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Webpage monitoring issue

There are several different websites that allow you to track specific web pages for any changes, such as watchthatpage.com or page2rss.com

I am wondering how these sites work, namely how they determine if any web page is being updated. Do they just copy all the text from the page, store it in memory and compare it later with the contents of the page? Or maybe they are looking for some specific html elements and comparing their values?

Please help me find the answer.

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3 answers

How it works: http://www.watchthatpage.com/information.jsp

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I suspect that they store all the contents, and every time they check, they compare. If different, send a warning, otherwise not.

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There are two ways that this can be done only from the head.

First, pull out the HTML and make a simple string.compare file.

The second way is to execute a HEAD request. See section 9.4 here

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