What is a web crawler and how does it work?

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Have you ever searched for something on Google and wondered, “How does it know where to look?” The answer is “web crawlers,” which search the web and index it so you can easily find things online. We will explain to you.

Search engines and crawlers

When you search using a keyword on a search engine like Google or Bing, the site sifts through billions of pages to generate a list of results related to that term. How exactly do these search engines have all these pages in their folders, know how to search for them, and generate these results in seconds?

The answer is web crawlers, also known as spiders. These are automated programs (often called “robots” or “bots”) that “crawl” or browse the web in order to be added to search engines. These crawlers index websites to create a list of pages that will eventually show up in your search results.

The crawlers also create and store copies of these pages in the engine’s database, allowing you to search almost instantly. This is also why search engines often include cached versions of sites in their databases.

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Sitemaps and selection


An illustration of a man in front of a flowchart.
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So how do crawlers choose which websites to crawl? Well, the most common scenario is that website owners want search engines to crawl their sites. They can achieve this by asking Google, Bing, Yahoo or another search engine to index their pages. This process varies from engine to engine. Also, search engines frequently select popular and well-linked websites to crawl by tracking the number of times a URL is linked on other public sites.

Website owners can use certain processes to help search engines index their websites, such as
download a sitemap. This is a file containing all the links and pages that are part of your website. It is normally used to indicate which pages you want to index.

After search engines have already crawls a website once, they automatically crawl that site again. Frequency varies based on a website’s popularity, among other metrics. Therefore, site owners frequently maintain updated sitemaps to notify engines of new websites to index.

Robots and the politeness factor

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What if a website doesn’t do you want some or all of its pages to appear on a search engine? For example, you might not want people searching for a members-only page or seeing your 404 error page. This is where the crawling exclusion list comes in, also known as robots.txt. This is a simple text file that tells web crawlers which web pages to exclude from indexing.

Another reason why robots.txt is important is that web crawlers can have a significant effect on site performance. Since crawlers essentially download all the pages on your website, they consume resources and can cause slowdowns. They arrive at unpredictable times and without approval. If you don’t need your pages to be indexed repeatedly, stopping crawlers can help reduce some of the load on your website. Fortunately, most crawlers stop crawling certain pages based on the site owner’s rules.

Metadata Magic

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Below the URL and title of each search result in Google, you will find a brief description of the page. These descriptions are called snippets. You may notice that a page’s snippet in Google doesn’t always match the actual content of the website. This is because many websites have what are called “meta tags”, which are custom descriptions that site owners add to their pages.

Site owners often come up with attractive metadata descriptions written to make you want to click on a website. Google also lists other meta information, such as prices and stock availability. This is especially useful for those who run e-commerce websites.

Your research

Web searching is an essential part of using the Internet. Searching the web is a great way to discover new websites, stores, communities, and interests. Every day, web crawlers visit millions of pages and add them to search engines. Although crawlers have some disadvantages, such as the use of site resources, they are invaluable to site owners and visitors.

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