Web Crawler
A web crawler is a program that automatically visits web pages, downloads their content and follows links to find more pages. Search engines use crawlers, also called spiders or bots, to discover content for their index, and AI companies use them to gather data and fetch pages for answers.
How a web crawler works
Crawling is the first step in how search engines work. A crawler starts with a list of known URLs, often from previous crawls and sitemaps. It then:
- Picks a URL from its queue, based on priority.
- Checks the site’s robots.txt file to see whether it is allowed to fetch that URL.
- Requests the page from the server and records the response code.
- Reads the content and extracts the links it contains.
- Adds new URLs to the queue and repeats the process.
Modern search crawlers can also render pages, running JavaScript as a browser would, so they see content that loads after the initial HTML. Rendering is costly, so it may happen later than the first fetch.
Common web crawlers
| Crawler | Operated by | Main purpose |
|---|---|---|
| Googlebot | Google Search indexing | |
| Bingbot | Microsoft | Bing search and services built on it |
| GPTBot | OpenAI | Collecting data for AI model training |
| Applebot | Apple | Siri and Spotlight suggestions |
| Screaming Frog, Sitebulb and similar | SEO tools | Auditing sites on request |
Each crawler identifies itself with a user-agent string, which servers record in their logs.
Google’s main crawler is Googlebot, which crawls mostly as a smartphone browser because of mobile-first indexing.
Bots run by AI companies are covered under AI crawlers. Some gather training data while others fetch pages live to answer a user’s question.
Why crawlers matter for SEO
If a search engine’s crawler cannot reach a page, that page cannot be indexed or ranked. Problems such as broken links, server errors, blocked resources or pages with no internal links all reduce what crawlers can find. On very large sites, wasting crawler effort on low-value URLs can also slow the discovery of important pages, an issue known as crawl budget.
How to manage crawlers on your site
- Use robots.txt to tell crawlers which areas they should not fetch. Reputable bots follow it, but it is not a security measure.
- Submit an XML sitemap so crawlers can find your important URLs.
- Link to every important page from somewhere on your site.
- Keep servers fast and reliable, since slow responses can make crawlers back off.
- Check server logs to see which bots visit and what they request.
Common mistakes
- Blocking CSS or JavaScript files that crawlers need to render pages properly.
- Using robots.txt to hide pages from search results: blocked pages can still be indexed if linked elsewhere. Use a noindex tag instead.
- Trusting any bot that claims to be Googlebot. Fake crawlers are common, so verify them with a reverse DNS lookup.
Further reading: Google’s overview of its crawlers and fetchers.
Key Takeaways
- A web crawler automatically fetches pages and follows links to find more.
- Search engines rely on crawlers to discover content for their index.
- AI companies run their own crawlers for training data and live answers.
- Robots.txt guides well-behaved crawlers but does not secure content.
- Pages that crawlers cannot reach cannot rank.
Related Terms
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