Semalt Islamabad Expert – What You Need To Know About A Web Crawler
A search engine crawler is an automated application, script or program that goes over the World Wide Web in a programmed manner to provide updated information for a particular search engine. Have you ever wondered why you get different sets of results each time you type the same keywords on Bing or Google? It is because webpages are being uploaded every minute. And as they are being uploaded web crawlers run over the new web pages.
Michael Brown, a leading expert from Semalt, tells that web crawlers, also known as automatic indexers and web spiders, work on different algorithms for different search engines. The process of web crawling begins with the identification of new URLs that should be visited either because they have just been uploaded or because some of their web pages have fresh content. These identified URLs are known as seeds in search engine term.
These URLs are eventually visited and re-visited depending on how often new content is uploaded to them and the policies guiding the spiders. During the visit, all the hyperlinks on each of the web pages are identified and added to the list. At this point, it is important to state in clear terms that different search engines use different algorithms and policies. This is why there will be differences from the Google results and Bing results for the same keywords even though there will be a lot of similarities too.
Web crawlers do tremendous jobs keeping search engines up-to-date. In fact, their job is very difficult because of three reasons below.
1. The volume of web pages on the internet at every given time. You know there are several millions of sites on the web and more are being launched every day. The more the volume of the website on the net, the harder it is for crawlers to be up-to-date.
2. The pace at which websites are being launched. Do you have any idea how many new websites are launched every day?
3. The frequency at which content are changed even on existing websites and the addition of dynamic pages.
These are the three issues that make it difficult for web spiders to be up-to-date. Instead of crawling websites on the first-come-first-served basis, a lot of web spiders prioritize web pages and hyperlinks. The prioritization is based on just 4 general search engine crawler policies.
1. The selection policy is used for selecting which pages are downloaded for crawling first.
2. The re-visit policy type is used for determining when and how often web pages are revisited for possible changes.
3. The parallelization policy is used to coordinate how crawlers are distributed for quick coverage of all the seeds.
4. The politeness policy is used determine how URLs are crawled to avoid overloading of websites.
For fast and accurate coverage of seeds, crawlers must have a great crawling technique that allows prioritization and narrowing down of web pages, and they must also have highly optimized architecture. These two will make it easier for them to crawl and download hundreds of millions of web pages in a few weeks.
In an ideal situation, each web page is pulled from the World Wide Web and taken through a multi-threaded downloader after which, the web pages or URLs are queued up before passing them through a dedicated scheduler for priority. The prioritized URLs are taken through multi-threaded downloader again so that their metadata and text are stored for proper crawling.
Currently, there are several search engine spiders or crawlers. The one used by Google is the Google Crawler. Without web spiders, search engine result pages will either return zero results or obsolete content since new web pages would never be listed. In fact, there will not be anything like online research.