The cold swap migration strategy implements the ProcessHub.Strategy.Migration.Base protocol. It provides a migration strategy where the local process is terminated before starting it on the remote node.

Cold swap is a safe strategy if we want to ensure that the child process is not running on multiple nodes at the same time.

This is the default strategy for process migration.

State Handover

When handover: true is set, ColdSwap will:

1. Query state from local processes before termination
2. Store states in ETS with TTL
3. Terminate local processes
4. Send start requests to new nodes
5. When new processes are registered, deliver stored state via hook

To use state handover, your GenServer must use ProcessHub.Strategy.Migration.ColdSwap:

defmodule MyServer do
  use GenServer
  use ProcessHub.Strategy.Migration.ColdSwap

  # Optionally override these callbacks:
  # def prepare_handover_state(state), do: state
  # def alter_handover_state(_current, handover), do: handover
end

State Handover with Replication

State handover is not supported when using the ProcessHub.Strategy.Redundancy.Replication strategy. With replication, multiple instances of a process run across the cluster, making state handover semantics undefined. If you attempt to use handover: true with replication, the hub will fail to start with {:error, {:invalid_config, :handover_with_replication_not_supported}}.