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 use of transport handlers.

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

Starting a listener

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/5 function. Before doing so however, you must ensure that the ranch application is started.

Starting 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.

Starting a listener for TCP connections on port 5555
{ok, _} = ranch:start_listener(tcp_echo,
        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 command:

Building and starting a Ranch example
$ make run

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.

Connecting to the example listener with telnet
$ telnet localhost 5555
Connected to localhost.
Escape character is '^]'.
It works!
It works!

telnet> quit
Connection closed.

Stopping a listener

All you need to stop a Ranch listener is to call the ranch:stop_listener/1 function with the listener’s name as argument. In the previous section we started the listener named tcp_echo. We can now stop it.

Stopping a listener

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/5.

Starting a listener for TCP connections on a random port
{ok, _} = ranch:start_listener(tcp_echo,
        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.

Customizing the maximum number of concurrent connections
{ok, _} = ranch:start_listener(tcp_echo,
        ranch_tcp, [{port, 5555}, {max_connections, 100}],
        echo_protocol, []

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

Disabling the limit for the number of connections
{ok, _} = ranch:start_listener(tcp_echo,
        ranch_tcp, [{port, 5555}, {max_connections, infinity}],
        echo_protocol, []

The maximum number of connections is a soft limit. In practice, it can reach max_connections + the number of acceptors.

When the maximum number of connections is reached, Ranch will stop accepting connections. This will not result in further connections being rejected, as the kernel option allows queueing incoming connections. The size of this queue is determined by the backlog option and defaults to 1024. Ranch does not know about the number of connections that are in the backlog.

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.

Removing a connection from the count of connections

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.

Upgrading the maximum number of connections
ranch:set_max_connections(tcp_echo, MaxConns).

The change will occur immediately.

Customizing the number of acceptor processes

By default Ranch will use 10 acceptor processes. Their role is to accept connections and spawn a connection process for every new connection.

This number can be tweaked to improve performance. A good number is typically between 10 or 100 acceptors. You must measure to find the best value for your application.

Specifying a custom number of acceptor processes
{ok, _} = ranch:start_listener(tcp_echo,
        ranch_tcp, [{port, 5555}, {num_acceptors, 42}],
        echo_protocol, []

When running out of file descriptors

Operating systems have limits on the number of sockets which can be opened by applications. When this maximum is reached the listener can no longer accept new connections. The accept rate of the listener will be automatically reduced, and a warning message will be logged.

=ERROR REPORT==== 13-Jan-2016::12:24:38 ===
Ranch acceptor reducing accept rate: out of file descriptors

If you notice messages like this you should increase the number of file-descriptors which can be opened by your application. How this should be done is operating-system dependent. Please consult the documentation of your operating system.

Using a supervisor for connection processes

Ranch allows you to define the type of process that will be used for the connection processes. By default it expects a worker. When the connection_type configuration value is set to supervisor, Ranch will consider that the connection process it manages is a supervisor and will reflect that in its supervision tree.

Connection processes of type supervisor can either handle the socket directly or through one of their children. In the latter case the start function for the connection process must return two pids: the pid of the supervisor you created (that will be supervised) and the pid of the protocol handling process (that will receive the socket).

Instead of returning {ok, ConnPid}, simply return {ok, SupPid, ConnPid}.

It is very important that the connection process be created under the supervisor process so that everything works as intended. If not, you will most likely experience issues when the supervised process is stopped.


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.

Upgrading the protocol options
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.

Retrieving the current protocol options
Opts = ranch:get_protocol_options(tcp_echo).

Obtaining information about listeners

Ranch provides two functions for retrieving information about the listeners, for reporting and diagnostic purposes.

The ranch:info/0 function will return detailed information about all listeners.

Retrieving detailed information

The ranch:procs/2 function will return all acceptor or listener processes for a given listener.

Get all acceptor processes
ranch:procs(tcp_echo, acceptors).
Get all connection processes
ranch:procs(tcp_echo, connections).

Ranch 1.3 User Guide


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