A protocol handler starts a connection process and defines the protocol logic executed in this process.
All protocol handlers must implement the ranch_protocol behavior
which defines a single callback, start_link/3. This callback is
responsible for spawning a new process for handling the connection.
It receives three arguments: the name of the listener, 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/3]).
-export([init/3]).
start_link(Ref, 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.
This problem can be addressed in several ways.
ranch:handshake/1,2 call in the enter
clause of the initial state. Check the tcp_reverse example for a complete
example.
next_event action in the return from init/1 and place the
ranch:handshake/1,2 call in the clause handling the event in the initial
state.
gen_statem:enter_loop/4 function and start your process with
proc_lib:spawn_link/3 or proc_lib:start_link/3,4,5. See below for an
example.
Using gen_statem:enter_loop/4 to start a protocol.
-module(my_protocol).
-behaviour(gen_statem).
-behaviour(ranch_protocol).
-export([start_link/3]).
-export([init/1]).
%% Exports of other gen_statem callbacks here.
start_link(Ref, 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.