ETags, IIS7, Kernel Cache Policy (enableKernelCache)

Question

Two-part question (parts are closely related): with the default OOTB ETag policy that IIS7 employs, why don't we see the If-None-Match/304 interaction as we navigate through pages?

The headers returned for an empty-cache request, for instance, are:

Content-Type    image/png
Last-Modified   Thu, 03 Dec 2009 15:51:56 GMT
Accept-Ranges   bytes
Etag    "a8a0628a3074ca1:0"
Server  Microsoft-IIS/7.0
X-Powered-By    ASP.NET
Date    Tue, 22 Dec 2009 19:47:36 GMT
Content-Length  1780

...and yet subsequent accesses to the page don't generate a 304 round-trip for the image?

Also, the default applicationHost file for IIS7 has the following (1):

   <caching enabled="true" enableKernelCache="true">
   </caching>

Does enableKernelCache='true' extend to all static files, freeing you of the need to register extensions explicitly to grant CacheUntilChange as the kernel policy (2):

<caching>
  <profiles>
    <add extension=".gif" policy="DontCache" kernelCachePolicy="CacheUntilChange" duration="0.00:01:00" location="Any" />
    <add extension=".png" policy="DontCache" kernelCachePolicy="CacheUntilChange" duration="0.00:01:00" location="Any" />
    <add extension=".js" policy="DontCache" kernelCachePolicy="CacheUntilChange" duration="0.00:01:00" location="Any" />
    <add extension=".css" policy="DontCache" kernelCachePolicy="CacheUntilChange" duration="0.00:01:00" location="Any" />
    <add extension=".jpg" policy="DontCache" kernelCachePolicy="CacheUntilChange" duration="0.00:01:00" location="Any" />
    <add extension=".jpeg" policy="DontCache" kernelCachePolicy="CacheUntilChange" duration="0.00:01:00" location="Any" />
  </profiles>
</caching>

(1) %systemroot%\System32\inetsrv\config\applicationHost.config

(2) http://labs.episerver.com/en/Blogs/Per/Archive/2009/3/Configuring-cache-expiration-on-IIS-7/

Answer 1

The handling of ETags and the associated If-None-Match / If-Modified-Since is somewhat browser dependent. You might try a few different browsers and see what happens -- in general, if you don't set an explicit expiration time, I would expect to see the 304's, as you said.

For kernel caching, it is enabled for static files by default. To help see what's happening, I've found it helpful to run the following command:

netsh http show cachestate

That will show information about the files that are currently in the cache.

Keep in mind that files normally have to be referenced a couple of times within a certain time window before the kernel will cache them.