What Is a Web Crawler and How Do Crawlers Work?

A web crawler, also known as a web spider because they crawl pages on the World Wide Web. It’s like a robot that travels around the internet and organizes what it finds. Its main job is to make lists of web pages for search engines, but it also does other things like helping with online shopping discounts, making websites easier to find, and gathering news feeds. These digital explorers move from website to website, collecting information like page titles, keywords, pictures and links. This information helps search engines make big lists of web pages, so when you search for something, you can find it faster. Besides making lists, web crawlers can also do tasks like taking information from websites, checking if websites change, seeing how well websites work, and collecting data for different reasons.

Search engines like Google, Bing, and Yahoo use web crawlers to discover and index web pages, enabling users to search for relevant information.

How Do Crawlers Work? 

  • Seed URLs: The web crawler starts with a list of seed URLs, which are the starting points for its crawling process. These URLs can be provided manually or generated algorithmically.
  • Requesting Web Pages: The crawler sends HTTP requests to web servers hosting the seed URLs and other URLs it discovers during the crawling process.
  • Retrieving Web Pages: Upon receiving a request, the web server sends back the requested web page along with its content (HTML, CSS, JavaScript, etc.).
  • Parsing: The crawler parses the content of the web page to extract useful information such as links, text, images, metadata, etc. It typically looks for HTML tags like <a> for links, <p> for paragraphs, <img> for images, etc.
  • Storing Information: The extracted information is then stored in a repository, often called an index, which is later used by search engines to serve search queries.
  • Following Links: The crawler follows the links found on the current page to discover new pages to crawl. This process continues looping, expanding the set of crawled pages.
  • Respecting Robots.txt: Web crawlers typically adhere to the rules specified in the robots.txt file, which is a text file hosted on a website that specifies rules for crawlers regarding which pages they are allowed or disallowed to crawl.
  • Crawl Budget Management: Crawlers have a limited amount of resources allocated for crawling, known as the crawl budget. They prioritize which pages to crawl based on various factors such as page relevance, importance, freshness, and popularity.
  • Recrawling: Crawlers periodically revisit previously crawled pages to check for updates and changes. The frequency of recrawling depends on factors like the update frequency of the page and its importance.

What are types of web crawlers?

Web crawlers can be categorized into various types based on their functionality, purpose, and behavior. Here are some common types of web crawlers:

  1. General-Purpose Crawlers: These crawlers are designed to crawl the entire web or a significant portion of it. They are used by search engines like Google, Bing, and Yahoo to index web pages for search results.
  1. Focused Crawlers: Also known as topical crawlers or vertical crawlers, these crawlers are designed to crawl specific areas of the web related to a particular topic or subject matter. They are used to gather information on niche topics or verticals.
  1. Incremental Crawlers: Incremental crawlers, crawl only new or updated content since the last crawl. Instead of recrawling the entire web, they focus on discovering changes to existing pages and indexing new content.
  1. Deep Web Crawlers: Deep web crawlers, also known as invisible web crawlers or hidden web crawlers, are designed to crawl content that is not indexed by traditional search engines. This includes content behind paywalls, databases, and dynamically generated pages.
  1. Enterprise Crawlers: These crawlers are used by organizations to index and search internal documents and resources within their intranets or internal networks. They help users find relevant information within an organization’s data repositories.
  1. Focused-Only Crawlers: Focused-only crawlers, crawl only specific websites or domains, ignoring all other content on the web. They are commonly used for data extraction, monitoring competitor websites, or gathering information from a selected set of sources.
  1. Mobile Crawlers: With the rise of mobile browsing, mobile crawlers are designed to crawl and index mobile-friendly versions of websites. They ensure that search engines can provide relevant search results tailored to mobile users.
  1. Social Media Crawlers: These crawlers are designed to crawl and index content from social media platforms such as Facebook, Twitter, and Instagram. They help search engines provide real-time updates and social media content in search results.
  1. Vertical Search Engine Crawlers: Vertical search engine crawlers are specialized crawlers used by vertical or specialized search engines focusing on specific industries or content types, such as travel, jobs, or real estate.
  1. Custom Crawlers: Organizations and developers may create custom crawlers suits to their specific needs and requirements. These crawlers can have unique features and functionalities depending on the goals of the project.

These are just some examples of the types of web crawlers, and there can be variations or combinations of these types depending on the specific use case and requirements.

Examples of Web Crawlers

Several web crawlers are used by search engines and other organizations for various purposes. Here are some examples:

  • Googlebot: Google’s web crawler, responsible for indexing web pages for the Google search engine. It follows links from one page to another and indexes the content for search results.
  • Bingbot: Microsoft’s web crawler, used by the Bing search engine to index web pages. Similar to Googlebot, it crawls the web and indexes pages for Bing’s search results.
  • Yahoo Slurp: Yahoo’s web crawler, used by the Yahoo search engine for indexing web pages. It operates similarly to other web crawlers, following links and indexing content.
  • Yandex Bot: Yandex’s web crawler, used for indexing web pages for the Yandex search engine, which is popular in Russia and other countries. It follows links and indexes content for search results.
  • Baidu Spider: Baidu’s web crawler, used for indexing web pages for the Baidu search engine, which is the leading search engine in China. It crawls Chinese and international websites.
  • DuckDuckBot: The web crawler used by DuckDuckGo, a privacy-focused search engine. It indexes web pages for DuckDuckGo’s search results while respecting user privacy.
  • Seznam Bot: Seznam’s web crawler, used for indexing web pages for the Seznam search engine, which is popular in the Czech Republic. It follows links and indexes content.
  • Screaming Frog SEO Spider: This is a desktop program used by SEO professionals to crawl websites for search engine optimization  analysis. It provides detailed information about a website’s structure, links, and other SEO-related factors.
  • Majestic Million: While not a traditional web crawler, Majestic Million is a dataset compiled by Majestic SEO that includes the top million websites on the internet. It provides information such as link counts and Trust Flow for these websites.

These are just a few examples of web crawlers used for indexing and analyzing web content. There are many other crawlers used by different organizations for various purposes, including research, data mining, and monitoring website changes.

In short, web crawlers play an important role in organizing and indexing the vast expanse of information on the internet.  They search the web to find and organize information for us. They’re used by search engines, companies, and other tools to help us find what we’re looking for online. These crawlers work hard to make sure we can easily access the information we need. These crawlers keep getting better and are changing how we find things online.

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