Web Robot

现在的互联网非常巨大，不可能通过一台或几台计算机服务器就能完成下载任务.因此，一个商业的网络爬虫需要有成千上万个服务器，并且由快速网络连接起来。如何建立这样复杂的网络系统，如何协调这些服务器的任务，就是网络设计和程序设计的艺术了。
The most common use for Web robots is to index a site for a search engine. But robots can be used for other purposes as well. Some of the more common uses are,for
example:
Change monitoring - There are services available on the Web that will tell you when a Web page has changed. These services are done by sending a robot to the page periodically to evaluate if the content has changed. When it is different, the robot would file a report.
another example:
Web site mirroring - Similar to the change monitoring robots, these robots evaluate a site, and when there is a change, the robot will transfer the changed information to the mirror site location.