Understanding HTTP Status Code 304 Not Modified Response and Output Caching Feature in IIS

Introduction
Caching plays a crucial role in optimizing web performance. When Output Caching is enabled in IIS for specific file extensions or URLs, it deliver 304 Not Modified status code. This status code is essential for managing caching, as it indicates whether a resource has been modified since the last time it was requested.

How it works

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

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

• Content-Location, Date, ETag, and Vary

• Cache-Control and Expires

For demonstration purposes, a site has been hosted in IIS with Output Caching enabled for the .js extension.

While browsing the site it is evident that requests for .js files return a 304 Not Modified status code, along with that the server sending an ETag which was matched with the Request Header If-None-Match.

Since the purpose of a 304 response is to minimize data transfer when the client already holds a cached version, the server avoids including metadata other than the required fields unless that metadata is necessary for cache management.

Conclusion
The HTTP 304 Not Modified status code is used for web caching and performance optimization. It notifies client to use an existing cached copy considering it is able to validate the specified condition catch condition in request header.

References

HTTP status code overview – Internet Information Services | Microsoft Learn

RFC 9110 – HTTP Semantics (ietf.org)

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