What Is a Web Crawler and How Does It Work to Your Benefit

Source: ScrapingHub

Have you ever wondered how search engines like Google find and display the most relevant web pages for your searches in a matter of seconds? The key is web crawlers. These sophisticated computer programs constantly scan the internet to discover, analyze and index billions of web pages. Web crawlers (also called search engine bots or spiders) are essential for making the wealth of information on the internet accessible through search engines.

As a web crawling and data scraping expert, I‘ve seen firsthand the power of crawlers for harvesting data and making information accessible. Whether you‘re a marketer, business owner, researcher or anyone with a website, understanding web crawlers is key to succeeding in today‘s digital landscape.

In this in-depth guide, we‘ll cover everything you need to know about web crawlers including:

• How crawlers work and are programmed
• The role of crawling in SEO and web search
• Key web crawler examples and statistics
• Tips to optimize your website for crawling
• Advanced topics like API crawling and the future of crawlers
• Applications and benefits of crawlers for business and marketing

By the end of this guide, you‘ll be equipped with expert-level knowledge and practical insights to make web crawlers work to your benefit. Let‘s dive in!

How Do Web Crawlers Work?

At the most basic level, a web crawler is a program that automatically fetches and indexes content from the internet in a methodical, automated manner. Crawlers are the main technology that powers search engines and are used to create a searchable index of the World Wide Web.

While the exact algorithms and architecture of commercial web crawlers are proprietary and vary, all crawlers share some key components and follow a similar multi-step process:

1. Seed URLs: Crawlers start from a fixed set of "seed" URLs which serve as entry points to begin the crawling process. The seed URLs are typically high-quality, popular web pages that are likely to contain many outbound links.

2. Crawl Frontier: URLs extracted from the seed pages are added to a queue called the crawl frontier. This ever-expanding list of discovered URLs serves as a "to-do list" for the crawler. URLs are prioritized and scheduled by algorithms.

3. Fetch: The crawler selects the next URL from the frontier, fetches the page content over HTTP, and analyzes the raw HTML and associated assets like images, CSS, and JavaScript.

4. Extract Links: The crawler extracts all the hyperlinks from the page and adds any new, unique ones back to the crawl frontier for future fetching. This process allows the crawler to continually discover new content.

5. Index: The fetched page content is processed and stored in a massive database called the search index. This index maps each word to the URLs containing it and allows for fast retrieval. Some additional data like page title, meta tags, and PageRank may also be extracted and indexed.

6. Re-crawl: Crawlers may periodically re-visit or "re-crawl" pages to check for updates or changes, especially for frequently updating or high-quality pages. Scheduling and deciding which pages to re-crawl is a key challenge in crawler design.

Googlebots Web Crawler Process. Source: Search Engine Journal

Web crawlers typically run on a distributed, scalable system comprised of hundreds or thousands of servers working in parallel. This allows them to fetch and process massive amounts of pages efficiently.

Some of the key considerations in web crawler architecture and design include:

• Robustness: Crawlers need to be resilient to issues like malformed HTML, server errors, network interruptions and other unexpected behaviors. They must be able to fetch and process a wide range of content types.
• Politeness: Crawlers should avoid overloading servers with requests and respect instructions in the robots.txt file. Well-behaved crawlers throttle their request rate and stagger visits to the same server.
• Freshness: Keeping the index up-to-date and determining an optimal re-crawl frequency is key to providing the best search results. Crawler scheduling algorithms aim to balance coverage and freshness.
• Prioritization: With limited resources, crawlers need to prioritize their frontier and decide which pages are most important to visit first. This may be based on factors like PageRank, update frequency, and link structure.
• Scale: Commercial web crawlers need to process hundreds of billions of pages and keep up with the ever-growing internet. This requires a massive, distributed system and smart techniques for URL canonicalization, data compression, and more.

Some of the most advanced and active web crawlers today are operated by major search engines like Google, Bing, Yandex, and Baidu. According to the latest data from Cloudflare, Google‘s crawlers alone are responsible for over 50% of all web traffic.

Web Crawler Traffic Share. Source: Cloudflare Radar 2022

Clearly, web crawlers operated by the major search engines are the most important ones to pay attention to as a website owner. However, there are also many specialized crawlers run by vertical search engines, research institutions, archival services, SEO agencies, and web data companies. Regardless of the source, it‘s important to understand and monitor what web crawlers are accessing your site.

