handler.rb

class MyCommandHandler
  def initialize
    make_threadsafe_by_stateless
  end

  def call(cmd)
    local_var = cmd.something
    output(local_var)
  end

  private

  def make_threadsafe_by_stateless
    freeze
  end

  def output(local_var)
    puts(local_var)
  end
end

CMD_HANDLER = MyCommandHandler.new