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A web crawler is an automated program or script that browses pages and files on the Internet in a methodical, orderly fashion.The process called crawling is mainly used to create a copy of all the visited pages for later processing by a search engine that will index the downloaded pages to provide fast searches. Also known as web spiders, bots, or scutters, crawlers are generally mutithreaded, and often run in an distributed environment, employing thousands of individual computers to crawl the web.
How does crawling work?
In the nineties, all webmasters needed to do was submit the address of a page to the various engines which would send a spider to crawl that page, extract links to other pages from it, and return information found on the page to be indexed. Today’s crawlers are more advanced and do not rely on humans to submit pages, rather automatically navigate pages frequently, the frequency depending on that of change of pages, among other things. When a search engine’s web crawler visits a web page, it reads the visible text, hyperlinks, and content of the various tags used in the site, such as keyword rich meta tags. Using the information gathered from the crawler, a search engine will then determine what the site is about and index the information. The website is then included in the search engine’s database and its page ranking process.
Googlebot and deep crawling
Googlebot is Google’s web crawling robot, which finds and retrieves pages on the web and hands them off to the Google indexer, which helps return relevant results from the entire web in a matter of seconds. The Googlebot functions much like the web browser: it sends a request to a web server for a web page, downloads the entire page, then hands it off to Google’s indexer, using an algorithmic process to determine which sites to crawl, how often, and how many pages to fetch from each site.
When Googlebot fetches a page, it culls all the links appearing on the page and adds them to a queue for subsequent crawling. By harvesting links from every page it encounters, Googlebot can quickly build a list of links that can cover broad reaches of the web. This technique, known as deep crawling, also allows Googlebot to probe deep within individual sites. Because of their massive scale, deep crawls can reach almost every page on the WWW. Because the web is vast, this can take some time, so some pages may be crawled only once a month.
Other uses of crawling
Preparing indexes to be used for search is not the only application crawlers find; they can also be used for automating maintenance tasks on a website such as checking links or validating (rendered) HTML code.
Linguists may use a web crawler to perform a textual analysis; i.e., they may comb the Internet to determine what words are commonly used today. Market researchers may use a web crawler to determine and assess trends in a given market.
There are numerous nefarious uses of web crawlers as well, they can be used to gather specific types of information from webpages, such as harvesting e-mail addresses. Such crawlers are called email spambots, twitter bots or forum bots depending on what they crawl upon.
SEO and crawling
Increasing the rate of crawling is one of the ways for Search Engine Optimization, and making your pages appear higher on search results. The higher rate can be obtained by updating the content of your site regularly, ensuring a fast server response, adding a sitemap, and getting more back links from regularly crawled sites. However, you cannot force Googlebot, or any other crawler, to visit your site more often, you can only invite them hoping they would oblige..