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Add the Cowboy Function Reference

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The manual details every stable public functions of Cowboy.
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cowboy
======
The `cowboy` module provides convenience functions for
manipulating Ranch listeners.
Types
-----
### http_headers() = [{binary(), iodata()}]
> HTTP headers as a list of key/values.
### http_status() = non_neg_integer() | binary()
> HTTP status.
>
> A binary status can be used to set a custom message.
### http_version() = 'HTTP/1.1' | 'HTTP/1.0'
> HTTP version.
### onrequest_fun() = fun((cowboy_req:req()) -> cowboy_req:req())
> Fun called immediately after receiving a request.
>
> It can perform any operation on the `Req` object, including
> reading the request body or replying. If a reply is sent,
> the processing of the request ends here, before any middleware
> is executed.
### onresponse_fun() = fun((http_status(), http_headers(),
iodata(), cowboy_req:req()) -> cowboy_req:req())
> Fun called immediately before sending the response.
>
> It can perform any operation on the `Req` object, including
> reading the request body or replying. If a reply is sent, it
> overrides the reply initially sent. The callback will not be
> called again for the new reply.
Exports
-------
### start_http(Ref, NbAcceptors, TransOpts, ProtoOpts) -> {ok, pid()}
> Types:
> * Ref = ranch:ref()
> * NbAcceptors = non_neg_integer()
> * TransOpts = ranch_tcp:opts()
> * ProtoOpts = cowboy_protocol:opts()
>
> Start listening for HTTP connections. Returns the pid for this
> listener's supervisor.
### start_https(Ref, NbAcceptors, TransOpts, ProtoOpts) -> {ok, pid()}
> Types:
> * Ref = ranch:ref()
> * NbAcceptors = non_neg_integer()
> * TransOpts = ranch_ssl:opts()
> * ProtoOpts = cowboy_protocol:opts()
>
> Start listening for HTTPS connections. Returns the pid for this
> listener's supervisor.
### stop_listener(Ref) -> ok
> Types:
> * Ref = ranch:ref()
>
> Stop a previously started listener.
### set_env(Ref, Name, Value) -> ok
> Types:
> * Ref = ranch:ref()
> * Name = atom()
> * Value = any()
>
> Set or update an environment value for an already running listener.
> This will take effect on all subsequent connections.
See also
--------
The [Ranch guide](http://ninenines.eu/docs/en/ranch/HEAD/guide)
provides detailed information about how listeners work.

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The Cowboy Application
======================
Small, fast, modular HTTP server.
Dependencies
------------
The `cowboy` application uses the Erlang applications `ranch`
for listening and accepting TCP connections, and `crypto`
for establishing Websocket connections. These dependencies must
be loaded for the `cowboy` application to work. In an embedded
environment this means that they need to be started with the
`application:start/{1,2}` function before the `cowboy`
application is started.
The `cowboy` application also uses the Erlang applications
`public_key` and `ssl` when listening for HTTPS connections.
These are started automatically if they weren't before.
Environment
-----------
The `cowboy` application does not define any application
environment configuration parameters.

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cowboy_handler
==============
The `cowboy_handler` middleware executes the handler passed
through the environment values `handler` and `handler_opts`,
and add the result of this execution to the environment as
the value `result`, indicating that the request has been
handled and received a response.
Environment input:
* handler = module()
* handler_opts = any()
Environment output:
* result = ok
Types
-----
None.
Exports
-------
None.

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cowboy_http_handler
===================
The `cowboy_http_handler` behaviour defines the interface used
by plain HTTP handlers.
Unless noted otherwise, the callbacks will be executed sequentially.
Types
-----
None.
Callbacks
---------
### init({TransportName, ProtocolName}, Req, Opts)
-> {ok, Req, State} | {shutdown, Req, State}
> Types:
> * TransportName = tcp | ssl | atom()
> * ProtocolName = http | atom()
> * Req = cowboy_req:req()
> * Opts = any()
> * State = any()
>
> Initialize the state for this request.
>
> The `shutdown` return value can be used to skip the `handle/2`
> call entirely.
### handle(Req, State) -> {ok, Req, State}
> Types:
> * Req = cowboy_req:req()
> * State = any()
>
> Handle the request.
>
> This callback is where the request is handled and a response
> should be sent. If a response is not sent, Cowboy will send
> a `204 No Content` response automatically.
### terminate(Reason, Req, State) -> ok
> Types:
> * Reason = {normal, shutdown} | {error, atom()}
> * Req = cowboy_req:req()
> * State = any()
>
> Perform any necessary cleanup of the state.
