Not Modified Response, Caching Optimization and Output Caching Feature in IIS

introduction
Caching plays a vital role in optimizing web performance. When you enable output caching for a specific file extension or URL in IIS, you will receive a 304 Not Modified status code. This status code is essential for managing caching because it indicates whether the resource has been modified since it was last requested.

How it works

The 304 Not Modified status code indicates that a conditional GET or HEAD request was made and that the resource would have received a 200 OK response if the specified condition had not evaluated to false. Essentially, this means that the server does not need to send the resource again because the client already has a valid copy. In effect, the server instructs the client to use the cached version as if it had received a 200 OK response.

When the server sends a 304 response, it must include the headers that would have been sent with a 200 OK response, such as .

• Content-location, date, ETag and variety

• Cache control and expiration

For demo purposes, the site was hosted on IIS with output caching enabled for the .js extension.

While browsing the site, you’ll notice that requests for .js files return a 304 Not Modified status code, and the server sends an ETag that matches the request header If-None-Match.

Since the purpose of a 304 response is to minimize data transfer when the client already has a cached version, the server does not include any metadata other than required fields that are not needed for cache management.

conclusion
The HTTP 304 Not Modified status code is used for web caching and performance optimization. It informs the client to use an existing cached copy, assuming that the conditional catch conditions specified in the request header can be verified.

References

HTTP Status Codes Overview – Internet Information Services | Microsoft Learn

RFC 9110 – HTTP Semantics (ietf.org)