The ability to browse through the content of World Wide Web in an automated manner is performed by web crawler. It is also referred to as the spiders, web robots, automatic indexers, bots and ants. The process that is implemented by a web crawler is often referred to as spidering or web crawling. Many search engines utilize the process in order to provide updated information about the number of websites that are added to the World Wide Web. A search engine uses the program in order to store the most visited pages of a website which are processed for later usage.
A web crawler is often used by major search engines as in automated maintenance process to check out a validation of HTML code. It also has the ability to check out for information from different WebPages in order to harvest e-mail addresses. The crawler usually visits the URLs of a website, which are often defined as hyperlinks in the page. It has the ability to download a limited quantity in a given set of time. Due to this reason, a web crawler is often made to prioritize the list of downloads. Any change in the websites will often create such a situation.
Due to the enormous expansion of World Wide Web, search engines are almost covering a little amount of the total content that is available publicly. A survey carried out in the year 2005 showed that major search engines index around a maximum of 70% of the total web area. A web crawler has the ability to download a web page. However, it should not be any a random sample, but should be relevant to the search that is being carried out by a customer using the search engine. The priority of the website is based on the functionality, the traffic, the popularity of links present in it and the page rank.
Web crawlers are also known to download WebPages that are very much in similar. This process is termed as topical crawlers or focused crawler. The performance of this particular procedure will greatly depend on the popularity of the link for the topic that is being searched. It usually depends on the search engine for the initial start of the search. Web crawler can also be used for restricting links, normalizing an URL, implement path ascending crawling and revisit policy that helps segregate the amount of information present over the Internet.