The Role of Web Crawlers in SEO

For SEO professionals and website owners, web crawlers are one of the most important considerations in driving organic traffic. After all, your content can only appear in the search results and drive traffic if it has first been discovered, crawled, and indexed by search engine bots.

Some of the key ways web crawlers impact SEO include:

• Indexation: Search engine crawlers are the gatekeepers to getting your webpages included in the search index. No crawling means no indexing and no organic search traffic, period. Making sure your most important content is being found and indexed by crawlers should be the top SEO priority.

• Ranking Signals: Web crawlers are used to gather many of the data points and signals that feed into search ranking algorithms. Things like page load time, meta data, mobile-friendliness, content freshness, and internal link structure are all assessed at the time of crawling.

• Crawl Budget: Even the most advanced crawlers have a limited "crawl budget" for each website based on its size, health, and authority. Wasting your budget on low-value or unnecessary pages means less depth and frequency of crawling for your key content. Optimizing your site‘s "crawl budget" is an important technical SEO consideration.

• Frequency: How often your pages are re-crawled impacts how quickly updates or new pages will be reflected in the search results. High-authority domains may see their pages crawled daily or even hourly, while a small site might only be re-crawled every few weeks. This has big implications for time-sensitive content.

So what can you do to optimize your website for crawling and improve your SEO? Here are some expert tips:

• Submit sitemaps: XML sitemaps are a standardized way to list all the important URLs on your site and signal to crawlers which pages are highest priority. Submitting a sitemap can speed up discovery of your content, especially for large or frequently changing sites.

• Clean up index bloat: Use the noindex tag and robots.txt file to prevent crawlers from wasting time on low-value pages like archives, search results, or user-specific content. Google recommends keeping your index size under 10 million URLs.

• Flatten your architecture: A "flat" website architecture minimizes the number of clicks required to reach any given page from the homepage. This makes it easier for crawlers (and users) to discover all your content and spreads more authority throughout the site. Aim for a "click depth" of 3 or less for your most important pages.

• Build authority: The amount and quality of external links pointing to your domain is one of the biggest factors in how frequently and deeply your site is crawled. Building authoritative backlinks from reputable websites is key for maximizing your site‘s "crawl budget" and SEO potential.

• Speed things up: Page load time is not only an important user experience factor but also a technical challenge for web crawlers. Bots may struggle to completely render and index pages that are very slow to load. Optimizing your page speed can encourage complete and frequent crawling.

Example Site Crawl Analytics from Moz

Of course, getting your content crawled and indexed is just the first step. Rankings and organic traffic ultimately depend on the relevance, quality, and competitiveness of your content compared to other websites. However, ensuring your site is optimally designed for crawling sets the stage for SEO success.

How Businesses Can Leverage Web Crawlers

Beyond powering search engines, web crawlers can be powerful tools for businesses and organizations to gather data and generate insights. Some common use cases include:

• Competitive analysis: Crawlers can be used to automatically monitor competitor websites and gather data points like keyword usage, pricing, content strategies and more. This real-time competitive intelligence can inform business strategies.

• Lead generation: Targeted web crawlers can gather contact information, social media profiles, and other details for sales leads. This is especially useful for recruiting and staffing firms looking to source candidates by crawling professional networking sites and job boards.

• Market research: By crawling forums, review sites, and social media, businesses can track consumer sentiment and extract insights to guide product development and branding. This unstructured web data offers a raw pulse of the market.

• Academic research: Crawlers allow researchers to gather large corpuses of text and multimedia content to analyze and support their work. Web data is behind many breakthroughs in machine learning, natural language processing, and the social sciences.

• Archival: Organizations like the Internet Archive use massive crawlers to create a historical record of the web and preserve online content for future generations. Over 720 billion web pages have been archived since 1996.

To leverage web crawlers for these applications and others, businesses essentially have two options: buying crawling services from a web data provider or building their own crawlers in-house.

Buying crawl data or crawling as a service can be a fast and easy way to get started without major up-front investment. Companies like Scrapy, Zyte, and WebSundew offer customizable crawling solutions for a variety of data needs.

