What Is a Web Crawler and How Does It Work?

  • 07/03/2020

What Is a Web Crawler And Indexing?

A web crawler (also known as a web spider, spider bot, web bot, or simply a crawler) is a computer software program that is used by a search engine to index web pages and content across the World Wide Web. 

What is Web Crawler, Web Spider, Web Crawling, Web Scraping, Crawler, Spider, Bot

Indexing is quite an essential process as it helps users find relevant queries within seconds. The search indexing can be compared to the book indexing. For instance, if you open the last pages of a textbook, you will find an index with a list of queries in alphabetical order and pages where they are mentioned in the textbook. The same principle underlines the search index, but instead of page numbering, a search engine shows you some links where you can look for answers to your inquiry. 

The significant difference between the search and book indices is that the former is dynamic, therefore, it can be changed, and the latter is always static.

How Does a Web Search Work?

Before plunging into the details of how a crawler robot works, let’s see how the whole search process is executed before you get an answer to your search query. 

For instance, if you type “What is the distance between Earth and Moon” and hit enter, a search engine will show you a list of relevant pages. Usually, it takes three major steps to provide users with the required information to their searches:

  • A web spider crawls content on websites
  • It builds an index for a search engine
  • Search algorithms rank the most relevant pages

Also, one needs to bear in mind two essential points:

  • You do not do your searches in real-time as it is impossible

There are plenty of websites on the World Wide Web, and many more are being created even now when you are reading this article. That is why it could take eons for a search engine to come up with a list of pages that would be relevant to your query. To speed up the process of searching, a search engine crawls the pages before showing them to the world.

  • You do not do your searches in the World Wide Web

Indeed, you do not perform searches in the World Wide Web but in a search index and this is when a web crawler enters the battlefield.

What Is a Web Crawler? How Does a Web Crawler Work?

There are many search engines out there − Google, Bing, Yahoo!, DuckDuckGo, Baidu, Yandex, and many others. Each of them uses its spider bot to index pages. 

They start their crawling process from the most popular websites. Their primary purpose of web bots is to convey the gist of what each page content is all about. Thus, web spiders seek words on these pages and then build a practical list of these words that will be used by a search engine next time when you want to find information about your query. 

All pages on the Internet are connected by hyperlinks, so site spiders can discover those links and follow them to the next pages. Web bots only stop when they locate all content and connected websites. Then they send the recorded information a search index, which is stored on servers around the globe. The whole process resembles a real-life spider web where everything is intertwined. 

Crawling does not stop immediately once pages have been indexed. Search engines periodically use web spiders to see if any changes have been made to pages. If there is a change, the index of a search engine will be updated accordingly.

Interesting Read: https://hirinfotech.com/ultimate-b2b-sales-guide-definition-steps-hacks/

What Are the Main Web Crawler Types?

Web crawlers are not limited to search engine spiders. There are other types of web crawling out there.

Email crawling, News crawling, Image crawling, Social Media Crawling, Video Crawling, Spider, Crawler

Email crawling 

Email crawling is especially useful in outbound lead generation as this type of crawling helps extract email addresses. It is worth mentioning that this kind of crawling is illegal as it violates personal privacy and can’t be used without user permission.  

News crawling

With the advent of the Internet, news from all over the world can be spread rapidly around the Web, and to extract data from various websites can be quite unmanageable. 

There are many web crawlers that can cope with this task. Such crawlers are able to retrieve data from new, old, and archived news content and read RSS feeds. They extract the following information: date of publishing, the author’s name, headlines, lead paragraphs, main text, and publishing language.

Image crawling

As the name implies, this type of crawling is applied to images. The Internet is full of visual representations. Thus, such bots help people find relevant pictures in a plethora of images across the Web. 

Social media crawling

Social media crawling is quite an interesting matter as not all social media platforms allow to be crawled. You should also bear in mind that such type of crawling can be illegal if it violates data privacy compliance. Still, there are many social media platform providers that are fine with crawling. For instance, Pinterest and Twitter allow spider bots to scan their pages if they are not user-sensitive and do not disclose any personal information. Facebook, LinkedIn are strict regarding this matter.

Video crawling  

Sometimes it is much easier to watch a video than read a lot of content. If you decide to embed Youtube, Soundcloud, Vimeo, or any other video content into your website, it can be indexed by some web crawlers.

Interesting Read: https://hirinfotech.com/top-8-python-based-web-crawling-and-web-scraping-libraries/

What Are Examples of Web Crawlers?

A lot of search engines use their own search bots. For instance, the most common web crawlers examples are:

Alexabot

Amazon web crawler Alexabot is used for web content identification and backlink discovery. If you want to keep some of your information private, you can exclude Alexabot from crawling your website.

Yahoo! Slurp Bot

Yahoo crawler Yahoo! Slurp Bot is used for indexing and scraping of web pages to enhance personalized content for users.

Bingbot

Bingbot is one of the most popular web spiders powered by Microsoft. It helps a search engine, Bing, to create the most relevant index for its users. 

DuckDuck Bot

DuckDuckGo is probably one of the most popular search engines that do not track your history and follow you on whatever sites you are visiting. Its DuckDuck Bot web crawler helps to find the most relevant and best results that will satisfy a user’s needs. 

Facebook External Hit

Facebook also has its crawler. For example, when a Facebook user wants to share a link to an external content page with another person, the crawler scrapes the HTML code of the page and provides both of them with the title, a tag of the video or images of the content. 

Baiduspider

This crawler is operated by the dominant Chinese search engine − Baidu. Like any other bot, it travels through a variety of web pages and looks for hyperlinks to index content for the engine. 

Exabot

French search engine Exalead uses Exabot for indexation of content so that it could be included in the engine’s index.

Yandex Bot

This bot belongs to the largest Russian search engine Yandex. You can block it from indexing your content if you are not planning to conduct business there. 

Web Crawler vs Web Scraper — What Is the Difference?

A lot of people use web crawlers and web scrapers interchangeably. Nevertheless, there is an essential difference between these two. If the former deals mostly with metadata of content, like tags, headlines, keywords, and other things, the latter “steals” content from a website to be hosted on someone else’s online resource. 

A web scraper also “hunts” for specific data. For instance, if you need to extract information from a website where there is information such as stock market trends, Bitcoin prices, or any other, you can retrieve data from these websites by using a web scraping bot.

If you crawl your website, and you want to submit your content for indexing or have an intention for other people to find it — it is perfectly legal, otherwise scraping of other people’s and companies’ websites is against the law.

Interesting Read: https://hirinfotech.com/a-detailed-overview-of-web-crawlers/

Custom Web Crawler — What Is It?

A custom web crawler is a bot that is used to cover a specific need. You can build your spider bot to cover any task that needs to be resolved. For instance, if you are an entrepreneur or marketer or any other professional who deals with content, you can make it easier for your customers and users to find the information they want on your website. You can create a variety of web bots for various purposes.

If you do not have any practical experience in building your custom web crawler, you can always contact a software development service provider that can help you with it. 

Wrapping Up

Website crawlers are an integral part of any major search engine that is used for indexing and discovering content. Many search engine companies have their bots, for instance, Googlebot is powered by the corporate giant Google. Apart from that, there are multiple types of crawling that are utilized to cover specific needs, like video, image, or social media crawling. 

Taking into account what spider bots can do, they are highly essential and beneficial for your business because web crawlers reveal you and your company to the world and can bring in new users and customers.


If you are looking to create a custom web crawler, contact Hir Infotech for more information.

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