What is a Web Crawler and How Does It Work?

Web Crawler Definition:

A web crawler is an internet bot/computer program that systematically browses the internet to discover, index and learn about new content.

Web crawlers (also called Search Engine Spiders or Spiderbots) are mostly used by search engines to crawl and index web pages (Web Indexing).

Using the gathered data, Google as well as many other search engines use their own search algorithms to qualify and rank the most relevant pages in their search results.

They do this because they want to ensure that the index gives an answer back for the intent of a query while also satisfying it.

How does a Web Crawler work?

Web spiders, starting from the original URL or seed of URLs travel to additional pages by following any hyperlinks in those web pages. When they find these hyperlinks to other web pages on the internet, they add them to their crawl queue and crawl those too.

The internet is huge, so these crawlers need rules on how many pages they can crawl or what hyperlinks they can follow, otherwise, one crawler could indefinitely crawl web pages.

These rules are contained in every websites' Robots.txt file.

This robots.txt file tells crawlers what directories on a website they can crawl or not crawl, how often they can crawl the website and also can direct them to the XML Sitemaps of the website for easier crawling.

History of Web Crawlers

The idea of web crawlers was born in early as 1993.

In 1993, four important (yet basic according to now) web crawlers were created:

• World Wide Web Wanderer
• Jump Station
• World Wide Web Worm
• RBSE Spider

These four spiders mainly collected information and statistics about the web using a set of seed URLs.

Since then, these computer programs have evolved and can now be used to do much more than just collect information.

Thanks to several algorithm updates like BERT (and soon MUM), Google is now able to actually understand what your page is about and rank your site accordingly.

What are the Most Common Web Crawlers?

The most common web crawlers are:

• Googlebot
• BingBot
• Yahoo! Slurp Bot
• DuckDuck Bot
• Baiduspider
• Yandex Bot
• Sogou Spider
• Exabot
• Alexa Crawler
  

What is the difference between Web Crawling and Web Scraping?

A web crawler is a software program that systematically browses the World Wide Web, following hyperlinks, in order to create an index of every document it finds.

A web scraper, on the other hand, does not work with such a large corpus and only focuses on specific websites or pages for data extraction purposes without actually browsing the website or clicking on its links.

Web scrapers can be operated by anyone while web crawlers are operated by a search engine.

There are also SEO Audit tools that act as web scrapers to collect data.

How do Web Crawlers affect Search Engine Optimization?

Web crawlers, like Googlebot, periodically crawl your web page and affect your SEO by determining how fresh the content is, whether it’s relevant to the specific search query in place and then rank your website in the search results accordingly.

You can optimize how spider bots affect your SEO by improving these things:

• Making your site easier to crawl.
  
  
• Regularly updating your content and changing the content "published date" in your Structured Data.
  
  
• Making sure your pages render properly.
  
  
• By having a proper robot.txt and disallowing parts you don't want to be crawled.
  

Is Crawling a Website Legal?

Yes, crawling a website is completely legal and isn't prohibited by law.

Web crawlers do not break any internet laws, as they are mostly used for the purpose of indexing websites and providing search engines with more content to rank in their algorithms.

How to Block Crawlers?

You can block any crawler by adding a Disallow directive to your Robots.txt.

You can learn how to do that in our Robots.txt guide.

Blocking a web crawler from accessing your website through the Robots.txt (Robots Exclusion Protocol) file can help prevent your web servers from becoming overloaded.

Website owners can take also take advantage of a website's Robots.txt file in order to give themselves a better crawl budget for search engine optimization purposes.

Keep in mind that although major search engines honour this request, a smaller search engine might not!

How Does a Web Crawler Find Your Website?

A web crawler finds your website through links from another website or if you manually submit your website for crawling.

How Can I tell if Google has Crawled My Site?

If you would like to see if Google has crawled your website or not, you will have to log into your Google Search Console, enter the URL you want to check. If your website was crawled, you will see a last crawl date in the "Coverage" section.

You can also check your server logs to see if Google bot has visited your website or not.

Checking your website with the "Site:" operator is also possible, but keep in mind that shows if your website is indexed or not. Your webpage can be crawled, but not indexed.

Different Types of Web Crawlers

There are several types of web crawlers and in this section, we will be looking at a few of them:

Focused Web Crawler

A focused crawler is a web crawler that searches for pages on the internet that meet specific criteria, by being selective about what areas it explores and how it links from one to another.

A focused web crawler focuses on crawling according to a specific goal for example, crawl pages from a .hu domain or crawl pages about cats.

If a focused crawlers mission is to crawl pages about a specific topic, like blue dogs, it then becomes a "topical crawler".

To save resources, a focused web crawler has to predict the relevancy of a page even before downloading the page. In the early days, this was done by looking at the anchor text of links, but now these focused crawlers have their own topical crawling algorithms which were trained using reinforcement learning.

A different type of a focused web crawler is a semantic focused crawler, which looks at domain ontologies to determine the websites topical map and then links web pages with relevant ontological concepts together.

Incremental Web Crawler

The internet is huge, there are new pages created and removed every second and the content of these pages or URLs can also change without notice.

Because of this, there will be always pages which of the content is not yet present in a search engines repository, or pages where there is an older version of a page present in that repository.

An incremental web crawler periodically prioritizes and revisits old URLs to make sure these pages/URLs are being discovered and recrawled. This is called incremental web crawling.

Distributed Web Crawler

A distributed web crawler is a computer program that is a part of a voluntary system, where there are many computers employed to help index the Internet and retrieve data. This is also called distributed web crawling.

A distributed web crawler is a part of this system and it can be a server or even a personal computer.

Google and Yahoo also use thousands of individual computers as a part of this system, where computers that are connected to the internet can crawl websites in the background.

When they visit a webpage, they download it, compress it and send it back to a central server with a status flag to notify it of its status, like if it's a new page, it was deleted, redirected or changed.

Parallel Web Crawler

A parallel web crawler is a web spider that runs in parallel with other web crawlers to crawl the web.

Many existing search engines already use parallel web crawlers to increase the efficiency of their crawling process.

A Parallel web crawler is very scalable and is superior to single-process crawlers.

Parallel web crawlers also decrease the network load for a search engine.

Is Web Crawling Secure?

Yes, web crawling is secure as the web crawler's search indexing is not a breach of any security.

Your data cannot be compromised or hacked because it does not access your private computer and files.

It only retrieves the content that you have made public on the Internet by way of HTML code markup, CSS styling and JavaScript code.

With that said, your website will be present in search engines after crawling which makes it foundable by anyone, hackers too.

What is Search Indexing?

Search indexing is the process of extracting information about documents from a database and storing them in an ordered list, a search index.

Today, search indexing is the process by which search engines organize information before a query to provide fast responses.

Google uses what is called an inverted index which is a data structure used for fast retrieval of search results.

The inverted index stores information about the words in each document, as well as pointers to each location where those occurrences can be found within that particular documents, also known as positions.

This way the Search Engine knows what word you are searching for and which part of your document you want to search.

Conclusion

We hope this article helped you explain how web crawler works and how you can make sure it's easier for Google to crawl your site!