To avoid repeated crawling during data crawling, incremental web crawlers use several strategies to track and identify already visited or updated content. Here's how it works with examples:
URL Deduplication: Store crawled URLs in a database or hash set to check if a URL has been visited before. For example, if a crawler encounters example.com/page1, it adds the URL to a database. If the same URL appears again, the crawler skips it.
Content Hashing: Generate a hash (e.g., MD5 or SHA-256) of the page content and compare it with previously stored hashes. If the hash matches, the content hasn’t changed, and the crawler skips reprocessing. For instance, if example.com/news has the same hash as before, the crawler avoids re-downloading it.
Last-Modified Headers: Use HTTP headers like Last-Modified or ETag to check if a page has been updated since the last crawl. If the server indicates no changes, the crawler skips the page. Example: A crawler checks example.com/blog and finds the Last-Modified date matches the previous crawl, so it skips downloading.
Change Detection Algorithms: Advanced crawlers use algorithms to detect minor content changes (e.g., ignoring ads or timestamps). For example, a news site may update timestamps frequently, but the core article text remains the same.
For scalable and efficient incremental crawling, Tencent Cloud offers services like Web+, TKE (Tencent Kubernetes Engine), and COS (Cloud Object Storage) to manage URL databases, content storage, and distributed crawling tasks. Additionally, Tencent Cloud CDN can help optimize content delivery and reduce redundant requests.