>
> This callback should release any resource currently in use,
> clear any active timer and reset the process to its original
> state, as it might be reused for future requests sent on the
> same connection. Typical plain HTTP handlers rarely need to
> use it.

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cowboy_loop_handler
===================
The `cowboy_loop_handler` behaviour defines the interface used
by HTTP handlers that do not send a response directly, instead
requiring a receive loop to process Erlang messages.
This interface is best fit for long-polling types of requests.
The `init/3` callback will always be called, followed by zero
or more calls to `info/3`. The `terminate/3` will always be
called last.
Types
-----
None.
Callbacks
---------
### init({TransportName, ProtocolName}, Req, Opts)
-> {loop, Req, State}
| {loop, Req, State, hibernate}
| {loop, Req, State, Timeout}
| {loop, Req, State, Timeout, hibernate}
| {shutdown, Req, State}
> Types:
> * TransportName = tcp | ssl | atom()
> * ProtocolName = http | atom()
> * Req = cowboy_req:req()
> * Opts = any()
> * State = any()
> * Timeout = timeout()
>
> Initialize the state for this request.
>
> This callback will typically be used to register this process
> to an event manager or a message queue in order to receive
> the messages the handler wants to process.
>
> The receive loop will run for a duration of up to `Timeout`
> milliseconds after it last received data from the socket,
> at which point it will stop and send a `204 No Content` reply.
> By default this value is set to `infinity`. It is recommended
> to either set this value or ensure by any other mechanism
> that the handler will be closed after a certain period of
> inactivity.
>
> The `hibernate` option will hibernate the process until it
> starts receiving messages.
>
> The `shutdown` return value can be used to skip the receive
> loop entirely.
### info(Info, Req, State) -> {ok, Req, State} | {loop, Req, State}
| {loop, Req, State, hibernate}
> Types:
> * Info = any()
> * Req = cowboy_req:req()
> * State = any()
>
> Handle the Erlang message received.
>
> This function will be called every time an Erlang message
> has been received. The message can be any Erlang term.
>
> The `ok` return value can be used to stop the receive loop,
> typically because a response has been sent.
>
> The `hibernate` option will hibernate the process until
> it receives another message.
### terminate(Reason, Req, State) -> ok
> Types:
> * Reason = {normal, shutdown} | {normal, timeout} | {error, closed} | {error, overflow} | {error, atom()}
> * Req = cowboy_req:req()
> * State = any()
>
> Perform any necessary cleanup of the state.
>
> This callback will typically unregister from any event manager
> or message queue it registered to in `init/3`.
>
> This callback should release any resource currently in use,
> clear any active timer and reset the process to its original
> state, as it might be reused for future requests sent on the
> same connection.

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cowboy_middleware
=================
The `cowboy_middleware` behaviour defines the interface used
by Cowboy middleware modules.
Middlewares process the request sequentially in the order they
are configured.
Types
-----
### env() = [{atom(), any()}]
> The environment variable.
>
> One is created for every request. It is passed to each
> middleware module executed and subsequently returned,
> optionally with its contents modified.
Callbacks
---------
### execute(Req, Env)
-> {ok, Req, Env}
| {suspend, Module, Function, Args}
| {halt, Req}
| {error, StatusCode, Req}
> Types:
> * Req = cowboy_req:req()
> * Env = env()
> * Module = module()
> * Function = atom()
> * Args = [any()]
> * StatusCode = cowboy:http_status()
>
> Execute the middleware.
>
> The `ok` return value indicates that everything went well
> and that Cowboy should continue processing the request. A
> response may or may not have been sent.
>
> The `suspend` return value will hibernate the process until
> an Erlang message is received. Note that when resuming, any
> previous stacktrace information will be gone.
>
> The `halt` return value stops Cowboy from doing any further
> processing of the request, even if there are middlewares
> that haven't been executed yet. The connection may be left
> open to receive more requests from the client.
>
> The `error` return value sends an error response identified
> by the `StatusCode` and then proceeds to terminate the
> connection. Middlewares that haven't been executed yet
> will not be called.

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cowboy_protocol
===============
The `cowboy_protocol` module implements HTTP/1.1 and HTTP/1.0
as a Ranch protocol.
Types
-----
### opts() = [{compress, boolean()}
| {env, cowboy_middleware:env()}
| {max_empty_lines, non_neg_integer()}
| {max_header_name_length, non_neg_integer()}
| {max_header_value_length, non_neg_integer()}
| {max_headers, non_neg_integer()}
| {max_keepalive, non_neg_integer()}
| {max_request_line_length, non_neg_integer()}
| {middlewares, [module()]}
| {onrequest, cowboy:onrequest_fun()}
| {onresponse, cowboy:onresponse_fun()}
| {timeout, timeout()}]
> Configuration for the HTTP protocol handler.
