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List Crawling Is a Type of Process That Involves a Computer Program Repeatedly Accessing a Webpage and Collecting Data From It. It’s Generally Done for Reasons Including Commercial Advertising, Heat Mapping, As a Research Tool, a Form of Data Mining and to Catalogue the Content on a Website for Indexing.
The number of possible URLs crawled being generated by server-side software has also made it difficult for web crawlers to avoid retrieving duplicate content. Endless combinations of HTTP GET (URL-based) parameters exist, of which only a small selection will actually return unique content. For example, a simple online photo gallery may offer three options to users, as specified through HTTP GET parameters in the URL. If there exist four ways to sort images, three choices of thumbnail size, two file formats, and an option to disable user-provided content, then the same set of content can be accessed with 48 different URLs, all of which may be linked on the site. This mathematical combination creates a problem for crawlers, as they must sort through endless combinations of relatively minor scripted changes in order to retrieve unique content.
Search engines like Google, Bing, and Yahoo use crawlers to properly index downloaded pages so that users can find them faster and more efficiently when they are searching. Without web crawlers, there would be nothing to tell them that your website has new and fresh content. Sitemaps also can play a part in that process. So web crawlers, for the most part, are a good thing. However there are also issues sometimes when it comes to scheduling and load as a crawler might be constantly polling your site. And this is where a robots.txt file comes into play. This file can help control the crawl traffic and ensure that it doesn't overwhelm your server. (Source: www.keycdn.com)