Change detection and notification (CDN) is the automatic detection of changes made to World Wide Web pages and notification to interested users by email or other means.[1]
Whereas search engines are designed to find web pages, CDN systems are designed to monitor changes to web pages. Before change detection and notification, it was necessary for users to manually check for web page changes, either by revisiting web sites or periodically searching again. Efficient and effective change detection and notification is hampered by the fact that most servers do not accurately track content changes through Last-Modified or ETag web-server headers. In 2019 a comprehensive analysis regarding CDN systems was published.