Why does HttpSendHttpResponse with CachePolicy not cache responses in HTTP.sys?

Question

I am writing an HTTP server in Delphi on top of HTTP.sys. I want to let the kernel cache certain GET responses so they are served directly without my code seeing the request again.

Here’s the simplified code:

AResponse.StatusCode := 200;
AResponse.Reason := 'OK';
AResponse.CachePolicyType := TALHttpServerResponse.TCachePolicyType.TimeToLive;
AResponse.CacheSecondsToLive := 500;
AResponse.BodyString := 'OK';
AResponse.Headers.ContentLength := '2';
HttpSendHttpResponse(
 FRequestQueueHandle,
 LRequestId,
 0,
 @LHttpResponse,
 @LCachePolicy, // points to the above cache policy
 nil,
 nil,
 0,
 nil,
 nil);

I expected the next request for the same URL to be served from the kernel cache. But when I request the URL again, HttpReceiveHttpRequest still fires and the request reaches my server code.

If I check with:

netsh http show cachestate url=http://localhost:23456/echo/
there are no entries in the cache.

Question

• Why isn’t my response cached in HTTP.sys ?

• What exact conditions must be met for HTTP.sys to actually store and serve the response from its kernel cache?

• Simon Mourier: This is documented here Kernel Mode Cache
• Lex Li: If you believe there wasn’t enough documentation to read, you might want to dig into github.com/dotnet/aspnetcore/tree/main/src/Servers/HttpSys as Microsoft uses the related API in ASP.NET Core.
• Remy Lebeau: @Lex but, if you actually search that repo, there is only 1 spot where HttpSendHttpResponse is called (HttpSysListener.cs line 364), and it doesn't use a caching policy.
• Remy Lebeau: @zeus in the code you have shown above, how is AResponse related to LHttpResponse or LCachePolicy? How are you actually populating LCachePolicy?
• zeus: @RemyLebeau I found the reason it's because i must set AResponse.Headers.ContentType := 'Content-Type: text/html; charset=UTF-8'; Without this the response is not cached :( this was explained nowhere in the microsoft doc :(