cowboy_rest - REST handlers
The module cowboy_rest implements the HTTP state machine.
Implementing REST handlers is not enough to provide a REST interface; this interface must also follow the REST constraints including HATEOAS (hypermedia as the engine of application state).
REST handlers implement the following interface:
init(Req, State)
-> {cowboy_rest, Req, State}
Callback(Req, State)
-> {Result, Req, State}
| {stop, Req, State}
| {{switch_handler, Module}, Req, State}
| {{switch_handler, Module, Opts}, Req, State}
terminate(Reason, Req, State) -> ok %% optional
Req :: cowboy_req:req()
State :: any()
Module :: module()
Opts :: any()
Reason :: normal
| {crash, error | exit | throw, any()}
Callback - see below
Result - see below
Default - see below
The init/2 callback is common to all handlers. To switch to the REST handler behavior, it must return cowboy_rest as the first element of the tuple.
The Callback/2 above represents all the REST-specific callbacks. They are described in the following section of this manual. REST-specific callbacks differ by their name, semantics, result and default values. The default value is the one used when the callback has not been implemented. They otherwise all follow the same interface.
The stop tuple can be returned to stop REST processing. If no response was sent before then, Cowboy will send a 204 No Content. The stop tuple can be returned from any callback, excluding expires, generate_etag, last_modified and variances.
A switch_handler tuple can be returned from these same callbacks to stop REST processing and switch to a different handler type. This is very useful to, for example, to stream the response body.
The optional terminate/3 callback will ultimately be called with the reason for the termination of the handler. Cowboy will terminate the process right after this. There is no need to perform any cleanup in this callback.
The following terminate reasons are defined for loop handlers:
The handler terminated normally.
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.
AcceptCallback(Req, State) -> {Result, Req, State}
Result :: true | {true, URI :: iodata()} | false}
Default - crash
Process the request body.
This function should create or update the resource using the request body.
For PUT requests, the body is a representation of the resource that is being created or replaced.
For POST requests, the body is typically application-specific instructions on how to process the request, but it may also be a representation of the resource. When creating a new resource with POST at a different location, return {true, URI} with URI the new location.
For PATCH requests, the body is a series of instructions on how to update the resource. Patch files or JSON Patch are examples of such media types.
A response body may be sent. The appropriate media type, charset and language for the response can be retrieved from the Req object using the media_type, charset and language keys, respectively. The body can be set using cowboy_req:set_resp_body(3).
allowed_methods(Req, State) -> {Result, Req, State}
Result :: [binary()] %% case sensitive
Default :: [<<"GET">>, <<"HEAD">>, <<"OPTIONS">>]
Return the list of allowed methods.
allow_missing_post(Req, State) -> {Result, Req, State}
Result :: boolean()
Default :: true
Return whether POST is allowed when the resource doesn't exist.
Returning true here means that a new resource will be created. The URI for the newly created resource should be returned from the AcceptCallback function.
charsets_provided(Req, State) -> {Result, Req, State}
Result :: [binary()] %% lowercase; case insensitive
Default - skip this step
Return the list of charsets the resource provides in order of preference.
During content negotiation Cowboy will pick the most appropriate charset for the client. The client advertises charsets it prefers with the accept-charset header. When that header is missing, Cowboy picks the first charset from the resource.
Cowboy will add the negotiated charset to the Req object after this step completes:
req() :: #{
charset => binary() %% lowercase; case insensitive
}
content_types_accepted(Req, State) -> {Result, Req, State}
Result :: [{binary() | ParsedMime, AcceptCallback :: atom()}]
ParsedMime :: {Type :: binary(), SubType :: binary(), '*' | Params}
Params :: [{Key :: binary(), Value :: binary()}]
Default - crash
Return the list of media types the resource accepts in order of preference.
A media type is made of different parts. The media type text/html;charset=utf-8 is of type text, subtype html and has a single parameter charset with value utf-8.
Cowboy will match the content-type request header against the media types the server accepts and select the appropriate callback. When that header is missing, or when the server does not accept this media type, the request fails and an error response is returned. Cowboy will execute the callback immediately otherwise.
An empty parameters list [] means that no parameters will be accepted. When any parameter is acceptable, the tuple form should be used with parameters as the atom '*'.
Cowboy treats all parameters as case sensitive, except for the charset parameter, which is known to be case insensitive. You should therefore always provide the charset as a lowercase binary string.
content_types_provided(Req, State) -> {Result, Req, State}
Result :: [{binary() | ParsedMime, ProvideCallback :: atom()}]
ParsedMime :: {Type :: binary(), SubType :: binary(), '*' | Params}
Params :: [{Key :: binary(), Value :: binary()}]
Default - [{{ <<"text">>, <<"html">>, '*'}, to_html}]
Return the list of media types the resource provides in order of preference.
A media type is made of different parts. The media type text/html;charset=utf-8 is of type text, subtype html and has a single parameter charset with value utf-8.
During content negotiation Cowboy will pick the most appropriate media type for the client. The client advertises media types it prefers with the accept header. When that header is missing, the content negotiation fails and an error response is returned.
The callback given for the selected media type will be called at the end of the execution of GET and HEAD requests when a representation must be sent to the client.
