gistfile1.rb

def all_servers
  # go figure...
  [:server_1, :server_2, :server_3]
end

def run(stuff)
  @active_servers.each {|s| puts "Running #{stuff} on #{s}" }
end

def on(server, &block)
  @active_servers = Array(server)
  yield
ensure
  @active_servers = all_servers
end

on :server1 do
  run "cat /dev/rand"
end