Web Engineering
An HTTP ETag gives a cached response a version to check
An ETag lets a browser ask whether its cached representation is still current. Follow a simple conditional request without confusing validation with an error.
SOFTWARE · SECURITY · DESIGN
HTTP caching
Web Development
The HTTP Vary Header Tells a Cache Which Request Differences Matter
Use an Accept-Language example to understand what Vary contributes to a cache key and why it does not replace an explicit cache-control policy.