# `PhoenixMountable.Singleton`
[🔗](https://github.com/hattoriai/phoenix_mountable/blob/v0.2.1/lib/phoenix_mountable/singleton.ex#L1)

Runs a process once per cluster.

Add it to a mount's children to run one instance of a process across all
nodes, such as a scheduler that must not run twice:

    def children(mount) do
      [
        {PhoenixMountable.Singleton,
         name: {:refresh_scheduler, PhoenixMountable.Mount.key(mount)},
         child: {MyLib.RefreshScheduler, mount: mount}}
      ]
    end

Every node runs a `Singleton`. The first to register `:name` globally owns
it and starts the child; the others watch the owner. When the owner's node
goes down, the next one registers and starts the child. A child that exits
is restarted by its owner. On a single node it simply runs the child.

`whereis/1` returns the child, on whichever node it runs.

## Options

  * `:name` (required) — a term naming the singleton across the cluster.
  * `:child` (required) — the child spec of the process.
  * `:retry` — milliseconds before restarting a child that failed to start
    or exited. Defaults to `1_000`.

# `whereis`

```elixir
@spec whereis(term()) :: pid() | nil
```

The pid of the singleton's child named `name`, on any node, or `nil`.

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*Consult [api-reference.md](api-reference.md) for complete listing*