An empty parameters list [] means that no parameters will be accepted. When any parameter is acceptable, the tuple form should be used with parameters as the atom '*'.
Cowboy treats all parameters as case sensitive, except for the charset parameter, which is known to be case insensitive. You should therefore always provide the charset as a lowercase binary string.
Cowboy will add the negotiated media_type to the Req object after this step completes:
req() :: #{
media_type => ParsedMime
}
delete_completed(Req, State) -> {Result, Req, State}
Result :: boolean()
Default :: true
Return whether the resource has been fully deleted from the system, including from any internal cache.
Returning false will result in a 202 Accepted response being sent instead of a 200 OK or 204 No Content.
delete_resource(Req, State) -> {Result, Req, State}
Result :: boolean()
Default :: false
Delete the resource.
Cowboy will send an error response when this function returns false.
expires(Req, State) -> {Result, Req, State}
Result :: calendar:datetime() | binary() | undefined
Default :: undefined
Return the resource's expiration date.
forbidden(Req, State) -> {Result, Req, State}
Result :: boolean()
Default :: 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(Req, State) -> {Result, Req, State}
Result :: binary() | {weak | strong, binary()}
Default - no etag value
Return the entity tag of the resource.
When a binary is returned, the value is automatically parsed to a tuple. The binary must be in the same format as the etag header, including quotes.
is_authorized(Req, State) -> {Result, Req, State}
Result :: true | {false, AuthHeader :: iodata()}
Default - 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.
When authentication fails, the AuthHeader value will be sent in the www-authenticate header for the 401 Unauthorized response.
is_conflict(Req, State) -> {Result, Req, State}
Result :: boolean()
Default :: false
Return whether the PUT request results in a conflict.
A 409 Conflict response is sent when true.
known_methods(Req, State) -> {Result, Req, State}
Result :: [binary()] %% case sensitive
Default :: [<<"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.
languages_provided(Req, State) -> {Result, Req, State}
Result :: [binary()] %% lowercase; case insensitive
Default - skip this step
Return the list of languages the resource provides in order of preference.
During content negotiation Cowboy will pick the most appropriate language for the client. The client advertises languages it prefers with the accept-language header. When that header is missing, Cowboy picks the first language from the resource.
Cowboy will add the negotiated language to the Req object after this step completes:
req() :: #{
language => binary() %% lowercase; case insensitive
}
last_modified(Req, State) -> {Result, Req, State}
Result :: calendar:datetime()
Default - no last modified value
Return the resource's last modification date.
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 to GET and HEAD requests.
malformed_request(Req, State) -> {Result, Req, State}
Result :: boolean()
Default :: false
Return whether the request is malformed.
A request is malformed when a component required by the resource is invalid. This may include the query string or individual headers. They should be parsed and validated in this function. The body should not be read at this point.
moved_permanently(Req, State) -> {Result, Req, State}
Result :: {true, URI :: iodata()} | false
Default :: false
Return whether the resource was permanently moved, and what its new location is.
moved_temporarily(Req, State) -> {Result, Req, State}
Result :: {true, URI :: iodata()} | false
Default :: false
Return whether the resource was temporarily moved, and what its new location is.
multiple_choices(Req, State) -> {Result, Req, State}
Result :: boolean()
Default :: false
Return whether the client should engage in reactive negotiation.
Return true when the server has multiple representations of a resource, each with their specific identifier, but is unable to determine which is best for the client. For example an image might have different sizes and the server is unable to determine the capabilities of the client.
When returning true the server should send a body with links to the different representations. If the server has a preferred representation it can send its link inside a location header.
options(Req, State) -> {ok, Req, State}
Respond to an OPTIONS request.
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(Req, State) -> {Result, Req, State}
Result :: boolean()
Default :: false
Return whether the resource existed previously.
ProvideCallback(Req, State) -> {Result, Req, State}
Result :: cowboy_req:resp_body()
Default - crash
Return the response body.
The response body can be provided either as the actual data to be sent or a tuple indicating which file to send.
This function is called for both GET and HEAD requests. For the latter the body is not sent, however.
Note that there used to be a way to stream the response body. It was temporarily removed and will be added back in a later release.
resource_exists(Req, State) -> {Result, Req, State}
Result :: boolean()
Default :: true
Return whether the resource exists.
service_available(Req, State) -> {Result, Req, State}
Result :: boolean()
Default :: true
Return whether the service is available.
A 503 Service Unavailable response will be sent when this function returns false.
uri_too_long(Req, State) -> {Result, Req, State}
Result :: boolean()
Default :: false
Return whether the requested URI is too long.
This function can be used to further restrict the length of the URI for this specific resource.
valid_content_headers(Req, State) -> {Result, Req, State}
Result :: boolean()
Default :: true
Return whether the content headers are valid.
This callback can be used to reject requests that have invalid content header values, for example an unsupported content-encoding.
valid_entity_length(Req, State) -> {Result, Req, State}
Result :: boolean()
Default :: 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(Req, State) -> {Result, Req, State}
Result :: [binary()] %% case insensitive
Default :: []
Return the list of request headers that affect the representation of the resource.
Cowboy automatically adds the accept, accept-charset and accept-language headers when necessary. It's also useful to note that some standard headers also do not need to be listed here, like the authorization header.
switch_handler return value was added.