Automatic cluster formation/healing for Elixir applications.
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README.md

skycluster

Hex.pm Version

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.

Deprecation warning

This package is deprecated in favor of bitwalker/libcluster implementation for service discovery and phoenixframework/firenest for messages broadcasting.

We think that this approach will be more reusable for community, because separate tasks should be done with a separate building blocks of your application.

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.)
  • easy pubsub for cluster events

Clustering

You have three choices with regards to cluster management. 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/0 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. skycluster starts the strategy process as part of it's supervision tree.

Currently it's required that strategies connect nodes via the Erlang distribution protocol, i.e. with Node.connect/1, :net_adm.connect_node/1, etc. In the future I plan on supporting alternate methods of clustering, but it's still unclear on how to properly do so.

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 :skycluster,
  strategy: {:system, :atom, LIBCLUSTER_STRATEGY, Cluster.Strategy.Gossip},
  port: {:system, :integer, LIBCLUSTER_MULTICAST_PORT, 45892},
  if_addr: {:system, LIBCLUSTER_MULTICAST_PORT_IF_ADDR, {0,0,0,0}},
  multicast_addr: {:system, LIBCLUSTER_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: {:system, :integer, LIBCLUSTER_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 :skycluster,
  strategy: { :system, "LIBCLUSTER_STRATEGY", Cluster.Strategy.Kubernetes },
  kubernetes_selector: { :system, "LIBCLUSTER_KUBERNETES_SELECTOR", "app=my_app" },
  kubernetes_node_basename: { :system, "LIBCLUSTER_KUBERNETES_NODE_BASENAME", "my_app" }

And in vm.args:

-name myapp@10.128.0.9
-setcookie test

Cluster Events

You can subscribe/unsubscribe a process to cluster events with Cluster.Events.subscribe(pid) and cluster.Events.unsubscribe(pid). Currently, only two events are published to subscribers:

  • {:nodeup, node} - when a node is connected, the node name is an atom
  • {:nodedown, node} - same as above, but occurs when a node is disconnected

Events are sent to subscribers with Kernel.send/2, so if subscribing a gen_* process, you'll receive them via the handle_info/2 callback.

License

MIT