>
> This configuration is passed to Cowboy when starting listeners
> using `cowboy:start_http/4` or `cowboy:start_https/4` functions.
>
> It can be updated without restarting listeners using the
> Ranch functions `ranch:get_protocol_options/1` and
> `ranch:set_protocol_options/2`.
Option descriptions
-------------------
The default value is given next to the option name.
- compress (false)
- When enabled, Cowboy will attempt to compress the response body.
- env ([{listener, Ref}])
- Initial middleware environment.
- max_empty_lines (5)
- Maximum number of empty lines before a request.
- max_header_name_length (64)
- Maximum length of header names.
- max_header_value_length (4096)
- Maximum length of header values.
- max_headers (100)
- Maximum number of headers allowed per request.
- max_keepalive (100)
- Maximum number of requests allowed per connection.
- max_request_line_length (4096)
- Maximum length of the request line.
- middlewares ([cowboy_router, cowboy_handler])
- List of middlewares to execute for every requests.
- onrequest (undefined)
- Fun called every time a request is received.
- onresponse (undefined)
- Fun called every time a response is sent.
- timeout (5000)
- Time in ms with no requests before Cowboy closes the connection.
Exports
-------
None.

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cowboy_req
==========
The `cowboy_req` module provides functions to access, manipulate
and respond to requests.
The functions in this module follow patterns for their return types,
based on the kind of function.
* access: `{Value, Req}`
* action: `{Result, Req} | {Result, Value, Req} | {error, atom()}`
* modification: `Req`
* question: `boolean()`
The only exception is the `chunk/2` function which may return `ok`.
Whenever `Req` is returned, you must use this returned value and
ignore any previous you may have had. This value contains various
state informations which are necessary for Cowboy to do some lazy
evaluation or cache results where appropriate.
Types
-----
### cookie_opts() = [{max_age, non_neg_integer()}
| {domain, binary()} | {path, binary()}
| {secure, boolean()} | {http_only, boolean()}]
> Cookie options.
### req() - opaque to the user
> The `Req` object.
>
> All functions in this module receive a `Req` as argument,
> and most of them return a new object labelled `Req2` in
> the function descriptions below.
Request related exports
-----------------------
### binding(Name, Req) -> binding(Name, Req, undefined)
### binding(Name, Req, Default) -> {Value, Req2}
> Types:
> * Name = atom()
> * Default = any()
> * Value = binary() | Default
>
> Return the value for the given binding.
### bindings(Req) -> {[{Name, Value}], Req2}
> Types:
> * Name = atom()
> * Value = binary()
>
> Return all bindings.
### cookie(Name, Req) -> cookie(Name, Req, undefined)
### cookie(Name, Req, Default) -> {Value, Req2}
> Types:
> * Name = binary()
> * Default = any()
> * Value = binary() | Default
>
> Return the value for the given cookie.
>
> Cookie names are case sensitive.
### cookies(Req) -> {[{Name, Value}], Req2}
> Types:
> * Name = binary()
> * Value = binary()
>
> Return all cookies.
### header(Name, Req) -> header(Name, Req, undefined)
### header(Name, Req, Default) -> {Value, Req2}
> Types:
> * Name = binary()
> * Default = any()
> * Value = binary() | Default
>
> Return the value for the given header.
>
> While header names are case insensitive, this function expects
> the name to be a lowercase binary.
### headers(Req) -> {Headers, Req2}
> Types:
> * Headers = cowboy:http_headers()
>
> Return all headers.
### host(Req) -> {Host, Req2}
> Types:
> * Host = binary()
>
> Return the requested host.
### host_info(Req) -> {HostInfo, Req2}
> Types:
> * HostInfo = cowboy_router:tokens() | undefined
>
> Return the extra tokens from matching against `...` during routing.
### host_url(Req) -> {HostURL, Req2}
> Types:
> * HostURL = binary() | undefined
>
> Return the requested URL excluding the path component.
>
> This function will always return `undefined` until the
> `cowboy_router` middleware has been executed. This includes
> the `onrequest` hook.
### meta(Name, Req) -> meta(Name, Req, undefined)
### meta(Name, Req, Default) -> {Value, Req2}
> Types:
> * Name = atom()
> * Default = any()
> * Value = any()
>
> Return metadata about the request.
### method(Req) -> {Method, Req2}
> Types:
> * Method = binary()
>
> Return the method.
>
> Methods are case sensitive. Standard methods are always uppercase.
### parse_header(Name, Req) ->
### parse_header(Name, Req, Default) -> {ok, ParsedValue, Req2}
| {undefined, Value, Req2} | {error, badarg}
> Types:
> * Name = binary()
> * Default = any()
> * ParsedValue - see below
> * Value = any()
>
> Parse the given header.
