What is Web Crawler?

How a web crawler works

A crawler starts from a set of seed URLs placed in a queue known as the frontier. It fetches the first page, parses out every link, filters and normalizes the discovered URLs, and appends the new ones to the frontier — repeating until the queue is empty or a stopping rule is reached. A visited set prevents fetching the same URL twice, and URL normalization (stripping tracking parameters, resolving relative paths) keeps that set accurate.

Four policies govern behavior: a selection policy decides which pages are worth fetching, a revisit policy decides how often to refresh known pages, a politeness policy limits request rate per host, and a parallelization policy coordinates many fetches at once across different domains. Well-behaved crawlers also read robots.txt before crawling a host and honor its disallow rules and crawl-delay hints.

Crawlers identify themselves through the User-Agent header. Search-engine and research crawlers announce their names so site owners can allow or restrict them; scraping crawlers often present a standard browser User-Agent instead.

Why crawlers matter for scraping and data collection

Crawling is the discovery layer of nearly all web data collection: before a scraper can extract fields from a page, something has to find that page. For a product catalog, a news archive, or an entire domain, the crawler builds the URL inventory that the extraction stage consumes.

Scale changes the architecture. Politeness limits cap how fast any single host can be crawled, and per-IP rate limits cap how much any single address can fetch, so large crawls parallelize across many domains and distribute requests across many IPs — rotating residential proxy ports such as ProxyOmega's are a common way to supply that IP diversity.

Practical notes and common misconceptions

Crawler and scraper are not synonyms: the crawler discovers and fetches pages, the scraper extracts structured data from them, and most production systems combine both. Watch for crawler traps — infinite calendar pages, session IDs embedded in URLs, and honeypot links can balloon a frontier indefinitely — and defend with depth limits, URL pattern rules, and per-domain page budgets.

Respecting robots.txt is not legally mandatory in most places, but it is the accepted convention, and ignoring it is a fast way to get an IP range or User-Agent blocked. For crawls meant to run long-term, courteous pacing preserves access better than aggressive fetching.