ranch - Socket acceptor pool


The module ranch provides functions for starting and manipulating Ranch listeners.









max_conns() = non_neg_integer() | infinity

Maximum number of connections allowed per connection supervisor.

This is a soft limit. The actual number of connections might be slightly above the limit due to concurrency when accepting new connections. Some connections may also be removed from this count explicitly by the user code.


opts() = any() | transport_opts(any())

Transport or socket options.


ref() = any()

Unique name used to refer to a listener.


transport_opts(SocketOpts) = #{
    alarms               => #{
                                term() => #{
                                    type := num_connections,
                                    treshold := non_neg_integer(),
                                    callback := fun((ref(), term(), pid(), [pid()]) -> any()),
                                    cooldown => non_neg_integer()
    connection_type      => worker | supervisor,
    handshake_timeout    => timeout(),
    max_connections      => max_conns(),
    logger               => module(),
    num_acceptors        => pos_integer(),
    num_conns_sups       => pos_integer(),
    post_listen_callback => fun((term()) -> ok | {error, term()}),
    shutdown             => timeout() | brutal_kill,
    socket_opts          => SocketOpts

Transport options.

The transport options are a combination of Ranch-specific options and transport-specific socket options.

None of the options are required.

alarms (#{})

Alarms to call a function when the number of connections tracked by one connection supervisor reaches or exceeds a defined treshold.

The map keys are the alarm names, which can be any term. The associated values are the respective alarm options, again in a map with the following keys:


Must be set to num_connections.


Treshold value, which must be a non_neg_integer. When the number of connections tracked by a single connection supervisor reaches or exceeds this value, The alarm will trigger and call the function defined in the callback key (see below).


The alarm function, which takes the listener name, the alarm name, the pid of the connection supervisor and a list of the pids of all connection processes under that supervisor as arguments. The return value is ignored.

cooldown (5000)

The minimum time after which the alarm can be triggered again, in milliseconds.

connection_type (worker)

Type of process that will handle the connection.

handshake_timeout (5000)

Maximum allowed time for the ranch:handshake/1,2 call to finish.

logger (logger)

The module that will be used to write log messages.

max_connections (1024)

Maximum number of active connections per connection supervisor. Soft limit. Use infinity to disable the limit entirely.

num_acceptors (10)

Number of processes that accept connections.

num_conns_sups - see below

Number of processes that supervise connection processes. If not specified, defaults to be equal to num_acceptors.

post_listen_callback (fun(_ListenSock) -> ok end)

A function which will be called after a listen socket has been successfully created, with the socket as argument. It can be used to perform any necessary setup steps on the socket.

If the callback function returns ok, the listener will start accepting connections on the socket. If it returns {error, Reason}, the listener will fail to start.

shutdown (5000)

Maximum allowed time for children to stop on listener shutdown.


Socket options to be used by Transport:listen/1. Please refer to the documentation of the transport module you are using for more details.


  • 2.1: The post_listen_callback transport option was added.
  • 2.0: The type transport_opts(SocketOpts) was added.
  • 2.0: The function ranch:accept_ack/1 was removed in favor of ranch:handshake(3).
  • 2.0: The option max_connections is now per connection supervisor.
  • 2.0: The num_conns_sup option was added.
  • 2.0: The socket option was removed.
  • 2.0: The logger option is no longer experimental. It now defaults to logger instead of error_logger.
  • 2.0: The opt() type was removed.
  • 1.6: The experimental logger option was added.
  • 1.6: The opt() type was deprecated in favor of the new opts() type.

See also


Ranch 2.1 Function Reference


Version select

Like my work? Donate!

Donate to Loïc Hoguin because his work on Cowboy, Ranch, Gun and Erlang.mk is fantastic:

Recurring payment options are also available via GitHub Sponsors. These funds are used to cover the recurring expenses like food, dedicated servers or domain names.