<commit_before>[[ws_handlers]]
== Handling Websocket connections

A special handler is required for handling Websocket connections.
Websocket handlers allow you to initialize the connection,
handle incoming frames from the socket, handle incoming Erlang
messages and then clean up on termination.

Websocket handlers essentially act as a bridge between the client
and the Erlang system. They will typically do little more than
socket communication and decoding/encoding of frames.

=== Initialization

First, the `init/2` callback is called. This callback is common
to all handlers. To establish a Websocket connection, this function
must return a `ws` tuple.

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

Upon receiving this tuple, Cowboy will switch to the code
that handles Websocket connections and perform the handshake
immediately.

If the sec-websocket-protocol header was sent with the request
for establishing a Websocket connection, then the Websocket
handler *must* select one of these subprotocol and send it
back to the client, otherwise the client might decide to close
the connection, assuming no correct subprotocol was found.

[source,erlang]
----
init(Req, State) ->
	case cowboy_req:parse_header(<<"sec-websocket-protocol">>, Req) of
		undefined ->
			{ok, Req, State};
		Subprotocols ->
			case lists:keymember(<<"mychat2">>, 1, Subprotocols) of
				true ->
					Req2 = cowboy_req:set_resp_header(<<"sec-websocket-protocol">>,
						<<"mychat2">>, Req),
					{ok, Req2, State};
				false ->
					{stop, Req, State}
			end
	end.
----

It is not recommended to wait too long inside the `init/2`
function. Any extra initialization may be done after returning by
sending yourself a message before doing anything. Any message sent
to `self()` from `init/2` is guaranteed to arrive before
any frames from the client.

It is also very easy to ensure that this message arrives before
any message from other processes by sending it before registering
or enabling timers.

// @todo This doesn't even work.

[source,erlang]
----
init(Req, State) ->
	self() ! post_init,
	%% Register process here...
	{cowboy_websocket, Req, State}.

websocket_info(post_init, State) ->
	%% Perform post_init initialization here...
	{ok, State}.
----

=== Handling frames from the client

Cowboy will call `websocket_handle/2` whenever a text, binary,
ping or pong frame arrives from the client. Note that in the
case of ping and pong frames, no action is expected as Cowboy
automatically replies to ping frames.

The handler can decide to send frames to the socket, stop
or just continue without sending anything.

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

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

=== Handling Erlang messages

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

The handler can decide to send frames to the socket, stop
or just continue without sending anything.

The following snippet forwards any `log` message to the socket
and ignores all others.

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

=== Sending frames to the socket

Cowboy allows sending either a single frame or a list of
frames to the socket, in which case the frames are sent
sequentially. Any frame can be sent: text, binary, ping,
pong or close frames.

The following example sends three frames using a single `reply`
tuple.

[source,erlang]
----
websocket_info(hello_world, State) ->
	{reply, [
		{text, "Hello"},
		{text, <<"world!">>},
		{binary, <<0:8000>>}
	], State};
%% More websocket_info/2 clauses here...
----

Note that the payload for text and binary frames is of type
`iodata()`, meaning it can be either a `binary()` or an
`iolist()`.

Sending a `close` frame will immediately initiate the closing
of the Websocket connection. Be aware that any additional
frames sent by the client or any Erlang messages waiting to
be received will not be processed. Also note that when replying
a list of frames that includes close, any frame found after the
close frame will not be sent.

=== Ping and timeout

The biggest performance improvement you can do when dealing
with a huge number of Websocket connections is to reduce the
number of timers that are started on the server. A common use
of timers when dealing with connections is for sending a ping
every once in a while. This should be done exclusively on the
client side. Indeed, a server handling one million Websocket
connections will perform a lot better when it doesn't have to
handle one million extra timers too!

Cowboy will automatically respond to ping frames sent by the
client. It will still forward the frame to the handler for
informative purpose, but no further action is required.

Cowboy can be configured to automatically close the Websocket
connection when no data arrives on the socket. It is highly
recommended to configure a timeout for it, as otherwise you
may end up with zombie "half-connected" sockets that may
leave the process alive forever.

A good timeout value is 60 seconds.

[source,erlang]
----
init(Req, State) ->
	{cowboy_websocket, Req, State, 60000}.
----

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

=== Hibernate

Most tuples returned from handler callbacks can include an
extra value `hibernate`. After doing any necessary operations
following the return of the callback, Cowboy will hibernate
the process.

It is highly recommended to hibernate processes that do not
handle much traffic. It is a good idea to hibernate all
connections by default and investigate only when you start
noticing increased CPU usage.

=== Supporting older browsers

Unfortunately Websocket is a relatively recent technology,
which means that not all browsers support it. A library like
https://github.com/ninenines/bullet[Bullet] can be used to
emulate Websocket connections on older browsers.<commit_msg>Update the Websocket handlers chapter<commit_after>[[ws_handlers]]
== 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:

[source,erlang]
----
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
be:

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

=== 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:

[source,erlang]
----
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:

[source,erlang]
----
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:

[source,erlang]
----
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
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:

[source,erlang]
----
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:

[source,erlang]
----
websocket_info(_Info, State) ->
	{ok, State}.
----

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

[source,erlang]
----
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:

[source,erlang]
----
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 60 seconds:

[source,erlang]
----
init(Req, State) ->
	{cowboy_websocket, Req, State, 60000}.
----

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

// @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:

[source,erlang]
----
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:

[source,erlang]
----
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.
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