::: cowboy_rest

The `cowboy_rest` module implements REST semantics on top of
the HTTP protocol.

This module is a sub protocol that defines many callbacks
be implemented by handlers. The `init/2` and `terminate/3`
callbacks are common to all handler types and are documented
in the manual for the ^cowboy_handler module.

All other callbacks are optional, though some may become
required depending on the return value of previous callbacks.

:: Meta values

: charset

Type: binary()

Negotiated charset.

This value may not be defined if no charset was negotiated.

: language

Type: binary()

Negotiated language.

This value may not be defined if no language was negotiated.

: media_type

Type: {binary(), binary(), '*' | [{binary(), binary()}]}

Negotiated media-type.

The media-type is the content-type, excluding the charset.

This value is always defined after the call to
`content_types_provided/2`.

:: Terminate reasons

The following values may be received as the terminate reason
in the optional `terminate/3` callback.

: normal

The connection was closed normally.

: {crash, Class, Reason}

A crash occurred in the handler. `Class` and `Reason` can be
used to obtain more information about the crash. The function
`erlang:get_stacktrace/0` can also be called to obtain the
stacktrace of the process when the crash occurred.

:: Callbacks

: Callback(Req, State) -> {Value, Req, State} | {stop, Req, State}

Types:

* Callback - one of the REST callbacks described below
* Req = cowboy_req:req()
* State = any()
* Value - see the REST callbacks description below

Please see the REST callbacks description below for details
on the `Value` type, the default value if the callback is
not defined, and more general information on when the
callback is called and what its intended use is.

The `stop` tuple can be returned to stop REST processing.
It is up to the resource code to send a reply before that,
otherwise a `204 No Content` will be sent.

:: REST callbacks description

: allowed_methods

* Methods: all
* Value type: [binary()]
* Default value: [<<"GET">>, <<"HEAD">>, <<"OPTIONS">>]

Return the list of allowed methods.

Methods are case sensitive. Standard methods are always uppercase.

: allow_missing_post

* Methods: POST
* Value type: boolean()
* Default value: true

Return whether POST is allowed when the resource doesn't exist.

Returning `true` here means that a new resource will be
created. The URL to the created resource should also be
returned from the `AcceptResource` callback.

: charsets_provided

* Methods: GET, HEAD, POST, PUT, PATCH, DELETE
* Value type: [binary()]
* Skip to the next step if undefined

Return the list of charsets the resource provides.

The list must be ordered in order of preference.

If the accept-charset header was not sent, the first charset
in the list will be selected. Otherwise Cowboy will select
the most appropriate charset from the list.

The chosen charset will be set in the `Req` object as the meta
value `charset`.

While charsets are case insensitive, this callback is expected
to return them as lowercase binary.

: content_types_accepted

* Methods: POST, PUT, PATCH
* No default

Types:

* Value = [{binary() | {Type, SubType, Params}, AcceptResource}]
* Type = SubType = binary()
* Params = '*' | [{binary(), binary()}]
* AcceptResource = atom()

Return the list of content-types the resource accepts.

The list must be ordered in order of preference.

Each content-type can be given either as a binary string or as
a tuple containing the type, subtype and parameters.

Cowboy will select the most appropriate content-type from the list.
If any parameter is acceptable, then the tuple form should be used
with parameters set to `'*'`. If the parameters value is set to `[]`
only content-type values with no parameters will be accepted. All
parameter values are treated in a case sensitive manner except the
`charset` parameter, if present, which is case insensitive.

This function will be called for POST, PUT and PATCH requests.
It is entirely possible to define different callbacks for different
methods if the handling of the request differs. Simply verify
what the method is with `cowboy_req:method/1` and return a
different list for each methods.

The `AcceptResource` value is the name of the callback that will
be called if the content-type matches. It is defined as follow.

* Value type: true | {true, URL} | false
* No default

Process the request body.

This function should create or update the resource with the
information contained in the request body. This information
may be full or partial depending on the request method.

If the request body was processed successfully, `true` must
be returned. If the request method is POST, `{true, URL}` may
be returned instead, and Cowboy will redirect the client to
the location of the newly created resource.

