libcluster

This library provides a mechanism for automatically forming clusters of Erlang nodes, with either static or dynamic node membership. It provides a publish/subscribe mechanism for cluster events so that you can easily be notified when cluster members join or leave, and provides a pluggable "strategy" system, with multicast UDP gossip, Kubernetes, and EPMD strategies all provided out of the box.

View the docs here.

Features

• automatic cluster formation/healing
• choice of multiple clustering strategies out of the box:
  • standard Distributed Erlang facilities (i.e. epmd)
  • multicast UDP gossip, using a configurable port/multicast address,
  • the Kubernetes API, via a configurable pod selector and node basename.
• provide your own clustering strategies (e.g. an EC2 strategy, etc.)
• provide your own topology plumbing (e.g. something other than standard Erlang distribution)

Installation

defp deps do
  [{:libcluster, "~> 2.1"}]
end

An example configuration

The following will help you understand the more descriptive text below. The configuration for libcluster can also be described as a spec for the clustering topologies and strategies which will be used.

config :libcluster,
  topologies: [
    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, []},
      # The function to use for disconnecting nodes. The node
      # name will be appended to the argument list. Optional
      disconnect: {:net_kernel, :disconnect, []},
      # The function to use for listing nodes. 
      # This function must return a list of node names. Optional
      list_nodes: {:erlang, :nodes, [:connected]},
      # A list of options for the supervisor child spec
      # of the selected strategy. Optional
      child_spec: [restart: :transient]
    ]
  ]

Clustering

You have three choices with regards to cluster management out of the box. You can use the built-in Erlang tooling for connecting nodes, by setting strategy: Cluster.Strategy.Epmd in the config. If set to Cluster.Strategy.Gossip it will make use of the multicast gossip protocol to dynamically form a cluster. If set to Cluster.Strategy.Kubernetes, it will use the Kubernetes API to query endpoints based on a basename and label selector, using the token and namespace injected into every pod; once it has a list of endpoints, it uses that list to form a cluster, and keep it up to date.

You can provide your own clustering strategy by setting strategy: MyApp.Strategy where MyApp.Strategy implements the Cluster.Strategy behaviour, which currently consists of exporting a start_link/1 callback. 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 (i.e. GenServer) is the ideal method, however any valid OTP process will work. libcluster starts the strategy process as part of it's supervision tree.

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, the Erlang distribution is used.

Clustering Strategies

The gossip protocol works by multicasting a heartbeat via UDP. The default configuration listens on all host interfaces, port 45892, and publishes via the multicast address 230.1.1.251. These parameters can all be changed via the following config settings:

config :libcluster,
  topologies: [
    gossip_example: [
      strategy: Cluster.Strategy.Gossip,
      config: [
        port: 45892,
        if_addr: {0,0,0,0},
        multicast_addr: {230,1,1,251},
        # a TTL of 1 remains on the local network,
        # use this to change the number of jumps the
        # multicast packets will make
        multicast_ttl: 1]]]

The Kubernetes strategy works by querying the Kubernetes API for all endpoints in the same namespace which match the provided selector, and getting the container IPs associated with them. Once all of the matching IPs have been found, it will attempt to establish node connections using the format <kubernetes_node_basename>@<endpoint ip>. You must make sure that your nodes are configured to use longnames, that the hostname matches the kubernetes_node_basename setting, and that the domain matches the IP address. Configuration might look like so:

config :libcluster,
  topologies: [
    k8s_example: [
      strategy: Cluster.Strategy.Kubernetes,
      config: [
        kubernetes_selector: "app=myapp",
        kubernetes_node_basename: "myapp"]]]

And in vm.args:

-name myapp@10.128.0.9
-setcookie test

License

MIT