What Is a Web Crawler/Spider and How Does It Work? (2024)

Search engines like Google are part of what makes the internet so powerful. With a few keystrokes and the click of a button, the most relevant answers to your question appear. But have you ever wondered how search engines work? Web crawlers are part of the answer.

So, what is a web crawler, and how does it work?

What Is a Web Crawler?

When you search for something in a search engine, the engine has to rapidly scan millions (or billions) of web pages to display the most relevant results. Web crawlers (also known as spiders or search engine bots) are automated programs that “crawl” the internet and compile information about web pages in an easily accessible way.

The word “crawling” refers to the way that web crawlers traverse the internet. Web crawlers are also known as “spiders.” This name comes from the way they crawl the web—like how spiders crawl on their spiderwebs.

Web crawlers assess and compile data on as many web pages as possible. They do this so that the data is easily accessible and searchable, hence why they are so important to search engines.

Think of a web crawler as the editor who compiles the index at the end of the book. The job of the index is to inform the reader where in the book each key topic or phrase appears. Likewise, a web crawler creates an index that a search engine uses to find relevant information on a search query quickly.

What Is Search Indexing?

As we’ve mentioned, search indexing is comparable to compiling the index at the back of a book. In a way, search indexing is like creating a simplified map of the internet. When someone asks a search engine a question, the search engine runs it through their index, and the most relevant pages appear first.

But, how does the search engine know which pages are relevant?

Search indexing primarily focuses on two things: the text on the page and the metadata of the page. The text is everything you see as a reader, while the metadata is information about that page input by the page creator, known as “meta tags.” The meta tags include things like the page description and meta title, which appear in search results.

Search engines like Google will index all of the text on a webpage (except for certain words like “the” and “a” in some cases). Then, when a term is searched into the search engine, it will swiftly scour its index for the most relevant page.

How Does a Web Crawler Work?

A web crawler works as the name suggests. They start at a known web page or URL and index every page at that URL (most of the time, website owners request search engines to crawl particular URLs). As they come across hyperlinks on those pages, they’ll compile a “to-do” list of pages that they’ll crawl next. The web crawler will continue this indefinitely, following particular rules about which pages to crawl and which to ignore.

Web crawlers do not crawl every page on the internet. In fact, it’s estimated that only 40-70% of the internet has been search indexed (which is still billions of pages). Many web crawlers are designed to focus on pages thought to be more “authoritative.” Authoritative pages fit a handful of criteria that makes them more likely to contain high-quality or popular information. Web crawlers also need to consistently revisit pages as they are updated, removed, or moved.

One final factor that controls which pages a web crawler will crawl is the robots.txt protocol or robots exclusion protocol. A web page’s server will host a robots.txt file that lays out the rules for any web crawler or other programs accessing the page. The file will rule out particular pages from being crawled and which links the crawler can follow. One purpose of the robots.txt file is to limit the strain that bots put on the website’s server.

To prevent a web crawler from accessing certain pages on your website, you can add the “disallow” tag via the robots.txt file or add the noindex meta tag to the page in question.

What’s the Difference Between Crawling and Scraping?

Web scraping is the use of bots to download data from a website without that website’s permission. Often, web scraping is used for malicious reasons. Web scraping often takes all of the HTML code from specific websites, and more advanced scrapers will also take the CSS and JavaScript elements. Web scraping tools can be used to quickly and easily compile information about particular topics (say, a product list) but can also wander into grey and illegal territories.

Web crawling, on the other hand, is the indexing of information on websites with permission so that they can appear easily in search engines.

Web Crawler Examples

Every major search engine has one or more web crawlers. For instance:

• Google has Googlebot
• Bing has Bingbot
• DuckDuckGo has DuckDuckBot.

Bigger search engines like Google have specific bots for different focuses, including Googlebot Images, Googlebot Videos, and AdsBot.

How Does Web Crawling Affect SEO?

If you want your page to appear in search engine results, the page must be accessible to web crawlers. Depending on your website server, you may want to allocate a particular frequency of crawling, which pages for the crawler to scan, and how much pressure they can put on your server.

Basically, you want the web crawlers to hone in on pages filled with content, but not on pages like thank you messages, admin pages, and internal search results.

Information at Your Fingertips

Using search engines has become second nature for most of us, yet most of us have no idea how they work. Web crawlers are one of the main parts of an effective search engine and effectively index information about millions of important websites every day. They are an invaluable tool for website owners, visitors, and search engines alike.