Web Crawlers

A crawler is a continuously expanding work pipeline#

The frontier owns priority, host grouping, schedule, retries, and freshness. Per-domain queues and rate limits prevent one site from dominating workers. Respect applicable robots directives and site policy rather than treating maximum fetch rate as the goal.

Normalize URLs carefully, use an exact set when exactness is required, or a Bloom filter as a memory-efficient hint where occasional false-positive skips are acceptable. Different URLs may return identical content, so content hashes can remove exact duplicates without pretending to solve near-duplicate similarity.