This behaviour can be used to provide custom modifications of relup instructions

when a release upgrade is built by edeliver.

By default the implementation from Edeliver.Relup.PhoenixModification is used for phoenix applications and for all others the implementation from Edeliver.Relup.DefaultModification.

Implementations can modify the relup instructions step by step by using modules implementing the Edeliver.Relup.Instruction behaviour.

The implementation returning the highest priority/0 or which is passed by the --relup-mod= command line option will be used unless the --skip-relup-mod option is set.

Example:

defmodule Acme.Relup.Modification do
  use Edeliver.Relup.Modification

  def modify_relup(instructions = %Instructions{}, config = %{}) do
    instructions |> Edeliver.Relup.DefaultModification.modify_relup(config) # use default modifications
                 |> log_upgrade # add custom modification which logs the upgrade
  end

  defp log_upgrade(instructions = %Instructions{up_instructions: up_instructions}) do
    log_instruction = {apply, {:Elixir.Logger, info, [<<"Upgraded successfully">>]}}
    %{instructions| up_instructions: [log_instruction|up_instructions]}
  end
end