What is Web Crawling?
Web crawling is a process which involves web crawlers to systematically browse the World Wide Web, for the purpose of Web indexing. Web crawlers often called as spiders or spider-bot. Search engines such as Google, Yahoo uses these web crawlers to update their web content or indices of others sites web content.
Web crawlers gather information such the URL of a particular website, the Web page content in it, information about Meta tag, various links provided in the web-page and the destinations from those links, the web page title and any other relevant information are extracted. Web crawlers keep track of the URLs which have already been downloaded to avoid downloading the same page again. A combination of policies such as selection policy, politeness policy, parallelization policy and re-visit policy decides the behavior of the Web crawler. There are many challenges for web crawlers, namely the large and continuously evolving World Wide Web, content selection trade-offs, dealing with adversaries and social obligations.
Web Crawling and indexing
When search engine crawlers visit any link is called crawling and when crawlers save or index that links in search engine database, it is called indexing
Indexing is the process of generating index for all the fetched web pages and keeping them into a huge database from where it can later be retrieved.
Index is another name for database use by a search engine. It contains information’s on all the websites the search engine was able to find. If a website is not in a search engine’s index, internet traffic will not be able to find it using that search engine therefore search engines regularly update their indexes.
How does it work?
In order to crawl a website bots need to know that your website exists so they can come to it. Back in the days one would have submit website to search engines in order to tell them your website was online. Now you can easily put links to your website
Once a crawler lands on your website it examine, inspect and analyses all your content line by line and follows each links you have whether they are internal or external links. This process goes until it lands on a page with no more links available.
From a more technical point of view a crawler works with a list of URLs(called seeds). This is then has to pass on to a Fetcher which will retrieve the content of a page. Next, the content should move on to a Link extractor which will parse the HTML and extract all the links present. These links are send to a Store processor to store them. URLs also go through a Page filter which will send all links to a URL-seen module. Analyzing of URL is checked by the module. If not it gets sent to the Fetcher which will retrieve the content of the page and so on the whole process is done.
Crawling the Deep Web
Each search engine constantly crawls the Internet. It then indexes the pages it crawls and ranks them according to relevance of data and content. Depending on algorithms, a search engine can either confirm the presence of a page without indexing it, or index the page content and look for hyperlinks on a page. The frequency of the crawling websites depends on the search engine’s discretion.
However, search engines have some limitations as they operate on some fixed algorithms. Often leading to irrelevant results because the search engine is sometimes not able to contextualize. Also, search engine bots only crawl fixed Web pages. The search results miss out on the data in several databases, such as those of universities and government organizations, among others. All this adds up to large numbers, making the search results a fraction of the total data available over it.
A vast amount of web pages lie in the invisible web or deep. These pages are typically only accessible by submitting queries to a database, and regular crawlers are unable to find these pages if there are no links that point to them
Web Crawling and Data Security
While most of the website owners are eager to have their pages indexed as broadly as possible to have more traffic in their sites. Web crawling can also have unintended consequences that may lead to a data breach. If a search engine indexes resources that it shouldn’t allow to publicly available, or pages disclosing vulnerable versions of software.
Apart from web security recommendations, owner of a website can reduce their exposure to hacking. By allowing search engines to index the public parts of their websites with commands like robots.txt and blocking them from indexing transaction parts such as private pages, login pages, etc.
This is where robots.txt file becomes very useful. It tells crawlers such as Google-bot or MSN Bot – what pages they cannot crawl. For example, you have navigation using facets, you do not want robots to crawl all of them as they have added value and will use crawl budget. Using this command line will help you prevent any robot from crawling over your data.
You can also use indication in HTML which tells robots not to follow that specific link using rel=”nofollow” tag in it. Some tests have shown using the rel=”nofollow” tag on a link won’t block Google-bot from crawling it. This is contradictory to its purpose, but will be useful in other related cases
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