= cowboy_rest(3) == Name 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 link:cowboy_handler.asciidoc[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:: 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.