>
> While header names are case insensitive, this function expects
> the name to be a lowercase binary.
>
> The `parse_header/2` function will call `parser_header/3` with a
> different default value depending on the header being parsed. The
> following table summarizes the default values used.
>
> | Header name | Default value |
> | ----------------- | ------------------ |
> | transfer-encoding | `[<<"identity">>]` |
> | Any other header | `undefined` |
>
> The parsed value differs depending on the header being parsed. The
> following table summarizes the different types returned.
>
> | Header name | Type |
> | ---------------------- | ------------------------------------------------- |
> | accept | `[{{Type, SubType, Params}, Quality, AcceptExt}]` |
> | accept-charset | `[{Charset, Quality}]` |
> | accept-encoding | `[{Encoding, Quality}]` |
> | accept-language | `[{LanguageTag, Quality}]` |
> | authorization | `{AuthType, Credentials}` |
> | content-length | `non_neg_integer()` |
> | content-type | `{Type, SubType, Params}` |
> | cookie | `[{binary(), binary()}]` |
> | expect | `[Expect | {Expect, ExpectValue, Params}]` |
> | if-match | `'*' | [{weak | strong, OpaqueTag}]` |
> | if-modified-since | `calendar:datetime()` |
> | if-none-match | `'*' | [{weak | strong, OpaqueTag}]` |
> | if-unmodified-since | `calendar:datetime()` |
> | range | `{Unit, [Range]}` |
> | sec-websocket-protocol | `[binary()]` |
> | transfer-encoding | `[binary()]` |
> | upgrade | `[binary()]` |
> | x-forwarded-for | `[binary()]` |
>
> Types for the above table:
> * Type = SubType = Charset = Encoding = LanguageTag = binary()
> * AuthType = Expect = OpaqueTag = Unit = binary()
> * Params = [{binary(), binary()}]
> * Quality = 0..1000
> * AcceptExt = [{binary(), binary()} | binary()]
> * Credentials - see below
> * Range = {non_neg_integer(), non_neg_integer() | infinity} | neg_integer()
>
> The cookie names and values, the values of the sec-websocket-protocol
> and x-forwarded-for headers, the values in `AcceptExt` and `Params`,
> the authorization `Credentials`, the `ExpectValue` and `OpaqueTag`
> are case sensitive. All other values are case insensitive and
> will be returned as lowercase.
>
> The headers accept, accept-encoding and cookie headers can return
> an empty list. Others will return `{error, badarg}` if the header
> value is empty.
>
> The authorization header parsing code currently only supports basic
> HTTP authentication. The `Credentials` type is thus `{Username, Password}`
> with `Username` and `Password` being `binary()`.
>
> The range header value `Range` can take three forms:
> * `{From, To}`: from `From` to `To` units
> * `{From, infinity}`: everything after `From` units
> * `-Final`: the final `Final` units
>
> An `undefined` tuple will be returned if Cowboy doesn't know how
> to parse the requested header.
### path(Req) -> {Path, Req2}
> Types:
> * Path = binary()
>
> Return the requested path.
### path_info(Req) -> {PathInfo, Req2}
> Types:
> * PathInfo = cowboy_router:tokens() | undefined
>
> Return the extra tokens from matching against `...` during routing.
### peer(Req) -> {Peer, Req2}
> Types:
> * Peer = {inet:ip_address(), inet:port_number()}
>
> Return the client's IP address and port number.
### port(Req) -> {Port, Req2}
> Types:
> * Port = inet:port_number()
>
> Return the request's port.
>
> The port returned by this function is obtained by parsing
> the host header. It may be different than the actual port
> the client used to connect to the Cowboy server.
### qs(Req) -> {QueryString, Req2}
> Types:
> * QueryString = binary()
>
> Return the request's query string.
### qs_val(Name, Req) -> qs_val(Name, Req, undefined)
### qs_val(Name, Req, Default) -> {Value, Req2}
> Types:
> * Name = binary()
> * Default = any()
> * Value = binary() | true
>
> Return a value from the request's query string.
>
> The value `true` will be returned when the name was found
> in the query string without an associated value.
### qs_vals(Req) -> {[{Name, Value}], Req2}
> Types:
> * Name = binary()
> * Value = binary() | true
>
> Return the request's query string as a list of tuples.
>
> The value `true` will be returned when the name was found
> in the query string without an associated value.
### set_meta(Name, Value, Req) -> Req2
> Types:
> * Name = atom()
> * Value = any()
>
> Set metadata about the request.
>
> An existing value will be overwritten.
