Should WebRequest.CachePolicy work from code running within IIS?

Question

I have some code running within an ApiController (ASP.Net Web API) that itself wants to make a GET request to another web service. The web service (also part of my app) returns Cache-Control headers indicating an expiry time for the content it returns.

I am using the new System.Net.Http.HttpClient, configured with a WebRequestHandler in order to use client-side caching (the default HttpClientHandler does not support cache configuration, although it does use System.Net.WebRequest as its underlying HTTP implementation):

var client = new HttpClient(new WebRequestHandler { 
  UseDefaultCredentials = true,
  CachePolicy = new RequestCachePolicy(RequestCacheLevel.Default)
});
var response = client.GetAsync("someUri").Result;
response.EnsureSuccessStatusCode();

On the server I am enabling caching within my controller action via...

var response = new HttpResponseMessage(HttpStatusCode.OK);
response.Headers.CacheControl = new CacheControlHeaderValue {
  Public = true,
  MaxAge = new TimeSpan(0, 5, 0); // Five minutes in this case
};

// Omitted, some content is added to the response
return response;

The above (abbreviated) code works correctly within a test; I make multiple calls to the service in this way and only the first call actually contacts the service (observed via log messages on the service in IIS); subsequent calls use the cache.

However, running the same code hosted on IIS itself, it seems the HttpClient ignores the caching result (I have also set up my IoC container such that only one instance of the HttpClient exists in the AppDomain) and calls the service each time. This is running as the AppPoolIdentity.

Interestingly, if I change the app pool to run as NetworkService, then the response has status code 401 Unauthorized (I have tried setting Preauthenticate = true on the WebRequestHandler but the status code is still 401). The same is true if I change the app pool to run under my own credentials.

So, is there something about running the App Pool under the NetworkService identity, and virtual AppPoolIdentity, that prevents them from using client-side caching. Where does the content cached by WebRequest physically exist anyway?

Answer 1

WinInet caching is not supported when running on IIS, please check the following support article from MS http://support.microsoft.com/kb/238425