Highlander allows you to run a single globally unique process in a cluster. (h/t @tuxified for the name)

Highlander uses erlang's :global module to ensure uniqueness, and uses child_spec.id as the uniqueness key.

Highlander will start its child process just once in a cluster. The first Highlander process will start its child, all other Highlander processes will monitor the first process and attempt to take over when it goes down.

Note: You can also use Highlander to start a globally unique supervision tree.

HighlanderPG

Highlander has a sister library called HighlanderPG, which is backed by Postgres advisory locks. If you need better guarantees of uniqueness or can not use erlang clustering (eg, in Heroku) then this library can be a good alternative.

Subscriptions to HighlanderPG support its maintenance and further development.

Usage

Simply wrap a child process with {Highlander, child}.

Before:

children = [
  child_spec
]

Supervisor.init(children, strategy: :one_for_one)

After:

children = [
  {Highlander, child_spec}
]

Supervisor.init(children, strategy: :one_for_one)

See the documentation on Supervisor.child_spec/1 for more information.

Determining global uniqueness

child_spec.id is used to determine global uniqueness. Check the debug logs if you are unsure what is being used.

Globally unique supervisors

You can also have Highlander run a supervisor:

children = [
  {Highlander, {MySupervisor, arg}},
]

Handling netsplits

If there is a netsplit in your cluster, then Highlander will think that the other process has died, and start a new one. When the split heals, :global will recognize that there is a naming conflict, and will take action to rectify that. To deal with this, Highlander simply terminates one of the two child processes with reason :shutdown.

To catch this, simply trap exits in your process and add a terminate/2 callback.

Note: The terminate/2 callback will also run when your application is terminating.

def init(arg) do
  Process.flag(:trap_exit, true)
  {:ok, initial_state(arg)}
end

def terminate(_reason, _state) do
  # this will run when the process receives an exit signal from its parent
end