recrawling

Recrawling is the process by which an information retrieval system, such as a search engine or a data-collection crawler, revisits pages it has previously crawled in order to refresh its understanding of the page and update its index or dataset. The goal is to maintain current, accurate results by detecting changes in content, structure, or availability, and to remove or deprecate pages that have become stale or unavailable.

In practice, recrawling uses a crawl queue with priorities. The crawler decides when to recrawl a page based on signals such as age since last fetch, observed update frequency, content volatility, and page importance. Signals may include last-modified headers, ETag values, sitemap change frequencies, and external signals like user traffic. The crawler respects robots.txt and crawl-delay and can use incremental methods to avoid re-fetching unchanged pages. Upon recrawl, the page is fetched, parsed, and compared to previous index data; changes may trigger index updates, redirects handling, or removal from the index if the page returns 404 or 410.

Scheduling and strategies vary: high-velocity sites may be recrawled frequently, while stable pages receive longer intervals. Adaptive systems adjust frequency based on observed volatility. Recrawling differs from a brand-new crawl in that the target is existing entries rather than discovery of new pages; however, recrawl can also reveal new internal links requiring discovery. Web archives and search indexes implement bespoke recrawl policies to balance freshness with resource limits.

Challenges include load on origin servers, crawl budget management, handling dynamic content and JavaScript-rendered pages, and avoiding overfitting to transient changes. Benefits include fresher results, better detection of removed or updated content, and improved indexing quality. Recrawling is a core component of maintaining up-to-date search indexes and data collections.