code: public static Message buildDiff(Zxid lastZxid) {
      ZabMessage.Zxid lz = toProtoZxid(lastZxid);
  
      Diff diff = Diff.newBuilder().setLastZxid(lz).build();
  
      return Message.newBuilder().setType(MessageType.DIFF)
                                 .setDiff(diff)
                                 .build();
    }
docs: Creates a DIFF message.
  
  @param lastZxid the last zxid of the server who initiates the sync.
  @return a protobuf message.