If a response body must be sent, the appropriate media-type, charset
and language can be retrieved using the `cowboy_req:meta/{2,3}`
functions. The respective keys are `media_type`, `charset`
and `language`. The body can be set using `cowboy_req:set_resp_body/2`.

: content_types_provided

* Methods: GET, HEAD, POST, PUT, PATCH, DELETE
* Default value: [{{<<"text">>, <<"html">>, '*'}, to_html}]

Types:

* Value = [{binary() | {Type, SubType, Params}, ProvideResource}]
* Type = SubType = binary()
* Params = '*' | [{binary(), binary()}]
* ProvideResource = atom()

Return the list of content-types the resource provides.

The list must be ordered in order of preference.

Each content-type can be given either as a binary string or as
a tuple containing the type, subtype and parameters.

Cowboy will select the most appropriate content-type from the list.
If any parameter is acceptable, then the tuple form should be used
with parameters set to `'*'`. If the parameters value is set to `[]`
only content-type values with no parameters will be accepted. All
parameter values are treated in a case sensitive manner except the
`charset` parameter, if present, which is case insensitive.

The `ProvideResource` value is the name of the callback that will
be called if the content-type matches. It will only be called when
a representation of the resource needs to be returned. It is defined
as follow.

* Methods: GET, HEAD
* Value type: iodata() | {stream, Fun} | {stream, Len, Fun} | {chunked, ChunkedFun}
* No default

Return the response body.

The response body may be provided directly or through a fun.
If a fun tuple is returned, the appropriate `set_resp_body_fun`
function will be called. Please refer to the documentation for
these functions for more information about the types.

The call to this callback happens a good time after the call to
`content_types_provided/2`, when it is time to start rendering
the response body.

: delete_completed

* Methods: DELETE
* Value type: boolean()
* Default value: true

Return whether the delete action has been completed.

This function should return `false` if there is no guarantee
that the resource gets deleted immediately from the system,
including from any internal cache.

When this function returns `false`, a `202 Accepted`
response will be sent instead of a `200 OK` or `204 No Content`.

: delete_resource

* Methods: DELETE
* Value type: boolean()
* Default value: false

Delete the resource.

The value returned indicates if the action was successful,
regardless of whether the resource is immediately deleted
from the system.

: expires

* Methods: GET, HEAD
* Value type: calendar:datetime() | binary() | undefined
* Default value: undefined

Return the date of expiration of the resource.

This date will be sent as the value of the expires header.

: forbidden

* Methods: all
* Value type: boolean()
* Default value: false

Return whether access to the resource is forbidden.

A `403 Forbidden` response will be sent if this
function returns `true`. This status code means that
access is forbidden regardless of authentication,
and that the request shouldn't be repeated.

: generate_etag

* Methods: GET, HEAD, POST, PUT, PATCH, DELETE
* Value type: binary() | {weak | strong, binary()}
* Default value: undefined

Return the entity tag of the resource.

This value will be sent as the value of the etag header.

If a binary is returned, then the value will be parsed
to the tuple form automatically. The value must be in
the same format as the etag header, including quotes.

: is_authorized

* Methods: all
* Value type: true | {false, AuthHeader}
* Default value: true

Return whether the user is authorized to perform the action.

This function should be used to perform any necessary
authentication of the user before attempting to perform
any action on the resource.

If the authentication fails, the value returned will be sent
as the value for the www-authenticate header in the
`401 Unauthorized` response.

: is_conflict

* Methods: PUT
* Value type: boolean()
* Default value: false

Return whether the put action results in a conflict.

A `409 Conflict` response will be sent if this function
returns `true`.

: known_methods

* Methods: all
* Value type: [binary()]
* Default value: [<<"GET">>, <<"HEAD">>, <<"POST">>, <<"PUT">>, <<"PATCH">>, <<"DELETE">>, <<"OPTIONS">>]

Return the list of known methods.

The full list of methods known by the server should be
returned, regardless of their use in the resource.

The default value lists the methods Cowboy knows and
implement in `cowboy_rest`.

Methods are case sensitive. Standard methods are always uppercase.

: languages_provided

* Methods: GET, HEAD, POST, PUT, PATCH, DELETE
* Value type: [binary()]
* Skip to the next step if undefined

Return the list of languages the resource provides.

