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== Websocket handlers

Websocket handlers provide an interface for upgrading HTTP/1.1
connections to Websocket and sending or receiving frames on
the Websocket connection.

As Websocket connections are established through the HTTP/1.1
upgrade mechanism, Websocket handlers need to be able to first
receive the HTTP request for the upgrade, before switching to
Websocket and taking over the connection. They can then receive
or send Websocket frames, handle incoming Erlang messages or
close the connection.

=== Upgrade

The `init/2` callback is called when the request is received.
To establish a Websocket connection, you must switch to the
`cowboy_websocket` module:

init(Req, State) ->
    {cowboy_websocket, Req, State}.

Cowboy will perform the Websocket handshake immediately. Note
that the handshake will fail if the client did not request an
upgrade to Websocket.

The Req object becomes unavailable after this function returns.
Any information required for proper execution of the Websocket
handler must be saved in the state.

=== Subprotocol

The client may provide a list of Websocket subprotocols it
supports in the sec-websocket-protocol header. The server *must*
select one of them and send it back to the client or the
handshake will fail.

For example, a client could understand both STOMP and MQTT over
Websocket, and provide the header:

sec-websocket-protocol: v12.stomp, mqtt

If the server only understands MQTT it can return:

sec-websocket-protocol: mqtt

This selection must be done in `init/2`. An example usage could

init(Req, State) ->
    case cowboy_req:parse_header(<<"sec-websocket-protocol">>, Req) of
        undefined ->
            {cowboy_websocket, Req, State};
        Subprotocols ->
            case lists:keymember(<<"mqtt">>, 1, Subprotocols) of
                true ->
                    Req2 = cowboy_req:set_resp_header(<<"sec-websocket-protocol">>,
                        <<"mqtt">>, Req),
                    {cowboy_websocket, Req2, State};
                false ->
                    {stop, Req, State}

=== Post-upgrade initialization

Cowboy has separate processes for handling the connection
and requests. Because Websocket takes over the connection,
the Websocket protocol handling occurs in a different
process than the request handling.

This is reflected in the different callbacks Websocket
handlers have. The `init/2` callback is called from the
temporary request process and the `websocket_` callbacks
from the connection process.

This means that some initialization cannot be done from
`init/2`. Anything that would require the current pid,
or be tied to the current pid, will not work as intended.
The optional `websocket_init/1` can be used instead:

websocket_init(State) ->
    erlang:start_timer(1000, self(), <<"Hello!">>),
    {ok, State}.

All Websocket callbacks share the same return values. This
means that we can send frames to the client right after
the upgrade:

websocket_init(State) ->
    {reply, {text, <<"Hello!">>}, State}.

=== Receiving frames

Cowboy will call `websocket_handle/2` whenever a text, binary,
ping or pong frame arrives from the client.

The handler can handle or ignore the frames. It can also
send frames back to the client or stop the connection.

The following snippet echoes back any text frame received and
ignores all others:

websocket_handle(Frame = {text, _}, State) ->
    {reply, Frame, State};
websocket_handle(_Frame, State) ->
    {ok, State}.

Note that ping and pong frames require no action from the
handler as Cowboy will automatically reply to ping frames.
They are provided for informative purposes only.

=== Receiving Erlang messages

Cowboy will call `websocket_info/2` whenever an Erlang message

The handler can handle or ignore the messages. It can also
send frames to the client or stop the connection.

The following snippet forwards log messages to the client
and ignores all others:

websocket_info({log, Text}, State) ->
    {reply, {text, Text}, State};
websocket_info(_Info, State) ->
    {ok, State}.

=== Sending frames

// @todo So yeah, reply makes no sense. Maybe change it to send. Sigh.

All `websocket_` callbacks share return values. They may
send zero, one or many frames to the client.

To send nothing, just return an ok tuple:

websocket_info(_Info, State) ->
    {ok, State}.

To send one frame, return a reply tuple with the frame to send:

websocket_info(_Info, State) ->
    {reply, {text, <<"Hello!">>}, State}.

You can send frames of any type: text, binary, ping, pong
or close frames.

To send many frames at once, return a reply tuple with the
list of frames to send:

websocket_info(_Info, State) ->
    {reply, [
        {text, "Hello"},
        {text, <<"world!">>},
        {binary, <<0:8000>>}
    ], State}.

They are sent in the given order.

=== Keeping the connection alive

Cowboy will automatically respond to ping frames sent by
the client. They are still forwarded to the handler for
informative purposes, but no further action is required.

Cowboy does not send ping frames itself. The handler can
do it if required. A better solution in most cases is to
let the client handle pings. Doing it from the handler
would imply having an additional timer per connection and
this can be a considerable cost for servers that need to
handle large numbers of connections.

Cowboy can be configured to close idle connections
automatically. It is highly recommended to configure
a timeout here, to avoid having processes linger longer
than needed.

The `init/2` callback can set the timeout to be used
for the connection. For example, this would make Cowboy
close connections idle for more than 30 seconds:

init(Req, State) ->
    {cowboy_websocket, Req, State, #{
        idle_timeout => 30000}}.

This value cannot be changed once it is set. It defaults to

// @todo Perhaps the default should be changed.

=== Saving memory

The Websocket connection process can be set to hibernate
after the callback returns.

Simply add an `hibernate` field to the ok or reply tuples:

websocket_init(State) ->
    {ok, State, hibernate}.

websocket_handle(_Frame, State) ->
    {ok, State, hibernate}.

websocket_info(_Info, State) ->
    {reply, {text, <<"Hello!">>}, State, hibernate}.

It is highly recommended to write your handlers with
hibernate enabled, as this allows to greatly reduce the
memory usage. Do note however that an increase in the
CPU usage or latency can be observed instead, in particular
for the more busy connections.

=== Closing the connection

The connection can be closed at any time, either by telling
Cowboy to stop it or by sending a close frame.

To tell Cowboy to close the connection, use a stop tuple:

websocket_info(_Info, State) ->
    {stop, State}.

Sending a `close` frame will immediately initiate the closing
of the Websocket connection. Note that when sending a list of
frames that include a close frame, any frame found after the
close frame will not be sent.