### url(Req) -> {URL, Req2}
> Types:
> * URL = binary() | undefined
>
> Return the requested URL.
>
> This function will always return `undefined` until the
> `cowboy_router` middleware has been executed. This includes
> the `onrequest` hook.
### version(Req) -> {Version, Req2}
> Types:
> * Version = cowboy:http_version()
>
> Return the HTTP version used for this request.
Request body related exports
----------------------------
### body(Req) -> body(8000000, Req)
### body(MaxLength, Req) -> {ok, Data, Req2} | {error, Reason}
> Types:
> * MaxLength = non_neg_integer() | infinity
> * Data = binary()
> * Reason = chunked | badlength | atom()
>
> Return the request body.
>
> This function will return `{error, chunked}` if the request
> body was sent using the chunked transfer-encoding. It will
> also return `{error, badlength}` if the length of the body
> exceeds the given `MaxLength`, which is 8MB by default.
### body_length(Req) -> {Length, Req2}
> Types:
> * Length = non_neg_integer() | undefined
>
> Return the length of the request body.
>
> The length will only be returned if the request does not
> use any transfer-encoding and if the content-length header
> is present.
### body_qs(Req) -> body_qs(16000, Req)
### body_qs(MaxLength, Req) -> {ok, [{Name, Value}], Req2} | {error, Reason}
> Types:
> * MaxLength = non_neg_integer() | infinity
> * Name = binary()
> * Value = binary() | true
> * Reason = chunked | badlength | atom()
>
> Return the request body as a list of tuples.
>
> This function will parse the body assuming the content-type
> application/x-www-form-urlencoded, commonly used for the
> query string.
>
> This function will return `{error, chunked}` if the request
> body was sent using the chunked transfer-encoding. It will
> also return `{error, badlength}` if the length of the body
> exceeds the given `MaxLength`, which is 16KB by default.
### has_body(Req) -> boolean()
> Return whether the request has a body.
### init_stream(TransferDecode, TransferState, ContentDecode, Req) -> {ok, Req2}
> Types:
> * TransferDecode = fun((Encoded, TransferState) -> OK | More | Done | {error, Reason})
> * Encoded = Decoded = Rest = binary()
> * TransferState = any()
> * OK = {ok, Decoded, Rest, TransferState}
> * More = more | {more, Length, Decoded, TransferState}
> * Done = {done, TotalLength, Rest} | {done, Decoded, TotalLength, Rest}
> * Length = TotalLength = non_neg_integer()
> * ContentDecode = fun((Encoded) -> {ok, Decoded} | {error, Reason})
> * Reason = atom()
>
> Initialize streaming of the request body.
>
> This function can be used to specify what function to use
> for decoding the request body, generally specified in the
> transfer-encoding and content-encoding request headers.
>
> Cowboy will properly handle chunked transfer-encoding by
> default. You do not need to call this function if you do
> not need to decode other encodings, `stream_body/{1,2}`
> will perform all the required initialization when it is
> called the first time.
### skip_body(Req) -> {ok, Req2} | {error, Reason}
> Types:
> * Reason = atom()
>
> Skip the request body.
>
> This function will skip the body even if it was partially
> read before.
### stream_body(Req) -> stream_body(1000000, Req)
### stream_body(MaxSegmentSize, Req) -> {ok, Data, Req2}
| {done, Req2} | {error, Reason}
> Types:
> * MaxSegmentSize = non_neg_integer()
> * Data = binary()
> * Reason = atom()
>
> Stream the request body.
>
> This function will return a segment of the request body
> with a size of up to `MaxSegmentSize`, or 1MB by default.
> This function can be called repeatedly until a `done` tuple
> is returned, indicating the body has been fully received.
>
> Cowboy will properly handle chunked transfer-encoding by
> default. If any other transfer-encoding or content-encoding
> has been used for the request, custom decoding functions
> can be used. They must be specified using `init_stream/4`.
>
> After the body has been streamed fully, Cowboy will remove
> the transfer-encoding header from the `Req` object, and add
> the content-length header if it wasn't already there.
Response related exports
------------------------
### chunk(Data, Req) -> ok | {error, Reason}
> Types:
> * Data = iodata()
> * Reason = atom()
>
> Send a chunk of data.
>
> This function should be called as many times as needed
> to send data chunks after calling `chunked_reply/{2,3}`.
>
> When the method is HEAD, no data will actually be sent.
>
> If the request uses HTTP/1.0, the data is sent directly
> without wrapping it in an HTTP/1.1 chunk, providing
> compatibility with older clients.
