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[[rest_handlers]]
== 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.

=== 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.

[source,erlang]
----
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.

=== 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 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.

[cols="<,^",options="header"]
|===
| Callback name          | Default value
| allowed_methods        | `[<<"GET">>, <<"HEAD">>, <<"OPTIONS">>]`
| allow_missing_post     | `true`
| charsets_provided      | skip
| content_types_accepted | none
// @todo Space required for the time being: https://github.com/spf13/hugo/issues/2398
| 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`
| 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`
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 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:

[cols="<,<",options="header"]
|===
| 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.

[cols="<,<",options="header"]
|===
| 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
|===