= cowboy_websocket(3)

== Name

cowboy_websocket - Websocket

== Description

The module `cowboy_websocket` implements Websocket
as a Ranch protocol. It also defines a callback interface
for handling Websocket connections.

== Callbacks

Websocket handlers must implement the following callback
interface:

[source,erlang]
----
init(Req, State)
    -> {cowboy_websocket, Req, State}
     | {cowboy_websocket, Req, State, Opts}

websocket_init(State)            -> CallResult  %% optional
websocket_handle(InFrame, State) -> CallResult
websocket_info(Info, State)      -> CallResult

terminate(Reason, undefined, State) -> ok       %% optional

Req        :: cowboy_req:req()
State      :: any()
Opts       :: cowboy_websocket:opts()
InFrame    :: {text | binary | ping | pong, binary()}
OutFrame   :: cow_ws:frame()
Info       :: any()

CallResult :: {ok, State}
            | {ok, State, hibernate}
            | {reply, OutFrame | [OutFrame], State}
            | {reply, OutFrame | [OutFrame], State, hibernate}
            | {stop, State}

Reason     :: normal | stop | timeout
            | remote | {remote, cow_ws:close_code(), binary()}
            | {error, badencoding | badframe | closed | atom()}
            | {crash, error | exit | throw, any()}
----

The `init/2` callback is common to all handlers. To upgrade
the connection to Websocket, it must return `cowboy_websocket`
as the first element of the tuple.

Any operation requiring the HTTP request must be done in the
`init/2` function, as the Req object will not be available
after it returns. Websocket sub-protocol selection should
therefore be done in this function.

The optional `websocket_init/1` callback will be called once
the connection has been upgraded to Websocket. It can be used
to perform any required initialization of the handler.

Note that the `init/2` function does not run in the same
process as the Websocket callbacks. Any Websocket-specific
initialization must be done in `websocket_init/1`.

The `websocket_handle/2` callback will be called for every
frame received. The `websocket_info/2` callback will be
called for every Erlang message received.

All three Websocket callbacks may send one or more frames
back to the client (by returning a `reply` tuple) or terminate
the connection (by sending a `close` frame or returning a `stop`
tuple).

The optional `terminate/3` callback will ultimately be called
with the reason for the termination of the connection. This
callback is common to all handlers. Note that Websocket has
no concept of requests so it sets the second argument to
undefined.

Cowboy will terminate the process right after closing the
Websocket connection. This means that there is no need to
perform any cleanup in the `terminate/3` callback.

The following terminate reasons are defined for Websocket
connections:

normal::
    The connection was closed normally before establishing a Websocket
    connection. This typically happens if an `ok` tuple is returned
    from the `init/2` callback.

remote::
    The remote endpoint closed the connection without giving any
    further details.

{remote, Code, Payload}::
    The remote endpoint closed the connection with the given
    `Code` and `Payload` as the reason.

stop::
    The handler requested to close the connection, either by returning
    a `stop` tuple or by sending a `close` frame.

timeout::
    The connection has been closed due to inactivity. The timeout
    value can be configured from `init/2`.

{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.

{error, badencoding}::
    A text frame was sent by the client with invalid encoding. All
    text frames must be valid UTF-8.

{error, badframe}::
    A protocol error has been detected.

{error, closed}::
    The socket has been closed brutally without a close frame being
    received first.

{error, Reason}::
    A socket error ocurred.

== Types

=== opts()

[source,erlang]
----
opts() :: #{
    compress => boolean(),
    idle_timeout => timeout(),
    req_filter => fun((cowboy_req:req()) -> map())
}
----

Websocket handler options.

This configuration is passed to Cowboy from the `init/2`
function:

[source,erlang]
----
init(Req, State) ->
    Opts = #{compress => true},
    {cowboy_websocket, Req, State, Opts}.
----

The default value is given next to the option name:

compress (false)::
    Whether to enable the Websocket frame compression
    extension. Frames will only be compressed for the
    clients that support this extension.

idle_timeout (60000)::
    Time in milliseconds that Cowboy will keep the
    connection open without receiving anything from
    the client.

req_filter::
    A function applied to the Req to compact it and
    only keep required information. The Req is only
    given back in the `terminate/3` callback. By default
    it keeps the method, version, URI components and peer
    information.

== Changelog

* *2.0*: The Req object is no longer passed to Websocket callbacks.
* *2.0*: The callback `websocket_terminate/3` was removed in favor of `terminate/3`.
* *1.0*: Protocol introduced.

== See also

link:man:cowboy(7)[cowboy(7)],
link:man:cowboy_handler(3)[cowboy_handler(3)],
link:man:cowboy_http(3)[cowboy_http(3)],
link:man:cowboy_http2(3)[cowboy_http2(3)]
