What are Crawlers?
Web crawlers are automated software tools that browse the internet to gather information about websites. They are also called spiders or bots.
Search engines like Google, Bing, and Yahoo use these crawlers to scan pages on websites. This allows the search engines to index the content and decide how relevant those pages are for users' searches.
How do web crawlers work?
Web crawlers work by following a set of predefined rules.
Here's a simplified overview of how they work:
The crawler starts by visiting a seed URL, typically the home page.
Before crawling a site, crawlers check for robots.txt or meta robots tags to see which pages on the site can be crawled and which should be avoided.
The crawler extracts all the links on the page and follows them to crawl pages on the same site or external sites.
On each page, the crawler extracts information such as text content, images, and metadata. This information is then stored in a database.
Crawlers are designed to avoid getting stuck in infinite loops or "trap" pages such as search infinite redirect pages.
The data collected by crawlers is used to update the search engines's large index that handles all searches.
Common examples of web crawlers
Here are some examples of web crawlers:
Googlebot: Googlebot is the name of Google's web crawler. It is one of the most widely used crawlers on the internet. Googlebot is responsible for finding and indexing billions of web pages to make them searchable in Google.
Bingbot: Bing's web crawler is another popular crawler used by the Bing search engine. It follows links on web pages and collects information about the content of each page.
Baidu Spider: Baidu, the leading search engine in China, uses the Baidu Spider to crawl and index web pages.
Yandex Bot: Yandex, the most popular search engine in Russia, uses the Yandex Bot.
Screaming Frog: Screaming Frog is software that website owners download and run on their computers to crawl their own sites. It scans pages to uncover common SEO problems like broken links, duplicate content, and missing page titles and descriptions.
Why are web crawlers also called 'spiders'?
Web crawlers get the name "spiders" because of how they explore websites, similar to how spiders crawl around their webs.
This comparison started back in 1993 with the first search engine called "WebCrawler."
The "spider" name for web crawlers makes sense because they chart out the complex web of internet content like a spider spinning an intricate web.
They systematically travel through the complex digital network of connected websites, gathering details as they go. This gets stored in a central database.
The collected information powers search engine indexes so people can easily find relevant sites.
Types of Crawlers
There are several types of web crawlers, each with its own characteristics and purposes. The most common ones are:
Search engine crawlers
Search engine crawlers, also known as spiders, are used by search engines to discover and index web pages. They follow links on websites to find new pages and collect information about the content of each page.
The data collected by search engine crawlers is used to create search engine indexes, which allow users to find relevant information on the web.
Focused crawlers
Focused crawlers are designed to scan only certain websites or content types. For example, one focused crawler may explore just news sites, while another may crawl e-commerce sites.
They are pointed at specific areas, unlike broader general crawlers used by search engines. Focused crawlers can gather valuable data and insights from targeted parts of the web.
Researchers may use them to collect information from niche sites related to their interests.
Incremental crawlers
Incremental crawlers are designed to crawl only new or updated content on a website since the last crawl. This allows them to be more efficient and avoid re-crawling content that has not changed.
Distributed crawlers
Distributed crawlers are designed to crawl the web using a distributed network of computers. This allows them to be more scalable and handle large volumes of data.
Deep web crawlers
Deep web crawlers are designed to access content that regular search engine crawlers cannot, such as content behind login screens, databases, or dynamic web pages.
These specialized crawlers let researchers, police, and companies gather hard-to-reach public data.
However, the deep web also contains lots of illegal and unethical content. So creating deep web crawlers brings up privacy issues and must be done carefully.
Academic research crawlers
These crawlers are designed for research purposes. They are often used by universities or research institutions to study the structure of the web or analyze the behaviour of web users.
Good vs. Bad Crawlers
Web crawlers can be either "good" or "bad".
Good crawlers are those that follow best practices and ethical standards when crawling websites.
They typically identify themselves in the user-agent field of their HTTP requests and obey the rules set out in a website's robots.txt file or in the robots meta tag.
Good crawlers are typically used by search engines to index websites and improve their search results.
Bad crawlers - also known as "malicious" or "rogue" crawlers - are those who violate ethical standards and laws.
They may ignore robots.txt files, hide their identity or engage in activities such as scraping or data theft.
