Web crawler

What is a web crawler?

A web crawler, also called a spider or bot, is a program that browses the web automatically. It starts from a list of known URLs, downloads each page, extracts the links it finds and adds them to a queue of pages to visit next. Googlebot, Bingbot and the bots used by AI assistants all work this way. Crawling is the first step of search: a page that is never crawled cannot be indexed, and a page that is not indexed cannot rank or be cited in an AI answer.

Crawlers also serve other purposes. SEO tools crawl your site to audit it, price comparison engines crawl product pages, and monitoring services crawl pages to detect changes. What they share is the same loop: fetch, parse, discover, repeat.

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How a web crawler works

A crawler follows a simple cycle. It picks a URL from its queue, checks the site's robots.txt file to see whether access is allowed, requests the page, reads the HTML and records the content. It then extracts every link and adds new URLs to the queue, prioritising them by signals such as popularity, freshness and depth.

• Seed URLs: the starting points, often your homepage and XML sitemap.
• Fetching: an HTTP request, with the response code (200, 301, 404, 500) telling the bot what to do next.
• Rendering: modern crawlers can execute JavaScript, but rendering is slower and queued separately from the initial fetch.
• Scheduling: each site gets a limited number of requests, known as crawl budget.

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Types of web crawlers

Not all bots serve the same goal, and treating them all alike is a common mistake.

• Search engine bots such as Googlebot and Bingbot, which feed the index.
• AI crawlers such as GPTBot, ClaudeBot and PerplexityBot, which collect content for training or for live answers.
• SEO audit crawlers such as Screaming Frog, Ahrefs Site Audit or Semrush, which you run on your own site to find technical issues.
• Scrapers and monitoring bots, which may or may not respect your rules.

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Web crawler vs web scraper

The two terms are often confused. A crawler discovers pages by following links, while a scraper extracts specific data from pages it already knows.

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How to control crawlers

You steer crawlers with a few well-known files and directives. The robots.txt file allows or blocks paths, for example:

User-agent: GPTBot
Disallow: /private/
Sitemap: https://www.example.com/sitemap.xml

• Use robots.txt to manage crawl access, not to hide pages from the index.
• Use noindex to keep a crawlable page out of search results.
• Submit an XML sitemap to help bots find important URLs.
• Keep internal linking clean so no key page sits more than three clicks from the homepage.

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Common crawl problems and how to fix them

Most crawl issues are simple to detect and cheap to fix. Redirect chains and 404 errors waste budget. Duplicate URLs created by parameters dilute signals. Heavy JavaScript can hide content from bots that do not render it. Slow servers make crawlers back off. A server log analysis shows exactly which URLs bots request and which they ignore, which is the most reliable way to check your assumptions.

AI crawlers add a new decision: whether to allow them. Blocking them protects content from training use but can reduce your visibility in AI-generated answers, so the choice should be deliberate.

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Web crawlers at BeBranded

Crawlability is the base of every technical audit we run. Our team checks robots.txt rules, sitemaps, redirects, rendering and log files, then fixes what blocks search engines and AI systems from reading your pages. See how we approach it in our SEO & GEO service.