schism

defmodule Schism do
  @moduledoc """
  Schism allows you to create network partitions in erlang nodes without
  needing to leave elixir.

  Let's say that we have 5 nodes and we want to test what happens when they
  disconnect from each other. We can use Schism like so:

  ```elixir
  test "netsplits" do
    [n1, n2, n3, n4, n5] = nodes

    # Partition our nodes
    Schism.partition([n1, n3])
    Schism.partition([n4])
    Schism.partition([n2, n5])

    # Test some stuff...

    # Heal our partitions
    Schism.heal([n1, n3])
    Schism.heal([n2, n4, n5])
  end
  ```

  This api is useful for testing and development in conjunction with tools like
  [local cluster](https://github.com/whitfin/local-cluster) and
  [propcheck](https://github.com/alfert/propcheck).
  """

  @doc """
  Creates a partition amongst a set of nodes. Any nodes in the partition
  will be able to see each other but no other nodes in the network. The
  partitioned nodes will still be able to see the node that induced the
  partition. Otherwise we would not be able to heal the partition.
  """
  @spec partition([Node.t], String.t) :: [Node.t] | none()
  def partition(nodes, id \\ random_string()) when is_binary(id) do
    manager = Node.self()

    for node <- nodes do
      # Set the remote nodes cookie to a different value
      true = :rpc.call(node, :erlang, :set_cookie, [node, String.to_atom(id)])

      # Force the node to disconnect from all other nodes that aren't us or in
      # the partition
      node
      |> :rpc.call(Node, :list, [])
      |> Enum.reject(fn n -> n == manager || n in nodes end)
      |> Enum.each(fn n -> :rpc.call(node, Node, :disconnect, [n]) end)

      # Ensure we can still talk to the node
      :pong = Node.ping(node)
    end

    nodes
  end

  @doc """
  Re-connects the nodes to the cluster.
  """
  @spec heal([Node.t]) :: [Node.t] | none()
  def heal(nodes) do
    for n <- nodes do
      # Reset the node and ensure we can still talk with it
      true = :rpc.call(n, :erlang, :set_cookie, [n, :erlang.get_cookie()])
      :pong = Node.ping(n)

      for other_node <- nodes do
        :rpc.call(n, Node, :connect, [other_node])
      end
    end
  end

  defp random_string do
    :crypto.strong_rand_bytes(10)
    |> Base.url_encode64
    |> binary_part(0, 10)
  end
end