A search engine spider, also known as a web crawler, is an internet bot that crawls websites and stores information for the search engine to index. Its purpose is to index the content of websites across the Internet so that those websites can appear in search engine results. A search engine spider is a program that works behind the scenes to deliver up-to-date web search results to users through a search engine. The search engine spider can work continuously or respond to user events.
Usually, the search engine spider uses specific and ordered methods to scan web text and index pages for ranking in engines. Web search engines and some other websites use web crawling software to update their web content or the web content indexes of other sites. Web crawlers copy pages for processing by a search engine, which indexes downloaded pages so that users can search more efficiently. A web crawler is also known as a spider, ant, automatic indexer or (in the context of the FOAF software) web browser. A web crawler starts with a list of URLs to visit.
Those first URLs are called seeds. As the crawler visits these URLs, by communicating with the web servers that respond to those URLs, it identifies all hyperlinks on the retrieved web pages and adds them to the list of URLs to visit, called the tracking border. Border URLs are recursively visited according to a set of policies. If the crawler performs website archiving (or web archiving), it copies and saves the information as it goes. Usually, files are stored in such a way that they can be viewed, read and browsed as if they were on the live web, but they are kept as “snapshots”.
The Web is very dynamic in nature, and crawling a fraction of the Web can take weeks or months. By the time a web crawler has finished its crawl, many events could have occurred, including creations, updates, and deletions. Cho and García-Molina demonstrated the surprising result that, in terms of average freshness, uniform policy outperforms proportional policy in both a simulated web crawl and a real web crawl. Intuitively, the reasoning is that since web crawlers have a limit on the number of pages they can crawl in a given period of time, (they will assign too many new crawls to pages that change quickly at the expense of updating them less frequently) and (the freshness of rapidly changing pages lasts a shorter period than changing pages less frequently. In other words, a proportional policy allocates more resources to crawling pages that are updated frequently, but experiences less overall refresh time. Web crawlers are a central part of search engines, and details about their algorithms and architecture are kept as trade secrets.
When tracker designs are published, there is often a significant lack of detail that prevents others from reproducing the work. Concerns are also emerging about search engine spam, which prevent major search engines from publishing their ranking algorithms. Web crawlers are usually identified on a web server by using the User-agent field in an HTTP request. Website administrators typically examine the log of their web servers and use the user agent field to determine which crawlers have visited the web server and how often. The user agent field may include a URL where the website administrator can find more information about the crawler. Examining the web server log is a tedious task and, therefore, some administrators use tools to identify, track, and verify web crawlers.
Spam bots and other malicious web crawlers are unlikely to place identifying information in the user agent field, or they may mask your identity as a browser or other known crawler. Website administrators prefer that web crawlers identify themselves so that they can contact the owner if necessary. In some cases, trackers may accidentally get caught in a tracker trap or they may be overloading a web server with requests, and the owner must stop the tracker. Identification is also useful for administrators who are interested in knowing when they expect their web pages to be indexed by a particular search engine. Search engine spiders are the foot soldiers of the search engine world. A search engine like Google has certain things that it wants to see on a highly ranked site.
The crawler moves around the web and carries out the will of the search engine. Search engines go from one web page to another, following the links found on each page. This process is known as web crawling. The program that is dragged is known as a spider or bot.
Once a spider finds a web page, it places it in the search engine index. When a search is performed, the results are extracted from the index and sorted according to the search engine's algorithm. Basically, you want web crawlers to focus on pages full of content, but not on pages like thank you messages, admin pages, and internal search results. The performance of a focused crawl depends primarily on the richness of links in the specific topic being searched, and a focused crawl generally relies on a general web search engine to provide starting points.
When you search for something on Google, those pages and pages of results can't materialize out of thin air. As search engines always want to deliver the latest and most relevant data, search engine spiders are constantly crawling the web for new information and updates to add to their library. Of course, Google is the big dog of the search engine world so when optimizing your website it's better to consider Google spiders above all. The importance of a page depends on its intrinsic quality, its popularity in terms of links or visits, and even its URL (the latter is especially true with vertical search engines restricted to one top-level domain or search engines restricted to one fixed website).