How a web crawler works
A crawler starts with one or more seed URLs. It fetches each page, parses its HTML, extracts links and adds unseen URLs to a queue called the crawl frontier. A scheduler chooses the next URL based on rules such as domain, priority and delay.
Seed URL → fetch page → parse links → queue new URLs → repeatThe crawler may store page content, response status, link relationships and metadata such as the title or last-modified date. Search engines use this information to build indexes. Site owners also run crawlers to find broken links and missing pages.
Web crawler vs web scraper
Crawling discovers pages. Web scraping extracts selected information from them.
| Web crawler | Web scraper | |
|---|---|---|
| Main job | Discover and fetch pages | Extract specific fields |
| Typical input | Seed URLs and crawl rules | Known pages or page types |
| Typical output | Pages, URLs and link graphs | Structured records |
One application can do both. A crawler might find every product page on a store, while a scraper extracts each product’s name, price and availability.
Crawl controls
A well-configured crawler limits what it visits and how quickly it sends requests:
- Scope: restrict crawling by hostname, path, file type or link depth.
- Site instructions: read robots.txt and avoid disallowed paths. These rules are guidance, not access control.
- Request pace: set per-host concurrency and delays, follow rate limits and honor
Retry-Afterafter a 429 response. - Duplicate handling: normalize URLs, track visited URLs and filter or limit query parameters to prevent crawl traps. Use content hashes after fetching to avoid processing duplicate content.
- Rendering: use a headless browser only when a page needs JavaScript, since a browser consumes more memory and bandwidth than a basic HTTP client.
How proxies fit
A proxy changes the source IP that a website sees. Crawlers may use proxies for authorized checks of location-specific public content or to keep separate collection jobs on separate addresses.
A rotating proxy can distribute requests across IPs, but it doesn’t reduce the total load placed on the website. Apply rate limits across the whole crawl, not just per proxy. Changing IPs also doesn’t fix unrealistic headers, browser fingerprints or aggressive request patterns that trigger bot detection.
FAQ
What is a web crawler used for?
A web crawler is used for search indexing, website audits, content monitoring and discovering pages for data collection. Its output may also feed a scraper that extracts structured fields.
Is a web crawler the same as a web scraper?
No. A crawler finds and fetches pages, while a scraper extracts selected data from those pages. Many collection systems combine both functions.
Are web crawlers legal?
It depends on the site, data, jurisdiction and purpose. Follow the site’s terms and robots.txt, respect access controls and don’t collect private or restricted information without authorization.