What is a Web Crawler / Spider and how does it work?

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?


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When you’re looking for something in a search engine, the engine needs to quickly crawl millions (or billions) of web pages to show the most relevant results. Web crawlers (also known as crawlers or search engine bots) are automated programs that “crawl” the Internet and compile information on web pages in an easily accessible manner.

The word “crawling” refers to the way web crawlers traverse the Internet. Web crawlers are also called “spiders”. This name comes from the way they explore the web, like the way spiders crawl on their cobwebs.

Web crawlers evaluate and compile data on as many web pages as possible. They do this so that the data is easily accessible and searchable, hence its importance for search engines.

Think of a web crawler as the publisher 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 topic or key phrase appears. Likewise, a web crawler creates an index that a search engine uses to quickly find relevant information about a search query.

What is search indexing?

As we mentioned, indexing search is like compiling the index at the end of a book. In a way, indexing searches is like creating a simplified map of the Internet. When someone asks a search engine a question, the search engine runs it in 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: text on the page and page metadata. Text is everything you see as a reader, while metadata is information about that page entered by the page creator, called “meta tags”. Meta tags include things like the page description and the meta title, which appear in search results.

Search engines like Google will index all of the text on a web page (with the exception of certain words like “the” and “a” in some cases). Then, when a term is searched for in the search engine, it quickly scans its index for the most relevant page.

How does a web crawler work?


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A web crawler works as the name suggests. They start at a known web page or URL and index every page to that URL (most of the time, website owners ask search engines to crawl particular URLs). When they come across hyperlinks on these pages, they will compile a “to-do” list of pages that they will then explore. The crawler will continue indefinitely, following specific rules regarding which pages to crawl and which to ignore.

Web crawlers do not crawl every page of the Internet. In fact, it is estimated that only 40 to 70% of the Internet has been indexed by searches (which is still billions of pages). Many web crawlers are designed to focus on pages considered to be more “authoritative”. Authoritative pages meet a handful of criteria that make them more likely to contain high-quality or popular information. Web crawlers should also systematically revisit pages as they are updated, deleted, or moved.

A final factor that controls which pages a crawler will crawl is the robots.txt protocol or the robot exclusion protocol. A web page’s server will host a robots.txt file that defines the rules for any web crawlers or other programs accessing the page. The file will exclude crawling of particular pages and any links the crawler may follow. One of the purposes of the robots.txt file is to limit the pressure exerted by the robots on the website’s server.

To prevent a crawler from accessing certain pages on your website, you can add the “disallow” tag through the robots.txt file or add the no index meta tag to the page in question.

What is the difference between crawling and scratching?

Web scraping is the use of robots to download data from a website without the permission of that website. Often, web scraping is used for malicious reasons. Web scraping often takes all the HTML from specific websites, and more advanced scrapers will take CSS and JavaScript elements as well. Web scraping tools can be used to quickly and easily compile information on particular topics (for example, a list of products), but can also roam gray and illegal territories.

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

Examples of web crawlers

Each major search engine has one or more crawlers. For example:

  • Google has Googlebot

  • Bing to Bingbot

  • DuckDuckGo a DuckDuckBot.

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

How does web crawling affect SEO?


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If you want your page to appear in search engine results, the page must be accessible to crawlers. Depending on your website’s server, you may want to assign a particular crawl frequency, which pages the crawler crawls, and how much strain it can put on your server.

Basically, you want web crawlers to focus on pages filled with content, but not on pages like thank you posts, admin pages, and internal search results.

Information at your fingertips

Using search engines has become second nature to most of us, but most of us have no idea how they work. Web crawlers are one of the main components of an efficient search engine and efficiently index information on millions of important websites every day. They are an invaluable tool for website owners, visitors, and search engines.


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