How web crawlers interact with websites.

What a web crawler does

A web crawler is software that retrieves resources from the web automatically. Search engines use crawlers to find pages and collect signals for later processing. Other crawlers support archiving, uptime monitoring, security testing, link analysis, accessibility checks, research, and content feeds.

A crawler normally starts with a list of known URLs. It requests a URL, reads the response, extracts useful links or references, places eligible URLs into a queue, and repeats the process under its own scheduling and policy rules.

A simplified crawl cycle

1. URL discovery: the crawler learns a URL from a link, sitemap, redirect, feed, previous crawl, manual submission, or another data source.
2. Policy check: it evaluates robots.txt and its own operator rules.
3. Request: it sends an HTTP request with headers that may include a user agent.
4. Response handling: it records the status code, headers, content type, body, redirect target, and timing.
5. Parsing and rendering: it extracts links and content. Some systems also render JavaScript.
6. Scheduling: it decides whether and when to revisit the URL and which newly found URLs should enter the queue.

Crawler, renderer, and indexer

These terms describe different functions. A crawler fetches a resource. A renderer may execute JavaScript and create a processed representation. An indexer analyses the result and decides how it should be stored or used. One product may combine these systems, but the distinction explains why a fetched page does not automatically become an indexed search result.

How crawlers find pages

Internal links remain one of the clearest discovery paths. XML sitemaps provide an additional list of preferred URLs. Redirects, external links, feeds, structured data references, and previous crawls can also expose URLs. A page that exists only behind a search form or client-side interaction may be harder to find.

How websites influence crawling

• Robots.txt requests that compliant crawlers avoid matching paths.
• Status codes explain whether a resource exists, moved, failed, or is unavailable.
• Internal links show relationships and provide discovery routes.
• Sitemaps list canonical URLs that a site wants crawled.
• Server performance affects how safely and efficiently a crawler can request pages.
• Authentication protects resources that should not be public.

What server logs can tell you

Access logs can show requested URLs, timestamps, status codes, response sizes, source addresses, user agents, and request volume. They help answer whether a crawler reached a page and how the server responded. They cannot prove that a search engine indexed the content or ranked it.

Signs of healthy crawlability

• Important pages return stable 200 responses.
• Internal links use crawlable URLs and normal anchor elements.
• Robots.txt does not block essential pages or required resources accidentally.
• Canonical tags, redirects, and sitemap URLs point to the same preferred versions.
• Error pages use accurate status codes.
• The server can handle legitimate crawler traffic without repeated failures.

Further reading

For the standard robots rules used during crawl-policy checks, see RFC 9309. Search-engine-specific behaviour should be checked in the operator's current documentation.