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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 listener's pid, the socket, the transport handler being used and the protocol options defined in the call to ranch:start_listener/6. 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 pid as argument.

ok = ranch:accept_ack(ListenerPid).

If your protocol code requires specific socket options, you should set them while initializing your connection process and before starting 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/.



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

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