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:handshake/1,2 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.

Perform the socket handshake. 

{ok, Socket} = ranch:handshake(Ref).

If your protocol code requires specific socket options, you should set them while initializing your connection process, after calling ranch:handshake/1,2. 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. 

-module(echo_protocol).
-behaviour(ranch_protocol).

-export([start_link/4]).
-export([init/3]).

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

init(Ref, Transport, _Opts = []) ->
    {ok, Socket} = ranch:handshake(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.

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:handshake/1,2 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. 

-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, Transport, Opts}])}.

init({Ref, Transport, _Opts = []}) ->
    %% Perform any required state initialization here.
    {ok, Socket} = ranch:handshake(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.