ASP.Net Core 2.0 - ResponseCaching Middleware - Not Caching on Server

Question

I want to use server-side response caching (output cache) with asp.net core 2.0 and found out about Response Caching Middleware and wanted to give it a try with a brand new asp.core mvc project.

Here is the description from the link above which makes me think this could be used like output cache.

The middleware determines when responses are cacheable, stores responses, and serves responses from cache.

Here is how my startup.cs looks like.

public class Startup
{
    public Startup(IConfiguration configuration)
    {
        Configuration = configuration;
    }

    public IConfiguration Configuration { get; }

    // This method gets called by the runtime. Use this method to add services to the container.
    public void ConfigureServices(IServiceCollection services)
    {
        services.AddResponseCaching();
        services.AddMvc();
    }

    // This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
    public void Configure(IApplicationBuilder app, IHostingEnvironment env)
    {
        app.UseResponseCaching();

        if (env.IsDevelopment())
        {
            app.UseBrowserLink();
            app.UseDeveloperExceptionPage();
        }
        else
        {
            app.UseExceptionHandler("/Home/Error");
        }

        app.UseStaticFiles();

        app.UseMvc(routes =>
        {
            routes.MapRoute(
                name: "default",
                template: "{controller=Home}/{action=Index}/{id?}");
        });
    }
}

and here is the HomeController.cs

[ResponseCache(Duration = 60)]
public class HomeController : Controller
{
    public IActionResult Index()
    {
        return View();
    }

    public IActionResult About()
    {
        ViewData["Message"] = "Your application description page.";

        return View();
    }

    public IActionResult Contact()
    {
        ViewData["Message"] = "Your contact page.";

        return View();
    }

    public IActionResult Error()
    {
        return View(new ErrorViewModel { RequestId = Activity.Current?.Id ?? HttpContext.TraceIdentifier });
    }
}

there is also a timestamp at the bottom of _Layout.cshtml file so i can tell when the page is rendered, like below.

<p>&copy; 2018 - ResponseCachingMiddleware - @DateTime.UtcNow</p>

Cache-Control headers seem to be fine, this is what I get in headers when I load the page but time stamp keeps getting updated on every refresh every second.

Cache-Control:public,max-age=60

What I'm understanding from MS documentations is Response Caching Middleware is the server-side caching mechanism that takes care of caching the response while Response Caching seems to be just a filter to manipulate response headers for caching.

Can't tell if there is something wrong with my understanding or code and I wanna complain that I'm feeling this way too often since I started prototyping with ASP.Net Core. Maybe you could also suggest better resources as a side topic.

I've checked out this post before ASP.NET Core 2.0 - Http Response Caching Middleware - Nothing cached

Also checked this out but it seems like the only difference is I'm using mvc. https://github.com/aspnet/ResponseCaching/blob/dev/samples/ResponseCachingSample/Startup.cs

Thanks

Edit: I'm seeing the message below in the output window, cannot find anything about it on google except the few places I already checked for response caching middleware.

Microsoft.AspNetCore.ResponseCaching.ResponseCachingMiddleware:Information: The response could not be cached for this request.

Note: I wish I could create #response-caching-middleware tag. Not sure #responsecache is relevant.

Answer 1

I had this same confusion recently.

ASP.Net Core's ResponseCaching does provide both client-side caching (through HTTP response headers) & server-side (through a memory cache'd middleware that short-circuits other middlewares if the response is in the cache). The server-side portion reads the HTTP response cache headers to determine if it should do server-side caching (similar to what an ISP or CDN might do).

Unfortunately, debugging the server-side ResponseCaching is tricky because it has weird rules & there's not adequate logging. In my case I pulled down Microsoft's source code to step through it & find the issue with my code.

The note you found in the output window "The response could not be cached for this request" is a clue.

There's 2 parts to the server-side caching of a request. The server has to prime the cache the first time the url is requested. It will serve the cached version the 2nd time it's requested. Pay attention to when the error message shows up, if it's on the 1st or 2nd request. That'll tell you if it couldn't be stored in the cache or if it couldn't be retrieved from the cache.

