HTTP methods for indicating when a resource has changed

HTTP header fields

• ETag   and   If-None-Match
• Last-Modified   and   If-Unmodified-Since
• Expires
• Cache-Control
• Date?

Other HTTP things related

• 304 Not Modified

Articles

• https://en.wikipedia.org/wiki/HTTP_ETag
• https://en.wikipedia.org/wiki/Web_cache
• https://en.wikipedia.org/wiki/List_of_HTTP_status_codes#304