libcluster

This library provides a mechanism for automatically forming clusters of Erlang nodes, with either static or dynamic node membership. It provides a pluggable "strategy" system, with a variety of strategies provided out of the box.

You can find supporting documentation here.

Features

• Automatic cluster formation/healing
• Choice of multiple clustering strategies out of the box:
  • Standard Distributed Erlang facilities (e.g. epmd, .hosts.erlang), which supports IP-based or DNS-based names
  • Multicast UDP gossip, using a configurable port/multicast address,
  • Kubernetes via its metadata API using via a configurable label selector and node basename; or alternatively, using DNS.
  • Rancher, via its metadata API
• Easy to provide your own custom clustering strategies for your specific environment.
• Easy to use provide your own distribution plumbing (i.e. something other than Distributed Erlang), by implementing a small set of callbacks. This allows libcluster to support projects like Partisan.

Installation

defp deps do
  [{:libcluster, "~> MAJ.MIN"}]
end

You can determine the latest version by running mix hex.info libcluster in your shell, or by going to the libcluster page on Hex.pm.

Usage

It is easy to get started using libcluster, simply decide which strategy you want to use to form a cluster, define a topology, and then start the Cluster.Supervisor module in the supervision tree of an application in your Elixir system, as demonstrated below:

defmodule MyApp.App do
  use Application

  def start(_type, _args) do
    topologies = [
      example: [
        strategy: Cluster.Strategy.Epmd,
        config: [hosts: [:"a@127.0.0.1", :"b@127.0.0.1"]],
      ]
    ]
    children = [
      {Cluster.Supervisor, [topologies, [name: MyApp.ClusterSupervisor]]},
      # ..other children..
    ]
    Supervisor.start_link(children, strategy: :one_for_one, name: MyApp.Supervisor)
  end
end

The following section describes topology configuration in more detail.

Example Configuration

You can configure libcluster either in your Mix config file (config.exs) as shown below, or construct the keyword list structure manually, as shown in the previous section. Either way, you need to pass the configuration to the Cluster.Supervisor module in it's start arguments. If you prefer to use Mix config files, then simply use Application.get_env(:libcluster, :topologies) to get the config that Cluster.Supervisor expects.

config :libcluster,
  topologies: [
    epmd_example: [
      # The selected clustering strategy. Required.
      strategy: Cluster.Strategy.Epmd,
      # Configuration for the provided strategy. Optional.
      config: [hosts: [:"a@127.0.0.1", :"b@127.0.0.1"]],
      # The function to use for connecting nodes. The node
      # name will be appended to the argument list. Optional
      connect: {:net_kernel, :connect_node, []},
      # The function to use for disconnecting nodes. The node
      # name will be appended to the argument list. Optional
      disconnect: {:erlang, :disconnect_node, []},
      # The function to use for listing nodes.
      # This function must return a list of node names. Optional
      list_nodes: {:erlang, :nodes, [:connected]},
    ],
    # more topologies can be added ...
    gossip_example: [
      # ...
    ]
  ]

Strategy Configuration

For instructions on configuring each strategy included with libcluster, please visit the docs on HexDocs, and look at the module doc for the strategy you want to use. The authoritative documentation for each strategy is kept up to date with the module implementing it.

Clustering

You have a handful of choices with regards to cluster management out of the box:

• Cluster.Strategy.Epmd, which relies on epmd to connect to a configured set of hosts.
• Cluster.Strategy.LocalEpmd, which relies on epmd to connect to discovered nodes on the local host.
• Cluster.Strategy.ErlangHosts, which uses the .hosts.erlang file to determine which hosts to connect to.
• Cluster.Strategy.Gossip, which uses multicast UDP to form a cluster between nodes gossiping a heartbeat.
• Cluster.Strategy.Kubernetes, which uses the Kubernetes Metadata API to query nodes based on a label selector and basename.
• Cluster.Strategy.Kubernetes.DNS, which uses DNS to join nodes under a shared headless service in a given namespace.
• Cluster.Strategy.Rancher, which like the Kubernetes strategy, uses a metadata API to query nodes to cluster with.

You can also define your own strategy implementation, by implementing the Cluster.Strategy behavior. This behavior expects you to implement a start_link/1 callback, optionally overriding child_spec/1 if needed. You don't necessarily have to start a process as part of your strategy, but since it's very likely you will need to maintain some state, designing your strategy as an OTP process (e.g. GenServer) is the ideal method, however any valid OTP process will work. See the Cluster.Strategy module for details on the callbacks you need to implement and the arguments they receive.

If you do not wish to use the default Erlang distribution protocol, you may provide an alternative means of connecting/ disconnecting nodes via the connect and disconnect configuration options, if not using Erlang distribution you must provide a list_nodes implementation as well. They take a {module, fun, args} tuple, and append the node name being targeted to the args list. How to implement distribution in this way is left as an exercise for the reader, but I recommend taking a look at the Firenest project currently under development. By default, libcluster uses Distributed Erlang.

Third-Party Strategies

The following list of third-party strategy implementations is not comprehensive, but are known to exist.

• libcluster_ec2 - EC2 clustering strategy based on tags
• libcluster_droplet - Digital Ocean Droplet clustering strategy
• libcluster_consul - Consul clustering strategy

Copyright and License

Copyright (c) 2016 Paul Schoenfelder

This library is MIT licensed. See the LICENSE.md for details.