What are web crawlers and how do they work?

Theo Vasilis
Theo Vasilis
Table of Contents

What is web spidering? What do Google crawlers do? And how do they affect SEO? We give you the answers to these and other questions about web crawlers.

Web crawlers are the librarians of the world wide web

What is a web crawler?

Imagine you’re looking for a book in an enormous library. You might search the shelves‘ categories, genres, and authors to find it. If you haven’t found what you’re looking for, you can ask the librarian to check the files to see if the book is there and where to get it. The librarian stores information about the books, such as title, description, category, and author. This makes it easy to find books when you need them.

The internet is a library, and even though it’s by far the most extensive library in history, it has no central filing system. So how do we get hold of the information we‘re looking for? The answer is web crawlers. These web crawlers, also known as web spiders or spiderbots, are the librarians of the world wide web. They index all the pages on the internet so we can find them. This web indexing is also known as web spidering.

How do web crawlers work?

Before crawling a webpage, web spiders check the page's robots.txt file to find out about the rules of that specific page. These rules define which pages the crawler can access and which links they can follow. Then they start to crawl by using a set of known URLs. The spiders follow the hyperlinks contained in those pages and repeat the cycle on the newly found websites. The crawler's goal is to download and index as much content as possible from the websites it visits.

What do Google web crawlers do?

If web crawlers are the librarians of the web, then Google is the closest thing we have to a central filing system. Google web crawlers explore publicly available web pages, follow the links on those pages, and crawl from link to link. They bring the data back to Google’s servers, and the information is organized by indexing.

Search indexing (or web spidering) is like creating a library catalog. This makes it possible for search engines like Google or Bing to know where to retrieve information when we search for it. The most extensive search index is the Google Search index. It contains hundreds of billions of web pages with entries for each word every page includes.

Web spidering is like creating a library catalog
Web spidering is like creating a library catalog


What does web crawling do for SEO?

SEO stands for search engine optimization. Content that makes it easy for web crawlers to identify the relevance of your page for search queries is search engine optimized content. Web crawlers determine whether your page has information relevant to the question that people insert into search engines. They can ascertain whether your content is likely to provide the answer you‘re looking for and whether it‘s a copy of other online materials.

If your page appears to answer those questions and contains hyperlinks relevant to those inquiries, it improves the likelihood that your content will appear on the first page of Google when you enter a query in the search engine.

One of the biggest obstacles to achieving this is broken links since following hyperlinks is essential for crawlers to index web pages. Google web crawlers are unforgiving creatures. When they identify broken links, they make digital notes and lower the website’s rating. That means there‘s less chance of people finding your web page.


Web crawlers vs. web scrapers

SEO is not the only thing web crawlers are good for. We also use them for web scraping. While both terms - web crawling and web scraping - are used for data collection, there‘s a difference. Web scraping is a more targeted form of crawling to extract structured data from web pages. This can be achieved either with a general web scraper like this one which crawls arbitrary websites or with a bot explicitly designed for the target website.

Even with a general scraper, it’s possible to extract data from websites without crawling them, assuming you already have a specific target website or list of URLs. For example, this web scraping tutorial for beginners shows you how to extract data from CNN.

Whether you want to crawl web pages or scrape data from specific sites, you have three options to fulfill your web scraping needs ⬇️

How to use web crawlers for data extraction


Use ready-made web crawlers and scrapers

Assuming you don’t have the tech skills or the time to create your own web crawlers, the best option is to use readymade scraping tools. Apify Store is full of these scrapers. They are ideal for crawling websites to extract data. This library of automation tools contains literally hundreds of servers for all your web scraping requirements. Some are custom-built for particular websites and applications. Others are universal scrapers that can be configured for any website, such as Web Scraper.

Get a custom web scraping solution

If any of the existing servers in Apify Store don’t meet your needs, get in touch with our experts. If you let them know your goals, they’ll have a chat with you and review your proposal, and then provide you with a solution or build a web scraper for your specific case.

Build your own web crawler

We recommend this option for developers. If you know how to create your own servers, you can make money from them by publishing them on the Apify platform. Just focus on building your code, and Apify will help you find customers for it, and will take care of running it for you.



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