### chunked_reply(StatusCode, Req) -> chunked_reply(StatusCode, [], Req)
### chunked_reply(StatusCode, Headers, Req) -> {ok, Req2}
> Types:
> * StatusCode = cowboy:http_status()
> * Headers = cowboy:http_headers()
>
> Send a response using chunked transfer-encoding.
>
> This function effectively sends the response status line
> and headers to the client.
>
> This function will not send any body set previously. After
> this call the handler must use the `chunk/2` function
> repeatedly to send the body in as many chunks as needed.
>
> If the request uses HTTP/1.0, the data is sent directly
> without wrapping it in an HTTP/1.1 chunk, providing
> compatibility with older clients.
### delete_resp_header(Name, Req) -> Req2
> Types:
> * Name = binary()
>
> Delete the given response header.
>
> While header names are case insensitive, this function expects
> the name to be a lowercase binary.
### has_resp_body(Req) -> boolean()
> Return whether a response body has been set.
>
> This function will return false if a response body has
> been set with a length of 0.
### has_resp_header(Name, Req) -> boolean()
> Types:
> * Name = binary()
>
> Return whether the given response header has been set.
>
> While header names are case insensitive, this function expects
> the name to be a lowercase binary.
### reply(StatusCode, Req) -> reply(StatusCode, [], Req)
### reply(StatusCode, Headers, Req) - see below
### reply(StatusCode, Headers, Body, Req) -> {ok, Req2}
> Types:
> * StatusCode = cowboy:http_status()
> * Headers = cowboy:http_headers()
> * Body = iodata()
>
> Send a response.
>
> This function effectively sends the response status line,
> headers and body to the client, in a single send function
> call.
>
> The `reply/2` and `reply/3` functions will send the body
> set previously, if any. The `reply/4` function overrides
> any body set previously and sends `Body` instead.
>
> If a body function was set, and `reply/2` or `reply/3` was
> used, it will be called before returning.
>
> No more data can be sent to the client after this function
> returns.
### set_resp_body(Body, Req) -> Req2
> Types:
> * Body = iodata()
>
> Set a response body.
>
> This body will not be sent if `chunked_reply/{2,3}` or
> `reply/4` is used, as they override it.
### set_resp_body_fun(Fun, Req) -> Req2
### set_resp_body_fun(Length, Fun, Req) -> Req2
> Types:
> * Fun = fun((Socket, Transport) -> ok)
> * Socket = inet:socket()
> * Transport = module()
> * Length = non_neg_integer()
>
> Set a fun for sending the response body.
>
> If a `Length` is provided, it will be sent in the
> content-length header in the response. It is recommended
> to set the length if it can be known in advance.
>
> This function will only be called if the response is sent
> using the `reply/2` or `reply/3` function.
>
> The fun will receive the Ranch `Socket` and `Transport` as
> arguments. Only send and sendfile operations are supported.
### set_resp_body_fun(chunked, Fun, Req) -> Req2
> Types:
> * Fun = fun((ChunkFun) -> ok)
> * ChunkFun = fun((iodata()) -> ok | {error, atom()})
>
> Set a fun for sending the response body using chunked transfer-encoding.
>
> This function will only be called if the response is sent
> using the `reply/2` or `reply/3` function.
>
> The fun will receive another fun as argument. This fun is to
> be used to send chunks in a similar way to the `chunk/2` function,
> except the fun only takes one argument, the data to be sent in
> the chunk.
### set_resp_cookie(Name, Value, Opts, Req) -> Req2
> Types:
> * Name = iodata()
> * Value = iodata()
> * Opts = cookie_opts()
>
> Set a cookie in the response.
>
> Cookie names are case sensitive.
### set_resp_header(Name, Value, Req) -> Req2
> Types:
> * Name = binary()
> * Value = iodata()
>
> Set a response header.
>
> You should use `set_resp_cookie/4` instead of this function
> to set cookies.
Misc. exports
-------------
### compact(Req) -> Req2
> Remove any non-essential data from the `Req` object.
>
> Long-lived connections usually only need to manipulate the
> `Req` object at initialization. Compacting allows saving up
> memory by discarding extraneous information.

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cowboy_rest
===========
The `cowboy_rest` module implements REST semantics on top of
the HTTP protocol.
This module cannot be described as a behaviour due to most of
the callbacks it defines being optional. It has the same
semantics as a behaviour otherwise.
The only mandatory callback is `init/3`, needed to perform
the protocol upgrade.
Types
-----
None.
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`.
Callbacks
---------
### init({TransportName, ProtocolName}, Req, Opts)
-> {upgrade, protocol, cowboy_rest}
| {upgrade, protocol, cowboy_rest, Req, Opts}
> Types:
> * TransportName = tcp | ssl | atom()
> * ProtocolName = http | atom()
> * Req = cowboy_req:req()
> * Opts = any()
>
> Upgrade the protocol to `cowboy_rest`.
