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## Chapter 19. REST handlers

REST is implemented in Cowboy as a sub protocol. 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 HTTP requests.

## 19.1. Initialization

First, the `init/2` callback is called. This callback is common to all handlers. To use REST for the current request, this function must return a `cowboy_rest` tuple.

    init(Req, State) ->
        {cowboy_rest, Req, State}.

Cowboy will then switch to the REST protocol and start executing the state machine.

After reaching the end of the flowchart, the `terminate/3` callback will be called if it is defined.

## 19.2. 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.

## 19.3. Callbacks

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.

All callbacks take two arguments, the Req object and the State, and return a three-element tuple of the form `{Value, Req, State}`.

Nearly all callbacks can also return `{stop, Req, State}` to stop execution of the request, and `{{switch_handler, Module}, Req, State}` or `{{switch_handler, Module, Opts}, Req, State}` to switch to a different handler type. The exceptions are `expires` `generate_etag`, `last_modified` and `variances`.

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.

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 name | Default value
--- | ---
allowed_methods | `[<<"GET">>, <<"HEAD">>, <<"OPTIONS">>]`
allow_missing_post | `true`
charsets_provided | skip
content_types_accepted | none
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_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`
ranges_provided | skip
range_satisfiable | `true`
rate_limited | `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`, `content_types_provided/2` or `ranges_provided/2`. They can take any name (except `auto` for range callbacks), 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.

## 19.4. Meta data

Cowboy will set informative values to the Req object at various points of the execution. You can retrieve them by matching the Req object directly. The values are defined in the following table:

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
range | The range selected for the ranged response

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

## 19.5. 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
--- | ---
accept-ranges | Range units accepted by the resource
allow | HTTP methods allowed by the resource
content-language | Language used in the response body
content-range | Range of the content found in the response
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
retry-after | Delay or time the client should wait before accessing the resource
vary | List of headers that may change the representation of the resource
www-authenticate | Authentication information to access the resource

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