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Listeners

Purpose

A listener is a set of processes whose role is to listen on a port for new connections. It manages a pool of acceptor processes, each of them indefinitely accepting connections. When it does, it starts a new process executing the protocol handler code. All the socket programming is abstracted through the user of transport handlers.

The listener takes care of supervising all the acceptor and connection processes, allowing developers to focus on building their application.

Starting and stopping

Ranch does nothing by default. It is up to the application developer to request that Ranch listens for connections.

A listener can be started and stopped at will.

When starting a listener, a number of different settings are required:

  • A name to identify it locally and be able to interact with it.
  • The number of acceptors in the pool.
  • A transport handler and its associated options.
  • A protocol handler and its associated options.

Ranch includes both TCP and SSL transport handlers, respectively ranch_tcp and ranch_ssl.

A listener can be started by calling the ranch:start_listener/6 function. Before doing so however, you must ensure that the ranch application is started.

To start the ranch application:

ok = application:start(ranch).

You are then ready to start a listener. Let's call it tcp_echo. It will have a pool of 100 acceptors, use a TCP transport and forward connections to the echo_protocol handler.

{ok, _} = ranch:start_listener(tcp_echo, 100,
    ranch_tcp, [{port, 5555}],
    echo_protocol, []
).

You can try this out by compiling and running the tcp_echo example in the examples directory. To do so, open a shell in the examples/tcp_echo/ directory and run the following commands:

% rebar get-deps compile
% ./start.sh
Listening on port 5555

You can then connect to it using telnet and see the echo server reply everything you send to it. Then when you're done testing, you can use the Ctrl+] key to escape to the telnet command line and type quit to exit.

% telnet localhost 5555
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
Hello!
Hello!
It works!
It works!
^]

telnet> quit
Connection closed.

Listening on a random port

You do not have to specify a specific port to listen on. If you give the port number 0, or if you omit the port number entirely, Ranch will start listening on a random port.

You can retrieve this port number by calling ranch:get_port/1. The argument is the name of the listener you gave in ranch:start_listener/6.

{ok, _} = ranch:start_listener(tcp_echo, 100,
    ranch_tcp, [{port, 0}],
    echo_protocol, []
).
Port = ranch:get_port(tcp_echo).

Listening on a port =< 1024

This is currently not possible. We recommend the use of load balancing or NAT firewall rules if the need arise. Proxies can sometimes also be used although that's a less efficient solution.

Limiting the number of concurrent connections

The max_connections transport option allows you to limit the number of concurrent connections. It defaults to 1024. Its purpose is to prevent your system from being overloaded and ensuring all the connections are handled optimally.

{ok, _} = ranch:start_listener(tcp_echo, 100,
    ranch_tcp, [{port, 5555}, {max_connections, 100}],
    echo_protocol, []
).

You can disable this limit by setting its value to the atom infinity.

{ok, _} = ranch:start_listener(tcp_echo, 100,
    ranch_tcp, [{port, 5555}, {max_connections, infinity}],
    echo_protocol, []
).

You may not always want connections to be counted when checking for max_connections. For example you might have a protocol where both short-lived and long-lived connections are possible. If the long-lived connections are mostly waiting for messages, then they don't consume much resources and can safely be removed from the count.

To remove the connection from the count, you must call the ranch_listener:remove_connection/1 from within the connection process, with the listener pid as the only argument.

ranch_listener:remove_connection(ListenerPid).

As seen in the chapter covering protocols, this pid is received as the first argument of the protocol's start_link/4 callback.

Upgrading

Ranch allows you to upgrade the protocol options. This takes effect immediately and for all subsequent connections.

To upgrade the protocol options, call ranch:set_protocol_options/2 with the name of the listener as first argument and the new options as the second.

ranch:set_protocol_options(tcp_echo, NewOpts).

All future connections will use the new options.

You can also retrieve the current options similarly by calling ranch:get_protocol_options/1.

Opts = ranch:get_protocol_options(tcp_echo).
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