def failed
      @brokers.inject([]) { |c, b| b.failed? ? c << b.identity : c }
    end
def failed
      @brokers.inject([]) { |c, b| b.failed? ? c << b.identity : c }
    end
Get serialized identity of failed broker clients, i.e., ones that were never successfully
 connected, not ones that are just disconnected

 === Return
 (Array):: Serialized identity of failed broker clients