The rules for both storage & retrieval are in this source code file: https://github.com/aspnet/ResponseCaching/blob/3bf5f6a1ce69b65c998d6f5c739822a9bed4a67e/src/Microsoft.AspNetCore.ResponseCaching/Internal/ResponseCachingPolicyProvider.cs

Your "Cache-Control:public,max-age=60" header should match these rules just fine.

My guess is you actually had it working, but didn't know how to test it correctly. There is a counter-intuitive portion of ResponseCaching noted in this issue: https://github.com/aspnet/Home/issues/2607 Essentially, if the browser sends a no-cache or no-store header (when you hit CTRL+F5 or have your debugger tools open), ASP.Net Core's ResponseCaching will honor the browser's request & re-generate the response.

So, to test if your code was working you probably loaded the page, which primed the cache, then you hit CTRL+F5 to force-refresh your browser & you expected the server-side to respond with a cached entry rather than running your WebAPI code. However, it honored the no-cache request header & bypassed the cache (& wrote that message in your output log).

The way to test this would be to clear your browser cache in-between requests (or switch to incognito), rather than using CTRL+F5.

On a side note, honoring the no-cache/no-store request headers was probably a poor design choice since ASP.Net Core's ResponseCache will most likely be used by a server who owns the response, rather than an intermediary cache like a CDN/ISP. I've extended the base ResponseCache with an option to disable honoring these headers (as well as serialize the cache to disk, rather than in-memory only). It's an easy drop-in replacement for the default cache.

You can find my extension here: https://github.com/speige/AspNetCore.ResponseCaching.Extensions https://www.nuget.org/packages/AspNetCore.ResponseCaching.Extensions

There are also a few other other gotchas with ResponseCaching to watch out for which you may have already read about in the blog urls you posted. Authenticated requests & responses with set-cookie won't be cached. Only requests using GET or HEAD method will be cached. If the QueryString is different, it'll make a new cache entry. Also, usually you'll want a "Vary" header to prevent caching if certain conditions of a request differ from the previously-cached request (example: user-agent, accept-encoding, etc). Finally, if a Middleware handles a request it'll short-circuit later Middlewares. Make sure your app.UseResponseCaching() is registered before app.UseMVC()

Answer 2

I had the same issue, I was about to pull my hairs over it, I'd set app.UseResponseCaching(); as well as services.AddResponseCaching(); and add ResponseCache on top of my action exactly like what was told in Microsoft official Docs, despite the the cache-controll header was set correctly on response returning from server but still nothing cached at server-side.

After couple of hours of sweating on this issue I figured out where the problem arises and why nothing cached at server.

Browsers by default set cache-controll value to max-age=0 for the request (if the request is not caused by back or forward) even though you set cache-controller correctly in your response by adding ResponseCache attribute on top of you action (or controller) since the cache-controller sent by request is set to max-age=0, the server is unable to cache response, I think this must be added to list of Response Caching limitation as well

Anyway you can override browser default behavior by adding few line of code right before calling app.UseResponseCaching(); on the other hand you need to add a custom middle-ware to modify request cache-control header value before calling app.UseResponseCaching();.

See code below, worked for me hope work for you too

 app.Use(async (ctx, next) =>
        {
            ctx.Request.GetTypedHeaders().CacheControl = new Microsoft.Net.Http.Headers.CacheControlHeaderValue()
            {
                Public = true,
                MaxAge = TimeSpan.FromSeconds(60)
            };
            await next();
        }
            );
        app.UseResponseCaching();

for ensuring that ResponseCaching works as expected you can also use postman but you must set 'Send no-cache Header' to off in the setting, see image below

Answer 3

If the Cache-Control header is coming through, then it's working. That's all the server can do from that perspective. The client ultimately makes the decision whether or not to actually cache the resource. Sending the header doesn't force the client to do anything; in fact, the server, in general, cannot force the client to do anything.