core object:
master:  queuedownloaderprocesser
worker:  queue(master) => worker

ظжʱ
ǰ(!downloading && !downloaded)صqueueǰ(!downloading && !downloaded)

url״̬
founddownloadingdownloaded

fllow:



                              Scheduler(queue)
							    /\    -
								|	      -
										       -
								|			        -
													     -
		                        |                            -+ \
															   downloader
          DatabasePipe      |                               -
														    -
								|					    -
												    -
								|				-
											-
								|		/+-
							 processer()



