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## Chapter 23. 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.

## 23.1. 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.

## 23.2. 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 be:

    init(Req0, State) ->
        case cowboy_req:parse_header(<<"sec-websocket-protocol">>, Req0) of
            undefined ->
                {cowboy_websocket, Req0, State};
            Subprotocols ->
                case lists:member(<<"mqtt">>, 1, Subprotocols) of
                    true ->
                        Req = cowboy_req:set_resp_header(<<"sec-websocket-protocol">>,
                            <<"mqtt">>, Req0),
                        {cowboy_websocket, Req, State};
                    false ->
                        Req = cowboy_req:reply(400, Req0),
                        {ok, Req, State}
                end
        end.

## 23.3. 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) ->
        {[{text, <<"Hello!">>}], State}.

## 23.4. 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) ->
        {[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.

## 23.5. Receiving Erlang messages

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

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) ->
        {[{text, Text}], State};
    websocket_info(_Info, State) ->
        {ok, State}.

## 23.6. Sending frames

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 the frame to be sent:

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

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

You can send many frames at the same time:

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

They are sent in the given order.

## 23.7. 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 `60000`.

## 23.8. Limiting frame sizes

Cowboy accepts frames of any size by default. You should limit the size depending on what your handler may handle. You can do this via the `init/2` callback:

    init(Req, State) ->
        {cowboy_websocket, Req, State, #{
            max_frame_size => 8000000}}.

The lack of limit is historical. A future version of Cowboy will have a more reasonable default.

## 23.9. Saving memory

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

Simply add an `hibernate` field to the returned tuple:

    websocket_init(State) ->
        {[], State, hibernate}.

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

    websocket_info(_Info, State) ->
        {[{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.

## 23.10. 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.

The following example sends a close frame with a reason message:

    websocket_info(_Info, State) ->
        {[{close, 1000, <<"some-reason">>}], State}.

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