== Protocols

A protocol handler starts a connection process and defines the
protocol logic executed in this process.

=== Writing a protocol handler

All protocol handlers must implement the `ranch_protocol` behavior
which defines a single callback, `start_link/4`. This callback is
responsible for spawning a new process for handling the connection.
It receives four arguments: the name of the listener, the socket, the
transport handler being used and the protocol options defined in
the call to `ranch:start_listener/5`. This callback must
return `{ok, Pid}`, with `Pid` the pid of the new process.

The newly started process can then freely initialize itself. However,
it must call `ranch:accept_ack/1` before doing any socket operation.
This will ensure the connection process is the owner of the socket.
It expects the listener's name as argument.

.Acknowledge accepting the socket

[source,erlang]
ok = ranch:accept_ack(Ref).

If your protocol code requires specific socket options, you should
set them while initializing your connection process, after
calling `ranch:accept_ack/1`. You can use `Transport:setopts/2`
for that purpose.

Following is the complete protocol code for the example found
in `examples/tcp_echo/`.

.Protocol module that echoes everything it receives

[source,erlang]
----
-module(echo_protocol).
-behaviour(ranch_protocol).

-export([start_link/4]).
-export([init/4]).

start_link(Ref, Socket, Transport, Opts) ->
	Pid = spawn_link(?MODULE, init, [Ref, Socket, Transport, Opts]),
	{ok, Pid}.

init(Ref, Socket, Transport, _Opts = []) ->
	ok = ranch:accept_ack(Ref),
	loop(Socket, Transport).

loop(Socket, Transport) ->
	case Transport:recv(Socket, 0, 5000) of
		{ok, Data} ->
			Transport:send(Socket, Data),
			loop(Socket, Transport);
		_ ->
			ok = Transport:close(Socket)
	end.
----

=== Using gen_server

Special processes like the ones that use the `gen_server` or `gen_fsm`
behaviours have the particularity of having their `start_link` call not
return until the `init` function returns. This is problematic, because
you won't be able to call `ranch:accept_ack/1` from the `init` callback
as this would cause a deadlock to happen.

Use the `gen_server:enter_loop/3` function. It allows you to start your process
normally (although it must be started with `proc_lib` like all special
processes), then perform any needed operations before falling back into
the normal `gen_server` execution loop.

.Use a gen_server for protocol handling

[source,erlang]
----
-module(my_protocol).
-behaviour(gen_server).
-behaviour(ranch_protocol).

-export([start_link/4]).
-export([init/1]).
%% Exports of other gen_server callbacks here.

start_link(Ref, Socket, Transport, Opts) ->
	{ok, proc_lib:spawn_link(?MODULE, init, [{Ref, Socket, Transport, Opts}])}.

init({Ref, Socket, Transport, _Opts = []}) ->
	%% Perform any required state initialization here.
	ok = ranch:accept_ack(Ref),
	ok = Transport:setopts(Socket, [{active, once}]),
	gen_server:enter_loop(?MODULE, [], {state, Socket, Transport}).

%% Other gen_server callbacks here.
----

Check the `tcp_reverse` example for a complete example.
