cowboy_rest man page

cowboy_rest — REST handlers

Description

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 = binary()

Negotiated charset.

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

language = binary()

Negotiated language.

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

media_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}

Callback

One of the REST callbacks described below.

Req = cowboy_req:req()

The Req object.

State = any()

Handler state.

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()]

Default behavior

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

Value type

[{binary() | {Type, SubType, Params}, AcceptResource}]

Default behavior

Crash if undefined.

With types:

· 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 follows.

Value type

true | {true, URL} | false

Default behavior

Crash if undefined.

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

Value type

[{binary() | {Type, SubType, Params}, ProvideResource}]

Default value

[{{<<"text">>, <<"html">>, '*'}, to_html}]

With types:

· 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}

Default behavior

Crash if undefined.

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

With types:

· AuthHead = iodata()

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()]

Default behavior

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

With types:

· URL = iodata()

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

With types:

· URL = iodata()

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

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.

Info

04/05/2016