to reprocess everything again.

To fix that, we will cut the reply mechanism in two steps, one that is basically some preprocessing of the response header to follow the protocol requirements, and then the actual response. The onresponse hook will fit in the middle, being called from the first step and calling the second step itself.

If a body streaming function is provided, the hook will also receive it (unlike today). It will not be able to inspect its contents however.

This should greatly simplify the code and allow users to do any operation they wish.

Low-level interface documented

A special chapter of the manual will document a low-level interface that may be used in middlewares or hooks (but nowhere else). This includes the Req access and update functions and the new response function described above.


We probably want to send something other than 500 when the max data read value has been reached. This happens when the client is misbehaving and is not a server error, but rather a safeguard.


The documentation for all REST callbacks will be updated to describe whether they can have side effects. This will allows us to build introspection tools on top of a working REST API.

Range support will be added.