View Source Process state handover

In this guide we will show a simple example of how to handover the state of a process from one node to another.

ProcessHub this scenario is happening automatically when the cluster has received new node event, meaning that the new node has joined the cluster or left.

Suppose we have the following GenServer process:

defmodule MyProcess do
  use GenServer

  def start_link(state) do
    GenServer.start_link(__MODULE__, state)
  end

  def init(state) do
    {:ok, state}
  end
end

In order to handover the state of the process from one node to another, we need to do the following:

1. Configure the hub

Configure the ProcessHub to use the Hotswap strategy.

defmodule MyApp.Application do
  use Application

  def start(_type, _args) do
    children = [process_hub()]

    opts = [strategy: :one_for_one, name: MyApp.Supervisor]
    Supervisor.start_link(children, opts)
  end

  defp process_hub() do
    {ProcessHub, %ProcessHub{
      hub_id: :my_hub,
      migration_strategy: %ProcessHub.Strategy.Migration.HotSwap{
        retention: 3000,
        handover: true
      },
    }}
  end
end

Pay attention to the retention and handover options. The retention option is the time in milliseconds that the process state will be kept in the hub after the process has been stopped. The handover option is a boolean that indicates whether the process state should be handed over to another node when the process is going to be stopped.

Retention option

The retention option does not mean that the old process will be kept that long, actually the old process will be terminated as soon as it has sent its state to the new process. The retention option is used to limit the time old process is kept alive.

2. Implement the neccessary callbacks

Our MyProcess have to also implement the ProcessHub.Strategy.Migration.Handover behaviour or define the necessary callbacks yourself.

defmodule MyProcess do
  use GenServer
  use ProcessHub.Strategy.Migration.HotSwap # This has been added

  # ...
end

3. That's it!

Now the ProcessHub will take care of the rest. When the process is going to be redirected to another node, the state of the process will be handed over to the new process on the new node.

More info can be found in the Hotswap module

Check out the Hotswap module for more information about the Hotswap strategy.