The list must be ordered in order of preference.

If the accept-language header was not sent, the first language
in the list will be selected. Otherwise Cowboy will select
the most appropriate language from the list.

The chosen language will be set in the `Req` object as the meta
value `language`.

While languages are case insensitive, this callback is expected
to return them as lowercase binary.

: last_modified

* Methods: GET, HEAD, POST, PUT, PATCH, DELETE
* Value type: calendar:datetime()
* Default value: undefined

Return the date of last modification of the resource.

This date will be used to test against the if-modified-since
and if-unmodified-since headers, and sent as the last-modified
header in the response of GET and HEAD requests.

: malformed_request

* Methods: all
* Value type: boolean()
* Default value: false

Return whether the request is malformed.

Cowboy has already performed all the necessary checks
by the time this function is called, so few resources
are expected to implement it.

The check is to be done on the request itself, not on
the request body, which is processed later.

: moved_permanently

* Methods: GET, HEAD, POST, PUT, PATCH, DELETE
* Value type: {true, URL} | false
* Default value: false

Return whether the resource was permanently moved.

If it was, its new URL is also returned and sent in the
location header in the response.

: moved_temporarily

* Methods: GET, HEAD, POST, PATCH, DELETE
* Value type: {true, URL} | false
* Default value: false

Return whether the resource was temporarily moved.

If it was, its new URL is also returned and sent in the
location header in the response.

: multiple_choices

* Methods: GET, HEAD, POST, PUT, PATCH, DELETE
* Value type: boolean()
* Default value: false

Return whether there are multiple representations of the resource.

This function should be used to inform the client if there
are different representations of the resource, for example
different content-type. If this function returns `true`,
the response body should include information about these
different representations using `cowboy_req:set_resp_body/2`.
The content-type of the response should be the one previously
negociated and that can be obtained by calling
`cowboy_req:meta(media_type, Req)`.

: options

* Methods: OPTIONS
* Value type: ok
* Default value: ok

Handle a request for information.

The response should inform the client the communication
options available for this resource.

By default, Cowboy will send a `200 OK` response with the
allow header set.

: previously_existed

* Methods: GET, HEAD, POST, PATCH, DELETE
* Value type: boolean()
* Default value: false

Return whether the resource existed previously.

: resource_exists

* Methods: GET, HEAD, POST, PUT, PATCH, DELETE
* Value type: boolean()
* Default value: true

Return whether the resource exists.

If it exists, conditional headers will be tested before
attempting to perform the action. Otherwise, Cowboy will
check if the resource previously existed first.

: service_available

* Methods: all
* Value type: boolean()
* Default value: true

Return whether the service is available.

This function can be used to test that all relevant backend
systems are up and able to handle requests.

A `503 Service Unavailable` response will be sent if this
function returns `false`.

: uri_too_long

* Methods: all
* Value type: boolean()
* Default value: false

Return whether the requested URI is too long.

Cowboy has already performed all the necessary checks
by the time this function is called, so few resources
are expected to implement it.

A `414 Request-URI Too Long` response will be sent if this
function returns `true`.

: valid_content_headers

* Methods: all
* Value type: boolean()
* Default value: true

Return whether the content-* headers are valid.

This also applies to the transfer-encoding header. This
function must return `false` for any unknown content-*
headers, or if the headers can't be understood. The
function `cowboy_req:parse_header/2` can be used to
quickly check the headers can be parsed.

A `501 Not Implemented` response will be sent if this
function returns `false`.

: valid_entity_length

* Methods: all
* Value type: boolean()
* Default value: true

Return whether the request body length is within acceptable boundaries.

A `413 Request Entity Too Large` response will be sent if this
function returns `false`.

: variances

* Methods: GET, HEAD, POST, PUT, PATCH, DELETE
* Value type: [binary()]
* Default value: []

Return the list of headers that affect the representation of the resource.

These request headers return the same resource but with different
parameters, like another language or a different content-type.

Cowboy will automatically add the accept, accept-language and
accept-charset headers to the list if the respective functions
were defined in the resource.

This operation is performed right before the `resource_exists/2`
callback. All responses past that point will contain the vary
header which holds this list.
