Maracuja

A microlibrary that helps you spawn a process that lives at most once per cluster.

It can cope with netsplits by checking if the count of the currently connected nodes would have an absolute majority in the historically biggest known cluster size.

How it handles net splits

With an exemplary amount of 5 nodes in a healthy cluster, imagine the following split scenarios:

5: No split - one of the nodes will host the singleton

4 - 1: The cluster with 4 nodes will host the singleton

3 - 2: The cluster with 3 nodes will host the singleton

1 - 2 - 2: None of the clusters will host the singleton because none of them have an absolute majority

1 - 2 - 1 - 1: Even though the cluster with 2 nodes would have the majority, it doesn't know that it's the cluster with the most nodes, so it doesn't host the singleton

1 - 1 - 1 - 1 - 1: None of these nodes will host the singleton either

As you can see, it is possible that in some net split situations, it is possible that no singleton is hosted. As such, this library is not for you if you often experience net splits and don't recover quickly from them.

Usage

In the start_link callback of your GenServer, instead of directly using the GenServer module to start the server, use Maracuja.start_link/3:

The Maracuja will call your module with start_server/2 when it has decided to start the module in this cluster.

def MySingleton do
  use GenServer

  def start_link(args) do
    Maracuja.start_link(__MODULE__, args, :my_global_name)
  end

  def start_server(args, name) do
    GenServer.start_link(__MODULE__, args, name: name)
  end

  # init and rest of the server
end

Maracuja uses :global to register the name, so you can find the pid of a singleton by passing its name to :global.whereis_name:

iex> :global.whereis_name(:my_global_name)
#Pid<0.170.0>

In the case the cluster is experiencing a net split and the current node is part of a faction that's too small, whereis_name will return :undefined!

Planned features

• Support for other behaviours
• Maybe support for other coping strategies

Installation

The package can be installed by adding maracuja to your list of dependencies in mix.exs:

def deps do
  [
    {:maracuja, "~> 0.2.0"}
  ]
end

Documentation can be found at https://hexdocs.pm/maracuja.