first draft, untested, and doing terrible things with negative indexes

gistfile1.rb

# desired_cluster is a hash of roles and their counts (e.g., {web: 2, redis: 1}
def equalize_servers fog, app, environment, desired_cluster
  current_cluster = filtered_servers(fog, app, environment).group_by { |server| server.tags["role"] }
  to_build = {}
  to_destroy = []

  current_cluster.each do |role, servers|
    if desired_cluster.has_key? role
      # we want some, but do we have the right amount?
      difference = desired_cluster[role] - servers.count
      if difference > 0
        to_build[role] = difference
      elsif difference < 0
        to_destroy += servers[difference..-1]
      end
    else
      # we don't want any
      to_destroy += servers
    end
  end

  fork { build_servers fog, app, environment, to_build }
  fork { kill fog, app, environment, to_destroy }
  Process.waitall
end