Bad crawlers can be used by hackers or spammers to collect data, launch attacks, or spam websites with unwanted requests.
Mobile vs. Desktop Crawler Versions
The mobile crawler is designed to render and index web pages as they would appear on mobile devices, to improve search results for mobile users.
It can understand and process mobile-friendly pages, responsive pages that adapt to different devices, AMP pages, etc. The desktop crawler, on the other hand, indexes pages based on how they would normally look on a computer screen.
Having separate mobile and desktop crawlers allows Google to better understand how each page works on those platforms. It lets them detect if a page has mobile usability issues, redirected mobile URLs, or design elements that should be recommended for mobile optimization.
It is also important for website owners and SEO professionals.
With the increasing popularity of mobile devices, websites need to be mobile-friendly and optimized for mobile crawlers.
This can improve the website's visibility in mobile search results and attract more traffic from mobile users.
Should web crawler bots always be allowed to access web properties?
Allowing web crawler bots to access web properties depends on the goals and priorities of website owners. Here are some factors to consider:
SEO benefits: Allowing web crawler bots to access web properties can improve a website's visibility. It leads to more traffic and potential customers.
Security risks: Web crawler bots can also be used by hackers to scrape sensitive data or launch attacks on websites. Allowing unchecked access can increase the risk of such attacks.
Bandwidth and server resources: Web crawler bots consume server resources such as bandwidth, CPU, and memory. Allowing too many bots can slow down the website or even crash the server.
To balance these factors, website owners can use various techniques to control the behaviour of web crawler bots.
For example, they can use a robots.txt file to limit the pages that bots can access, set rate limits to control the frequency of bot requests or use CAPTCHAs to distinguish between human and bot traffic.
Is there a difference between web crawling and web scraping?
Web crawling and web scraping are two related but distinct techniques for collecting data from the web.
Web crawling is typically performed by software programs called web crawlers or spiders. It is often used by search engines.
Web scraping, on the other hand, means extracting particular information from websites, usually by analyzing the HTML code. It goes beyond just reading what's visible.
Scrapers can pull data that are typically only available behind login screens or tucked in databases. So they access deeper sets of a site's information.
Web scraping can be done manually but is typically automated using specialized scraping tools or scripts.
In short, web crawling is a more general process of exploring and indexing the web, while web scraping is a more targeted process of extracting specific data.
Why web crawlers are important for SEO?
Web crawlers are important for SEO for several reasons:
Indexing: Crawlers discover pages so search engines can list them for relevant searches. No crawling means no indexing.
Site structure analysis: Web crawlers map out the structure of a website, including the hierarchy of pages, the internal linking structure, and the sitemap. This data helps optimize site navigation.
Keyword analysis: Bots scan to see keywords used on each page. This helps match pages to search queries.
Technical SEO analysis: Web spiders detect problems like broken links, duplicate content, and slow loading. Fixing these can improve SEO.
Competitor analysis: Crawlers can also be used for competitor analysis. This information can help SEO professionals identify opportunities to improve their own ranking.
Best practices for a crawl-friendly website
To ensure that your website is crawl-friendly, here are some best practices to follow:
Use a clear site structure
Use a clear and organized site structure, with a logical hierarchy of pages and a clear navigation menu.
Optimize your URLs
Use descriptive and readable URLs that accurately reflect the content of each page.
Use a sitemap
Use a sitemap to provide search engine crawlers with a list of all the pages on your website.
Optimize your content
Use clear and concise content that is optimized for relevant keywords and provides value to your users. Use descriptive headers and subheaders, and avoid keyword stuffing or duplicating content.
Use meta tags
Use descriptive meta tags, including title tags and meta descriptions, to provide search engines with a summary of the content on each page.
Optimize your images
Optimize your images by using descriptive file names and alt text, and compressing them to reduce page load times. This improves the user experience and can improve your website's visibility in search results.
Monitor your crawl errors
Website owners should regularly check for any crawl errors and fix them quickly. This allows crawlers to fully index all content.
Takeaway
Web crawlers are automated tools that browse websites and collect data about the pages. Search engines rely on them to index sites and decide rankings.
They are important for SEO because they help search engines and SEO experts analyze site structure, keyword usage, and technical issues.