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What is crawling in information retrieval?

by Time Project
What is crawling in information retrieval?

It is hard to go through a day without searching for information online. The internet is full of useful and easily accessible data, but we rarely think about how search engines work. There is a lot we can benefit from understanding them well, and web crawling is a key term for this.

Information retrieval

Information retrieval is the activity of collecting relevant data from a system where it’s stored unstructured. Most commonly, the data is textual, but it can also take the form of a video, image, or sound.

An information retrieval system is a set of algorithms that ranks the search results and displays them to the user. The system can pinpoint to the user where the needed information is located, also allowing easy browsing, organizing, and representing of data. All this is possible by associating keywords with data points.

Although these terms seem alien at first, millions of people use information retrieval on a daily basis. Searching various databases of government institutions, libraries, or academic indexes is only a small part of the searches happening every minute. Most of the information retrieval is happening on the world wide web.

Since it is the largest and most widely used database, effective searches require some extraordinary measures. Web crawling is the very first stage of any information retrieval system online. Only after crawling is done can such systems use ranking algorithms and display the results to users. 

What is web crawling?

Web crawling is a method of browsing and indexing the world wide web with the help of online bots. Bots, such as the famous Googlebot, crawl the internet constantly and note website content. Ranking algorithms can use these archives to associate them with keywords and complete search queries.

The bots used for web crawling are also half-jokingly called “spider bots,” as the process resembles that of a spider crawling its web. The bot starts by gathering a list of URLs in the specific order they should be crawled. Then it fetches and indexes all the content while looking for new URLs to crawl.

Most websites are welcoming of such spiders as it means that their websites are indexed and possible to find on search engines. These days search engines are the best source of internet traffic.

However, some boundaries are being set constantly as too much load from crawler bots can significantly reduce the website’s usability or even crash it completely. Rules for crawlers are outlined in a document called the robots exclusion protocol or robots.txt.

Despite search engines, other companies and individuals also have the incentive to crawl the web, and they also must obey the exclusion protocol or risk getting their IP address banned. Therefore using proxies to change your IPs is essential.

How does crawling differ from scraping?

One important pitfall to avoid is not differentiating web crawling vs. web scraping. While crawling can be important for collecting data, as you must have an index before you can extract the data, it’s not the same as scraping. Both use bots, but only web scraping is a method of extracting data.

You do not necessarily need a scraper bot to extract the data. The simplest method of data collection is to copy and paste what you need to your hard drive. It is a viable option for smaller data sets, but it gets inefficient when you need more data.

Therefore, web scraping is usually an automated process that uses scraper bots to load websites, find the needed text, CSS, or JavaScrip elements, and then convert them to a convenient format. Usually, it is a spreadsheet format (CSV), but it can also be XML or JSON.

Differently from crawling, scraping does not target the whole website. Instead, it can focus only on the needed data, but such a possibility is available when the website is crawled first. That’s why the web crawling vs. web scraping comparison raises some confusion. If you are interested to learn more about web crawling vs. web scraping, you can read the article here.

Information retrieval systems – such as web search engines, do not necessarily scrape the data. Googlebot, for example, only crawls and makes an index, which gives less load to websites. Scraping the websites requires more precautions, such as proxies or HTTP header optimization.

Use cases

There are two primary use cases of web crawling, and we already cover the main one – internet indexing, where search engines use crawler bots to enable web search. Knowing crawlers function in information retrieval allows marketers to reverse engineer their principles and build search engine optimization (SEO) tools.

SEO is a practice of increasing the quality and quantity of web traffic from search engines. Understanding crawling is crucial here because your website can be optimized for crawlers to visit. Successful site optimization leads to it being better archived and more visible online.

Building a correct sitemap, robots.txt, and ensuring that all the hyperlinks work are some actions you can take to make the website more available for search engine crawlers. Of course, these are only a few of the steps. More work is usually put into correct content, keywords more specifically.

Web crawling (along with web scraping) is used to collect data from search engine result pages (SERP). They reflect what keywords people use most often. Knowing them can help build strategies for placing your webpage on top of the results page. SERPs are constantly changing, so web crawlers must monitor them.

To complicate things more, different SERPs appear to different users depending on their geolocation, browsing habits, used devices, and other metrics. You must use additional tools with your crawlers to collect such data. Proxies are the main tool allowing you to change your location and show a different device.


Web crawling is a fundamental concept for online information retrieval. Without a complete index, no ranking can be established, and no search results can be displayed. By understanding how it works, we can use it to our advantage and make our websites more visible to search engines. It is a topic worth discovering.

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