cowboy/guide/rest_handlers.md

REST handlers

Purpose

REST is a set of constraints that, when applied to HTTP, dictates how resources must behave. It is the recommended way to handle requests with Cowboy.

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.

As the REST handler is still subject to change, the documentation is still thin. This state of affair will be improved in the coming weeks.

Usage

Like Websocket, REST is a sub-protocol of HTTP. It therefore requires a protocol upgrade.

init({tcp, http}, Req, Opts) ->
    {upgrade, protocol, cowboy_rest}.

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

Flow diagram

Not done yet. Feel free to use the one that is currently being worked on.

• https://github.com/extend/cowboy/pull/364

Methods

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.

Callbacks

All callbacks are optional. Some may become mandatory depending on what other defined callbacks return. The flow diagram should be a pretty good resource 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 flow diagram 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, an empty column means there is no default value for this callback.

Callback name	Default value
allowed_methods	[<<"GET">>, <<"HEAD">>, <<"OPTIONS">>]
allow_missing_post	true
charsets_provided	skip
content_types_accepted
content_types_provided	[{{<<"text">>, <<"html">>, '*'}, to_html}]
delete_completed	true
delete_resource	false
expires	undefined
forbidden	false
generate_etag	undefined
is_authorized	true
is_conflict	false
known_content_type	true
known_methods	[<<"GET">>, <<"HEAD">>, <<"POST">>, <<"PUT">>, <<"PATCH">>, <<"DELETE">>, <<"OPTIONS">>]
languages_provided	skip
last_modified	undefined
malformed_request	false
moved_permanently	false
moved_temporarily	false
multiple_choices	false
options	ok
previously_existed	false
resource_exists	true
service_available	true
uri_too_long	false
valid_content_headers	true
valid_entity_length	true
variances	[]

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 key	Details
media_type	The content-type negotiated for the response entity.
language	The language negotiated for the response entity.
charset	The 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 name	Details
content-language	Language used in the response body
content-type	Media type and charset of the response body
etag	Etag of the resource
expires	Expiration date of the resource
last-modified	Last modification date for the resource
location	Relative or absolute URI to the requested resource
vary	List of headers that may change the representation of the resource