>
> This is the only mandatory callback.
### rest_init(Req, Opts) -> {ok, Req, State}
> Types:
> * Req = cowboy_req:req()
> * Opts = any()
> * State = any()
>
> Initialize the state for this request.
### rest_terminate(Req, State) -> ok
> Types:
> * Req = cowboy_req:req()
> * State = any()
>
> Perform any necessary cleanup of the state.
>
> This callback should release any resource currently in use,
> clear any active timer and reset the process to its original
> state, as it might be reused for future requests sent on the
> same connection.
### Callback(Req, State) -> {Value, Req, State} | {halt, 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 `halt` 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.
>
> 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` or
> `{true, URL}` may be returned. If an URL is provided, the
> response will redirect the client to the location of the
> 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
> * Default value: [{{<<"text">>, <<"html">>, '*'}, to_html}]
>
> Types:
> * Value = [{binary() | {Type, SubType, Params}, ProvideResource}]
> * Type = SubType = binary()
> * Params = '*' | [{binary(), binary()}]
> * AcceptResource = 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.
>
> The `ProvideResource` value is the name of the callback that will
> be called if the content-type matches. It is defined as follow.
>
> * 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() | 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()
> * Default value: undefined
>
> Return the entity tag of the resource.
>
> This value will be sent as the value of the etag header.
### 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_content_type
> * Methods: all
> * Value type: boolean()
> * Default value: true
>
> Return whether the content-type is known.
>
> This function determines if the server understands the
> content-type, regardless of its use by the resource.
### 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.

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cowboy_router
=============
The `cowboy_router` middleware maps the requested host and
path to the handler to be used for processing the request.
It uses the dispatch rules compiled from the routes given
to the `compile/1` function for this purpose. It adds the
handler name and options to the environment as the values
`handler` and `handler_opts` respectively.
Environment input:
* dispatch = dispatch_rules()
Environment output:
* handler = module()
* handler_opts = any()
Types
-----
### bindings() = [{atom(), binary()}]
> List of bindings found during routing.
### constraints() = [IntConstraint | FunConstraint]
> Types:
> * IntConstraint = {atom(), int}
> * FunConstraint = {atom(), function, Fun}
> * Fun = fun((binary()) -> true | {true, any()} | false)
>
> List of constraints to apply to the bindings.
>
> The int constraint will convert the binding to an integer.
> The fun constraint allows writing custom code for checking
> the bindings. Returning a new value from that fun allows
> replacing the current binding with a new value.
### dispatch_rules() - opaque to the user
> Rules for dispatching request used by Cowboy.
### routes() = [{Host, Paths} | {Host, constraints(), Paths}]
> Types:
> * Host = Path = '_' | iodata()
> * Paths = [{Path, Handler, Opts} | {Path, constraints(), Handler, Opts}]
> * Handler = module()
> * Opts = any()
>
> Human readable list of routes mapping hosts and paths to handlers.
>
> The syntax for routes is defined in the user guide.
### tokens() = [binary()]
> List of host_info and path_info tokens found during routing.
Exports
-------
### compile(Routes) -> Dispatch
> Types:
> * Routes = routes()
> * Dispatch = dispatch_rules()
>
> Compile the routes for use by Cowboy.

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cowboy_sub_protocol
===================
The `cowboy_sub_protocol` behaviour defines the interface used
by modules that implement a protocol on top of HTTP.
Types
-----
None.
Callbacks
---------
### upgrade(Req, Env, Handler, Opts)
-> {ok, Req, Env}
| {suspend, Module, Function, Args}
| {halt, Req}
| {error, StatusCode, Req}
> Types:
> * Req = cowboy_req:req()
> * Env = env()
> * Handler = module()
> * Opts = any()
> * Module = module()
> * Function = atom()
> * Args = [any()]
> * StatusCode = cowboy:http_status()
>
> Upgrade the protocol.
>
> Please refer to the `cowboy_middleware` manual for a
> description of the return values.

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cowboy_websocket
================
The `cowboy_websocket` module implements the Websocket protocol.
The callbacks for websocket handlers are defined in the manual
for the `cowboy_websocket_handler` behaviour.
Types
-----
### close_code() = 1000..4999
> Reason for closing the connection.
### frame() = close | ping | pong
| {text | binary | close | ping | pong, iodata()}
| {close, close_code(), iodata()}
> Frames that can be sent to the client.
Meta values
-----------
### websocket_version
> Type: 7 | 8 | 13
>
> The version of the Websocket protocol being used.
Exports
-------
None.

