ETag
The ETag (or entity tag ) HTTP response header is an identifier for a specific version of a resource. It lets caches be more efficient and save bandwidth, as a web server does not need to resend a full response if the content was not changed. Additionally, etags help to prevent simultaneous updates of a resource from overwriting each other ( "mid-air collisions" ). If the resource at a given URL changes, a new Etag value must be generated. A comparison of them can determine whether two representations of a resource are the same. Syntax ETag: W/"<etag_value>" ETag: "<etag_value>" Directives W/ (Optional ) 'W/' (case-sensitive) indicates that a weak validator is used. Weak etags are easy to generate, but are far less useful for comparisons. Strong validators are ideal for comparisons but can be very difficult to generate efficiently. Weak ETag values of two representations of the same reso...