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doc/src/guide/ws_handlers.ezdoc

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+::: 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/3` callback is called. This callback is common
+to all handlers. To establish a Websocket connection, this function
+must return an `upgrade` tuple.
+
+``` erlang
+init(_, Req, Opts) ->
+	{upgrade, protocol, cowboy_websocket}.
+```
+
+It is also possible to return an update Req object and options
+using the longer form of this tuple.
+
+``` erlang
+init(_Type, Req, Opts) ->
+	{upgrade, protocol, cowboy_websocket, Req, Opts}.
+```
+
+Upon receiving this tuple, Cowboy will switch to the code
+that handles Websocket connections. It does not immediately
+perform the handshake however. First, it calls the `websocket_init/3`
+callback.
+
+This function must be used to initialize the state, and can
+also be used to register the process, start a timer, etc.
+As long as the function returns an `ok` tuple, then Cowboy
+performs the Websocket handshake.
+
+``` erlang
+websocket_init(_Type, Req, _Opts) ->
+    {ok, Req, #state{}}.
+```
+
+A `shutdown` tuple can be returned to refuse to perform the
+handshake. When doing so, Cowboy will send a `400 Bad Request`
+response to the client and close the connection.
+
+``` erlang
+websocket_init(_Type, Req, _Opts) ->
+	{shutdown, Req}.
+```
+
+It is also possible to perform a `cowboy_req:reply/{2,3,4}`
+before returning a `shutdown` tuple, allowing you to override
+the response sent back to the client.
+
+Note that browser support for handling Websocket connection
+failures may vary.
+
+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.
+
+``` erlang
+websocket_init(_Type, Req, _Opts) ->
+	case cowboy_req:parse_header(<<"sec-websocket-protocol">>, Req) of
+		{ok, undefined, Req2} ->
+			{ok, Req, #state{}};
+		{ok, Subprotocols, Req2} ->
+			case lists:keymember(<<"mychat2">>, 1, Subprotocols) of
+				true ->
+					Req3 = cowboy:set_resp_header(<<"sec-websocket-protocol">>,
+						<<"mychat2">>, Req2),
+					{ok, Req3, #state{}};
+				false ->
+					{shutdown, Req2}
+			end
+	end.
+```
+
+It is not recommended to wait too long inside the `websocket_init/3`
+function. Any extra initialization may be done after returning by
+sending yourself a message before doing anything. Any message sent
+to `self()` from `websocket_init/3` 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.
+
+``` erlang
+websocket_init(_Type, Req, _Opts) ->
+	self() ! post_init,
+	%% Register process here...
+	{ok, Req, #state{}}.
+
+websocket_info(post_init, Req, State) ->
+	%% Perform post_init initialization here...
+	{ok, Req, State}.
+```
+
+:: Handling frames from the client
+
+Cowboy will call `websocket_handle/3` 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, shutdown
+or just continue without sending anything.
+
+The following snippet echoes back any text frame received and
+ignores all others.
+
+``` erlang
+websocket_handle(Frame = {text, _}, Req, State) ->
+	{reply, Frame, Req, State};
+websocket_handle(_Frame, Req, State) ->
+	{ok, Req, State}.
+```
+
+:: Handling Erlang messages
+
+Cowboy will call `websocket_info/3` whenever an Erlang message
+arrives.
+
+The handler can decide to send frames to the socket, shutdown
+or just continue without sending anything.
+
+The following snippet forwards any `log` message to the socket
+and ignores all others.
+
+``` erlang
+websocket_info({log, Text}, Req, State) ->
+	{reply, {text, Text}, Req, State};
+websocket_info(_Info, Req, State) ->
+	{ok, Req, State}.
+```
+
+:: Sending frames to the socket
+
+Cowboy allows sending either a single frame or a list of
+frames to the socket. Any frame can be sent: text, binary, ping,
+pong or close frames.
+
+The following example sends three frames using a single `reply`
+tuple.
+
+``` erlang
+websocket_info(hello_world, Req, State) ->
+	{reply, [
+		{text, "Hello"},
+		{text, <<"world!">>},
+		{binary, <<0:8000>>}
+	], Req, State};
+%% More websocket_info/3 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.
+
+``` erlang
+websocket_init(_Type, Req, _Opts) ->
+	{ok, 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
+^"Bullet^https://github.com/extend/bullet^ can be used to
+emulate Websocket connections on older browsers.