A web crawler is a computer program that uses a search engine to find websites and extract data. Crawlers are used by website owners to improve their website traffic and make money from advertising.

Search Engines and Crawlers

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

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

Crawlers also create and store copies of these pages in the engine’s database, which allows you to make searches almost instantly. It’s also the reason why search engines often include cached versions of sites in their databases.

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Site Maps and Selection

So, how do crawlers pick 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 requesting Google, Bing, Yahoo, or another search engine to index their pages. This process varies from engine to engine. Also, search engines frequently select popular, well-linked websites to crawl by tracking the number of times that a URL is linked on other public sites.

Website owners can use certain processes to help search engines index their websites, such as uploading a site map. This is a file containing all the links and pages that are part of your website. It’s normally used to indicate what pages you’d like indexed.

Once search engines have already crawled a website once, they will automatically crawl that site again. The frequency varies based on how popular a website is, among other metrics. Therefore, site owners frequently keep updated site maps to let engines know which new websites to index.

Robots and the Politeness Factor

What if a website doesn’t want some or all of its pages to appear on a search engine? For example, you might not want people to search for a members-only page or see your 404 error page. This is where the crawl exclusion list, also known as robots.txt, comes into play. This is a simple text file that dictates to 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. Because crawlers are essentially downloading 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 indexed repeatedly, then stopping crawlers might help reduce some of your website load. Fortunately, most crawlers stop crawling certain pages based on the rules of the site owner.

Metadata Magic

Under the URL and title of every search result in Google, you will find a short description of the page. These descriptions are called snippets. You might notice that the snippet of a page in Google doesn’t always line up with the website’s actual content. This is because many websites have something called “meta tags,” which are custom descriptions that site owners add to their pages.

Site owners often come up with enticing 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 running e-commerce websites.

Your Searching

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. While crawlers have some downsides, like taking up site resources, they’re invaluable to both site owners and visitors.

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