config.ru

class BalancerState
  InvalidStatus = Class.new(StandardError)

  def initialize(disk_access = File)
    @disk_access = disk_access
  end

  def call(env)
    if balancer_member?
      [200, {'Content-Type' => 'application/json'}, ['{"status": "ok", "member": true}']]
    else
      [503, {'Content-Type' => 'application/json'}, ['{"status": "ok", "member": false}']]
    end
  rescue InvalidStatus
    [500, {'Content-Type' => 'application/json'}, ['{"status": "unknown status"}']]
  end

  private

  def balancer_member?
    state = @disk_access.read(File.expand_path('../config/balancer_state', __FILE__)).strip
    raise InvalidStatus, "Invalid state: #{state}" unless %w(on off).include?(state)
    state == "on"
  end
end

app = Rack::Builder.app do
  map '/load_balancer_status' do
    run BalancerState.new
  end

  map '/' do
    run lambda { |env| [200, {'Content-Type' => 'text/plain'}, ['Je hebt mij bereikt!']] }
  end
end

run app