REST handlers

REST is implemented in Cowboy as a protocol upgrade. Once upgraded, the request is handled as a state machine with many optional callbacks describing the resource and modifying the machine's behavior.

The REST handler is the recommended way to handle requests.


First, the init/3 callback is called. This callback is common to all handlers. To use REST for the current request, this function must return an upgrade tuple.

Cowboy will then switch to the REST protocol and start executing the state machine, starting from rest_init/2 if it's defined, and ending with rest_terminate/2 also if defined.


The REST component has code for handling the following HTTP methods: HEAD, GET, POST, PATCH, PUT, DELETE and OPTIONS.

Other methods can be accepted, however they have no specific callback defined for them at this time.


All callbacks are optional. Some may become mandatory depending on what other defined callbacks return. The various flowcharts in the next chapter should be a useful to determine which callbacks you need.

When the request starts being processed, Cowboy will call the rest_init/2 function if it is defined, with the Req object and the handler options as arguments. This function must return {ok, Req, State} where State is the handler's state that all subsequent callbacks will receive.

At the end of every request, the special callback rest_terminate/2 will be called if it is defined. It cannot be used to send a reply, and must always return ok.

All other callbacks are resource callbacks. They all take two arguments, the Req object and the State, and return a three-element tuple of the form {Value, Req, State}.

The following table summarizes the callbacks and their default values. If the callback isn't defined, then the default value will be used. Please look at the flowcharts to find out the result of each return value.

All callbacks can also return {halt, Req, State} to stop execution of the request, at which point rest_terminate/2 will be called.

In the following table, "skip" means the callback is entirely skipped if it is undefined, moving directly to the next step. Similarly, "none" means there is no default value for this callback.

Callback nameDefault value
allowed_methods[<<"GET">>, <<"HEAD">>, <<"OPTIONS">>]
content_types_provided[{{<<"text">>, <<"html">>, '*'}, to_html}]
known_methods[<<"GET">>, <<"HEAD">>, <<"POST">>, <<"PUT">>, <<"PATCH">>, <<"DELETE">>, <<"OPTIONS">>]

As you can see, Cowboy tries to move on with the request whenever possible by using well thought out default values.

In addition to these, there can be any number of user-defined callbacks that are specified through content_types_accepted/2 and content_types_provided/2. They can take any name, however it is recommended to use a separate prefix for the callbacks of each function. For example, from_html and to_html indicate in the first case that we're accepting a resource given as HTML, and in the second case that we send one as HTML.

Meta data

Cowboy will set informative meta values at various points of the execution. You can retrieve them using cowboy_req:meta/{2,3}. The values are defined in the following table.

Meta keyDetails
media_typeThe content-type negotiated for the response entity.
languageThe language negotiated for the response entity.
charsetThe charset negotiated for the response entity.

They can be used to send a proper body with the response to a request that used a method other than HEAD or GET.

Response headers

Cowboy will set response headers automatically over the execution of the REST code. They are listed in the following table.

Header nameDetails
content-languageLanguage used in the response body
content-typeMedia type and charset of the response body
etagEtag of the resource
expiresExpiration date of the resource
last-modifiedLast modification date for the resource
locationRelative or absolute URI to the requested resource
varyList of headers that may change the representation of the resource


See also

Version select