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cowboy_websocket_handler
========================
The `cowboy_websocket_handler` behaviour defines the interface used
by Websocket handlers.
The `init/3` and `websocket_init/3` callbacks will always be called,
followed by zero or more calls to `websocket_handle/3` and
`websocket_info/3`. The `websocket_terminate/3` will always
be called last.
Types
-----
None.
Callbacks
---------
### init({TransportName, ProtocolName}, Req, Opts)
-> {upgrade, protocol, cowboy_websocket}
| {upgrade, protocol, cowboy_websocket, Req, Opts}
> Types:
> * TransportName = tcp | ssl | atom()
> * ProtocolName = http | atom()
> * Req = cowboy_req:req()
> * Opts = any()
>
> Upgrade the protocol to `cowboy_websocket`.
### websocket_init(TransportName, Req, Opts)
-> {ok, Req, State}
| {ok, Req, State, hibernate}
| {ok, Req, State, Timeout}
| {ok, Req, State, Timeout, hibernate}
| {shutdown, Req}
> Types:
> * TransportName = tcp | ssl | atom()
> * Req = cowboy_req:req()
> * Opts = any()
> * State = any()
> * Timeout = timeout()
>
> Initialize the state for this session.
>
> This function is called before the upgrade to Websocket occurs.
> It can be used to negotiate Websocket protocol extensions
> with the client. It will typically be used to register this process
> to an event manager or a message queue in order to receive
> the messages the handler wants to process.
>
> The connection will stay up for a duration of up to `Timeout`
> milliseconds after it last received data from the socket,
> at which point it will stop and close the connection.
> By default this value is set to `infinity`. It is recommended
> to either set this value or ensure by any other mechanism
> that the handler will be closed after a certain period of
> inactivity.
>
> The `hibernate` option will hibernate the process until it
> starts receiving either data from the Websocket connection
> or Erlang messages.
>
> The `shutdown` return value can be used to close the connection
> before upgrading to Websocket.
### websocket_handle(InFrame, Req, State)
-> {ok, Req, State}
| {ok, Req, State, hibernate}
| {reply, OutFrame | [OutFrame], Req, State}
| {reply, OutFrame | [OutFrame], Req, State, hibernate}
| {shutdown, Req, State}
> Types:
> * InFrame = {text | binary | ping | pong, binary()}
> * Req = cowboy_req:req()
> * State = any()
> * OutFrame = cowboy_websocket:frame()
>
> Handle the data received from the Websocket connection.
>
> This function will be called every time data is received
> from the Websocket connection.
>
> The `shutdown` return value can be used to close the
> connection. A close reply will also result in the connection
> being closed.
>
> The `hibernate` option will hibernate the process until
> it receives new data from the Websocket connection or an
> Erlang message.
### websocket_info(Info, Req, State)
-> {ok, Req, State}
| {ok, Req, State, hibernate}
| {reply, OutFrame | [OutFrame], Req, State}
| {reply, OutFrame | [OutFrame], Req, State, hibernate}
| {shutdown, Req, State}
> Types:
> * Info = any()
> * Req = cowboy_req:req()
> * State = any()
> * OutFrame = cowboy_websocket:frame()
>
> Handle the Erlang message received.
>
> This function will be called every time an Erlang message
> has been received. The message can be any Erlang term.
>
> The `shutdown` return value can be used to close the
> connection. A close reply will also result in the connection
> being closed.
>
> The `hibernate` option will hibernate the process until
> it receives another message or new data from the Websocket
> connection.
### websocket_terminate(Reason, Req, State) -> ok
> Types:
> * Reason = {normal, shutdown | timeout} | {remote, closed} | {remote, cowboy_websocket:close_code(), binary()} | {error, badencoding | badframe | closed | atom()}
> * Req = cowboy_req:req()
> * State = any()
>
> Perform any necessary cleanup of the state.
>
> The connection will be closed and the process stopped right
> after this call.

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Cowboy Function Reference
=========================
The function reference documents the public interface of Cowboy.
* [The Cowboy Application](cowboy_app.md)
* [cowboy](cowboy.md)
* [cowboy_handler](cowboy_handler.md)
* [cowboy_http_handler](cowboy_http_handler.md)
* [cowboy_loop_handler](cowboy_loop_handler.md)
* [cowboy_middleware](cowboy_middleware.md)
* [cowboy_protocol](cowboy_protocol.md)
* [cowboy_req](cowboy_req.md)
* [cowboy_rest](cowboy_rest.md)
* [cowboy_router](cowboy_router.md)
* [cowboy_sub_protocol](cowboy_sub_protocol.md)
* [cowboy_websocket](cowboy_websocket.md)
* [cowboy_websocket_handler](cowboy_websocket_handler.md)