However, for applications requiring real-time data or very specific crawling logic, many companies choose to develop their own crawlers. Common programming languages and tools used for DIY crawlers include:

• Python libraries like Scrapy, BeautifulSoup, and Requests
• Node.js tools like node-crawler and Headless Chrome
• Apache Nutch, an extensible open source crawler in Java
• Crawling-as-a-service platforms like Diffbot and Apify

Regardless of the technical approach, companies looking to leverage web crawlers and web data need to be aware of the legal and ethical considerations. Most legitimate crawlers identify themselves with a unique user agent string and respect the instructions laid out in robots.txt on each domain. Failure to crawl responsibly could result in IP blocking, CAPTCHAs, or even legal action.

Businesses should also be careful about crawling any content behind a login wall or that is copyrighted, as this may violate terms of service. When in doubt, check with the website owner before crawling or scraping any data.

The Future of Web Crawlers

The web today is a massive, complex, and ever-evolving information ecosystem. By some estimates, there are over 1.9 billion websites and more than 5 billion web pages currently live. The web is also getting more dynamic and application-like, with JavaScript frameworks, single-page apps, and interactive experiences becoming the norm.

In this environment, traditional web crawlers face some big challenges:

• Scale: With the size of the web doubling every few years and the number of connected devices exploding, web crawlers need to be more efficient and scalable than ever. This means distributed architectures, optimized algorithms, and smart techniques for data compression and de-duplication.

• Dynamic content: Many modern websites are built with JavaScript frameworks like React that render content on the fly. This client-side content is invisible to traditional HTML-only crawlers. Search engines are investing in new crawling technology that can fully render and interact with web pages as a browser would.

• Structured data: As more websites adopt structured data formats like schema.org and JSON-LD, crawlers are evolving to better understand and extract this semantic information. This machine-readable data is powering new applications like voice search, knowledge panels, and entity-based search.

• Quality: With the rise of fake news, content farms, and AI-generated spam, search engines are doubling down on quality and authority signals to separate the wheat from the chaff. Web crawlers play a key role in assessing factors like expertise, authenticity, and trustworthiness.

• Privacy: With new privacy regulations like GDPR and CCPA, crawlers must be transparent about their data collection and usage. Search engines are moving away from individual tracking and towards more anonymous and aggregate data signals.

Looking further ahead, many experts predict a major evolution of the web from a collection of pages to a more structured, entity-centric knowledge graph. In this vision, web crawlers will evolve into intelligent agents that can understand and extract knowledge from multiple modalities like text, images, and video.

We‘re also likely to see more specialized crawlers for specific verticals and use cases beyond web search, like e-commerce, news, social media, and the Internet of Things. As the types and volume of web data continue to explode, crawlers will need to become smarter and more adaptive.

Conclusion

Web crawlers are the unsung heroes of the internet age, quietly powering the search engines and data pipelines that we depend on every day. As a web crawling and data expert, I‘ve seen firsthand the massive impact that crawlers have had on everything from business to science to culture.

For website owners and marketers, understanding how web crawlers work and what they value is key to search visibility and traffic. By optimizing your site architecture, content, and technical factors for crawlability, you can ensure that your most important pages are discovered, indexed, and ranked.

More broadly, web crawlers offer a powerful toolkit for businesses to gather data, generate insights, and automate competitive intelligence. As the web continues to evolve in size and complexity, crawlers will also need to adapt and become smarter.

One thing is certain: web crawlers will continue to be essential infrastructure for the internet ecosystem for years to come. As an expert in this space, I‘m excited to see how crawlers and web data extraction will evolve and unlock new possibilities across industries.

The key takeaways from this in-depth guide:

• Web crawlers are the core technology that powers search engines by discovering, fetching, and indexing web pages
• Crawlers follow links to continually find new content and use system of URL prioritization and scheduling
• Search engine crawlers are the most important bots to pay attention to for SEO and website owners
• Optimizing your site‘s crawlability and index bloat are key technical SEO considerations
• Web crawlers also power many data mining, archival, and business intelligence applications
• As the web gets bigger and more complex, crawlers face challenges of scale, quality, and structured data
• Web crawlers and data extraction will continue to evolve and be critical infrastructure for the internet

I hope you found this guide informative and comprehensive. Feel free to reach out with any questions! Happy crawling!