# 7 How to Implement an Alternative Node Discovery for Erlang Distribution

This section describes how to implement an alternative node discovery mechanism for Erlang distribution. Node discovery is normally done using DNS and the Erlang Port Mapper Daemon (EPMD) for port registration and lookup.

Note

Support for alternative node discovery mechanisms was added in Erlang/OTP 21.

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7.1 Introduction

To implement your own node discovery module you have to write your own EPMD module. The EPMD module is responsible for providing the location of another node. The distribution modules (inet_tcp_dist/inet_tls_dist) call the EPMD module to get the IP address and port of the other node. The EPMD module that is part of Erlang/OTP will resolve the hostname using DNS and uses the EPMD unix process to get the port of another node. The EPMD unix process does this by connecting to the other node on a well-known port, port 4369.

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7.2 Discovery module

The discovery module needs to implement the same API as the regular EPMD module. However, instead of communicating with EPMD you can connect to any service to find out connection details of other nodes. A discovery module is enabled by setting -epmd_module when starting erlang. The discovery module must implement the following callbacks:

start_link/0
  Start any processes needed by the discovery module.
names/1
  Return node names held by the registrar for the given host.
register_node/2
  Register the given node name with the registrar.
port_please/3
  Return the distribution port used by the given node.

The discovery module may implement the following callback:

address_please/3

  Return the address of the given node. If not implemented, erl_epmd:address_please/3 will be used instead.

  This callback may also return the port of the given node. In that case port_please/3 may be omitted.

listen_port_please/2

  Return the port the local node should listen to. If not implemented, erl_epmd:listen_port_please/2 will be used instead.