Simply Explained: Unravelling the Mystery of Web Crawlers

If you’ve spent even a little time in the world of SEO, you’ve probably heard of web crawlers. Carrying on with the metaphor of the internet as a web, crawlers are the little buggers that Search Engine Overlords like Google disperse to investigate every last thread. 

To be more specific, web crawlers are internet bots that perform automatic indexing for their developers. These bots scan info through the internet to build an ever-growing mass of indexed data. 

“Crawling,” then, is a term for the process that sees these bots automatically scoping out websites and loading up on data using scraping tools.

A few examples of web crawlers include Googlebot, Bingbot, Baiduspider, Slurp Bot, Yandex Bot, and DuckDuckBot. As wacky as some of these sound, web crawlers actually foundational to digital business as we know it – the crawlers report site content to search engines, the search engines use this info to help prioritize search results, and search results contain sites with ads that compete for space on high-priority sites.

That’s a simplified version of how things generally work, but the internet ad economy is pretty much the driving force behind a massive percentage of modern business, which means that web crawlers and search engines are to websites as health inspectors are to restaurants. Using web crawlers, search engines more effectively provide relevant search results that respond to your input.

The Secret Life of a Web Crawler

Now you’ve got the basic idea, the next question is how web crawlers work. Web crawlers act like librarians, indexing and updating online content for search engine companies. A crawler will go through lots and lots of web pages, checking the words on the page and cross-referencing those words with the other locations where those words are being used. The crawler will construct a large index in which it stores its findings. 

Basically, a crawler’s index is a list of words and any web pages it deems related to those words – that’s a big list! 

So let’s say you type “ham sandwich” into Google. The search engine looks at its own index (provided by the crawlers), and returns the most relevant findings it can, based on the URLs from the above list. Web crawlers begin their routines by visiting a series of familiar websites – in other words, it revisits places it’s already been. During these visits, a web crawler checks for other, related websites that a user might consider visiting. 

Through this perpetual visitation, web crawlers actually discover new URLs, make index updates to account for changes, and note any dead links. So if a search result returns you a bad link, it may be that a crawler hasn’t been sent to that page yet. When a web crawler visits a given page, it will thoroughly scan that page’s content and communicate its findings to its database. Once the page’s data is recorded, the words on that page get dropped into the relevant search engine’s working index. 

It helps to imagine that index as a massive library of words and where they appear across the internet. 

Of course, after indexing a specific web page, crawlers don’t simply stop – they will still check back every now and again to look for changes. If the crawler spots something different from what it recorded during its last visit, the index gets an update.

As the crawlers work, they follow these basic steps:

1. Pick a URL from within a group of candidates
2. Download related pages from the crawler’s index
3. Pull the URLs from the related pages
4. Add these new URLs to the candidate

Sounds easy, right? Not so much. 

Leave it to a tireless bot to take on a job like this one – there are countless sites full of different, changing pages out there. The number of people (and bots) working to change and update the web is staggering. We’re talking about large numbers of changes happening every fraction of a second. 

To compensate for this, search engines send out crawlers according to certain rules. For instance, a frequently updated page will get crawlers sent to it more often than one that updates less often. So if you’re looking to move up in those search results by adding relevant, high-quality content to your site, make sure you update it often. 

The more updates you make, the more often it gets crawled… 

And the more often your site gets crawled, the more of its content a search engine is able to consider as it delivers results to users everywhere.