What are Web Crawlers & How Do Crawlers Work?

Before a page can appear in search results or AI-generated answers, a program has to visit it and read it. That program is a web crawler. Understanding crawlers helps you make sure your important pages get found, and lets you decide which bots can access your content.

In this article, we explain what web crawlers are, how they work, the new wave of AI crawlers, and how to optimize your website for them.

What are Web Crawlers?

1. Definition:

• Web crawlers, also called spiders or bots, are automated programs that browse the web methodically. Search engines use them to collect pages and build an index, a giant library they search when you type a query. Well-known examples are Googlebot (Google) and Bingbot (Microsoft Bing).

2. Key Functions:

• Discovery: Crawlers find new and updated pages by following links and reading sitemaps.
• Fetching and Rendering: They download pages and, for modern sites, run JavaScript to see the final content.
• Feeding the Index: The content they collect is processed and stored so search engines can show it in results.
• Monitoring: They revisit pages to catch changes and keep the index fresh.

How Do Web Crawlers Work?

1. Starting Point: Crawlers begin with a list of known URLs. New URLs come from links on other pages and from XML sitemaps you submit, for example in Google Search Console.

2. Following Links: On each page, crawlers find links and add them to their queue. This is why internal linking matters: a page with no links pointing to it is hard to discover.

3. Analyzing Content: Crawlers read the HTML, text, metadata, images and links on each page. Search engines then process this data to understand what the page is about.

4. Handling Instructions: Site owners give crawlers instructions through a robots.txt file (which URLs a crawler may access) and robots meta tags such as noindex (whether a page may appear in results).

5. Managing Crawl Budget: Search engines limit how much they crawl per site to avoid overloading servers. For most websites this is not a concern: Google says crawl budget mainly matters for sites with 1 million+ pages that change weekly, or 10,000+ pages that change daily (Google).

Googlebot and mobile-first indexing

Googlebot has two main versions: Googlebot Smartphone and Googlebot Desktop (Google crawler documentation). Google now uses the smartphone crawler for most crawling and indexing, and announced in October 2023 that the move to mobile-first indexing was complete (Google Search Central Blog).

In practice, the mobile version of your site is the version Google judges. If content, links or structured data are missing on mobile, Google may never see them.

AI crawlers and how to control them

Search engines are no longer the only bots visiting your site. AI companies run crawlers to train models and to power AI search features. Each can be managed separately in robots.txt:

• Google-Extended: A robots.txt token that controls whether content Google crawls may be used to train future Gemini models. Google states it "does not impact a site's inclusion in Google Search nor is it used as a ranking signal" (Google).
• GPTBot: OpenAI's crawler for improving its generative AI models. Blocking it opts your content out of training.
• OAI-SearchBot: OpenAI's crawler for ChatGPT search results. Allow it if you want your pages to be eligible to appear there.
• ChatGPT-User: Visits pages when a ChatGPT user asks it to. OpenAI says robots.txt rules may not apply because these visits are user-initiated (OpenAI).

For example, this robots.txt blocks model training by OpenAI and Google while keeping search access open:

User-agent: GPTBot
Disallow: /

User-agent: Google-Extended
Disallow: /

User-agent: *
Allow: /

Sitemap: https://example.com/sitemap.xml

Think before blocking. Blocking training crawlers doesn't affect Google Search rankings, but blocking search crawlers like Googlebot or OAI-SearchBot removes you from those search experiences.

Importance of Web Crawlers for SEO

1. Ensuring Visibility: If crawlers can't reach a page, it can't rank. Crawlability is the first step of every SEO strategy.

2. Supporting Rankings: Crawlers collect the content and signals that ranking systems evaluate, such as page content, links, structured data and mobile experience.

3. Monitoring Site Health: SEO crawling tools that work like search engine bots reveal broken links, redirect chains, duplicate content and other issues. Google Search Console's Page indexing and Crawl stats reports show how Googlebot sees your site.

4. Keeping Content Fresh: When you update pages and your sitemap shows an accurate last-modified date, crawlers can pick up changes sooner.

How to Optimize for Web Crawlers

1. Create an XML Sitemap: List your important, indexable URLs with accurate lastmod dates. Google ignores the priority and changefreq values (Google Search Central Blog).

2. Configure Robots.txt Carefully: Use robots.txt to keep crawlers out of low-value areas like internal search results. Remember that robots.txt "is not a mechanism for keeping a web page out of Google"; use noindex or password protection for that (Google). Check for errors in Search Console's robots.txt report.

3. Use Descriptive URLs and Meta Tags: Clear URLs, titles and descriptions help search engines understand your pages.

4. Improve Site Structure: Keep important pages within a few clicks of the homepage and link related pages together. Fix broken links.

5. Ensure Mobile-Friendliness: With mobile-first indexing, make sure your mobile site has the same content, links and structured data as desktop.

6. Make JavaScript Content Crawlable: Use real <a href> links, and make sure key content appears in the rendered HTML. Server-side rendering helps many crawlers, including AI bots that may not run JavaScript.

7. Regularly Update Content: Keep important pages accurate and current so crawlers find something worth revisiting.

Conclusion on What are Web Crawlers & How Do Crawlers Work?

Web crawlers are how search engines and AI tools discover your content. Googlebot now crawls mainly as a smartphone, and AI crawlers add new choices about who can use your content.

Make your site easy to crawl, use robots.txt and noindex for their intended purposes, and decide deliberately which AI bots you allow.

Take Control of Your SEO Strategy!

Not sure whether crawlers can reach your key pages, or which AI bots to allow? Softhat IT Solutions reviews crawlability, sitemaps and robots.txt as part of technical SEO for businesses across Pakistan.

Explore our SEO services or book a free strategy call to check your site.