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 just 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 an indexing robot?

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When you search for something in a search engine, the engine must quickly crawl millions (or billions) of web pages to show the most relevant results. Web crawlers (also known as spiders or search engine robots) 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 web crawlers crawl the internet. Web crawlers are also called “spiders”. This name comes from the way they crawl the web, like how 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 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 topic or key phrase appears. Similarly, 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 mentioned, search indexing is like compiling the index at the end of a book. In a way, indexing search is like creating a simplified map of the Internet. When someone asks a search engine a question, the search engine crawls it through its 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: page text 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 page description and meta title, which show up in search results.

Search engines like Google will index all text on a webpage (except certain words like “the” and “a” in some cases). Then, when a term is looked up in the search engine, it quickly scans its index to find the most relevant page.

How does a web crawler work?

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A web crawler works as the name suggests. They start with a known web page or URL and index every page at that URL (most of the time website owners ask search engines to crawl particular URLs). As they come across hyperlinks on these pages, they compile a “to-do” list of the pages they will explore 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 indexed for search (that’s still billions of pages). Many web crawlers are designed to focus on pages that are considered 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 regularly review pages as they are updated, deleted or moved.

A final factor that controls which pages a web crawler will crawl is the robots.txt protocol or robot exclusion protocol. A webpage’s server will host a robots.txt file that lays out the rules for any web crawlers or other programs accessing the page. The file will prevent certain pages from being crawled and links the crawler can follow. One of the purposes of the robots.txt file is to limit the pressure exerted by bots on the website server.

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

What is the difference between crawl 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 purposes. Web scraping often takes all of the HTML from specific websites, and more advanced scrapers will also take CSS and JavaScript elements. Web scraping tools can be used to quickly and easily compile information on particular topics (eg, a product listing), but can also stray into gray and illegal territory.

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

Examples of web crawlers

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

  • Google has Googlebot
  • Bing has Bingbot
  • DuckDuckGo has 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 allocate a particular crawling frequency, the pages the crawler crawls, and the pressure it can put on your server.

Basically, you want web crawlers to focus on content-filled pages, but not 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 parts of an effective search engine and efficiently index information on millions of major websites every day. They are an invaluable tool for website owners, visitors and search engines.

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