ranch/guide/listeners.md

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.

Default transport options

By default the socket will be set to return binary data, with the options {active, false}, {packet, raw}, {reuseaddr, true} set. These values can't be overriden when starting the listener, but they can be overriden using Transport:setopts/2 in the protocol.

It will also set {backlog, 1024} and {nodelay, true}, which can be overriden at listener startup.

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 privileged ports

Some systems limit access to ports below 1024 for security reasons. This can easily be identified by an {error, eacces} error when trying to open a listening socket on such a port.

The methods for listening on privileged ports vary between systems, please refer to your system's documentation for more information.

We recommend the use of port rewriting for systems with a single server, and load balancing for systems with multiple servers. Documenting these solutions is however out of the scope of this guide.

Accepting connections on an existing socket

If you want to accept connections on an existing socket, you can use the socket transport option, which should just be the relevant data returned from the connect function for the transport or the underlying socket library (gen_tcp:connect, ssl:connect). The accept function will then be called on the passed in socket. You should connect the socket in {active, false} mode, as well.

Note, however, that because of a bug in SSL, you cannot change ownership of an SSL listen socket prior to R16. Ranch will catch the error thrown, but the owner of the SSL socket will remain as whatever process created the socket. However, this will not affect accept behaviour unless the owner process dies, in which case the socket is closed. Therefore, to use this feature with SSL with an erlang release prior to R16, ensure that the SSL socket is opened in a persistant process.

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:remove_connection/1 from within the connection process, with the name of the listener as the only argument.

ranch:remove_connection(Ref).

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

You can modify the max_connections value on a running listener by using the ranch:set_max_connections/2 function, with the name of the listener as first argument and the new value as the second.

ranch:set_max_connections(tcp_echo, MaxConns).

The change will occur immediately.

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).