webcrawl

A webcrawl, or web crawler, is an automated software program that traverses the World Wide Web by downloading web pages, following hyperlinks, and collecting data for purposes such as indexing or analysis. Web crawls typically begin with a set of seed URLs and iteratively fetch pages, extract hyperlinks, and add new targets to a crawl frontier. The collected content is stored and often processed by a separate indexing or data-analysis pipeline. The process is guided by policies that determine which sites to visit, how deeply to crawl, and how often to recrawl.

In practice, crawlers respect robots.txt files and meta tags, implement rate limits and politeness delays to avoid overloading servers, and manage bandwidth usage. They may also employ techniques to render or parse content, including handling dynamic pages that rely on JavaScript or using headless browsers for rendering.

Key components include a fetcher, a parser or link extractor, a storage or indexing component, and a scheduler or frontier manager. Crawlers can be general-purpose, aiming to cover the broad web, or focused, collecting content on a specific topic, region, or language. Large-scale crawlers support incremental crawling, deduplication, and parallelism to scale efficiently.

Common uses include powering search engines, enabling data mining and research, web archiving, and price or content monitoring. Legal and ethical considerations encompass copyright, privacy, and site policies, as well as compliance with robots exclusion standards and applicable laws.