== 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_statem
Special processes like the ones that use the `gen_statem` or `gen_server`
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_statem:enter_loop/4` 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_statem` execution loop.
.Use a gen_statem for protocol handling
[source,erlang]
----
-module(my_protocol).
-behaviour(gen_statem).
-behaviour(ranch_protocol).
-export([start_link/4]).
-export([init/1]).
%% Exports of other gen_statem 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_statem:enter_loop(?MODULE, [], state_name, {state_data, Socket, Transport}).
%% Other gen_statem callbacks here.
----
Check the `tcp_reverse` example for a complete example.
