A web crawler, also known as a spider or bot, is an automated program that systematically browses the internet by following links from page to page. Search engines like Google use crawlers to discover new content, update their index of existing pages, and remove pages that are no longer available.
Googlebot, Google's web crawler, operates by maintaining a queue of URLs to visit. It fetches each page, extracts the links, adds new URLs to the queue, and stores the page content for indexing. The crawler respects robots.txt directives, follows redirect chains, and adjusts its crawl rate based on server responsiveness to avoid overloading websites. Search engine books on Amazon explain crawler architecture.
For URL shortening services, web crawlers interact with short URLs during the crawling process. When a crawler encounters a short URL on a web page, it follows the redirect to discover the destination URL. The crawler then indexes the destination page, not the short URL itself. This behavior is desirable because it ensures the original content receives the SEO benefit.
Crawl budget, the number of pages a search engine will crawl on a site within a given timeframe, is an important consideration for large sites. Ensuring that crawlers can efficiently discover and access important pages through clean internal linking and sitemaps maximizes the use of crawl budget. SEO crawl optimization books on Amazon discuss these strategies.