Singleton

Convenience wrapper library around Erlang's global module to ensure a single instance of a process is kept running on a cluster of nodes.

Installation

The package can be installed as:

1. Add singleton to your list of dependencies in mix.exs:

def deps do
  [{:singleton, "~> 1.0"}]
end

2. Ensure Singleton.Supervisor is added to your application's supervision tree:

If your application includes a supervision tree in application.ex, you can simply add Singleton.Supervisor to the list of children.

children = [
  # ...,
  {Singleton.Supervisor, name: MyApp.Singleton}
]

supervisor = Supervisor.start_link(children, opts)

Usage

Use Singleton.start_child/3 to start a unique GenServer process.

Singleton.start_child(MyApp.Singleton, MyServer, [1], {:myserver, 1})

Execute this command on all nodes. The MyServer GenServer is now globally registered under the name {:global, {:myserver, 1}}.

As soon as you connect nodes together, you'll see logger messages like:

04:56:29.003 [info]  global: Name conflict terminating {MyServer, #PID<12501.68.0>}

When you now stop (or disconnect) the node on which the singleton process runs, you'll see it get started on one of the other nodes.

Troubleshooting

More than 3 singleton processes [info] Application singleton exited: shutdown

In case you run more than 3 singleton you'll need to increase the max_restarts of DynamicSupervisor.

config :singleton,
  dynamic_supervisor: [max_restarts: 100]

or add max_restarts to your singleton supervisor spec:

  {Singleton.Supervisor, name: MyApp.Singleton, max_restarts: 100}