Response caching in ASP.NET Core

By Rick Anderson and Kirk Larkin

View or download sample code (how to download)

Response caching reduces the number of requests a client or proxy makes to a web server. Response caching also reduces the amount of work the web server performs to generate a response. Response caching is set in headers.

The ResponseCache attribute sets response caching headers. Clients and intermediate proxies should honor the headers for caching responses under RFC 9111: HTTP Caching.

For server-side caching that follows the HTTP 1.1 Caching specification, use Response Caching Middleware. The middleware can use the ResponseCacheAttribute properties to influence server-side caching behavior.

The Response caching middleware:

• Enables caching server responses based on HTTP cache headers. Implements the standard HTTP caching semantics. Caches based on HTTP cache headers like proxies do.
• Is typically not beneficial for UI apps such as Razor Pages because browsers generally set request headers that prevent caching. Output caching, which is available in ASP.NET Core 7.0 and later, benefits UI apps. With output caching, configuration decides what should be cached independently of HTTP headers.
• May be beneficial for public GET or HEAD API requests from clients where the Conditions for caching are met.

To test response caching, use Fiddler, or another tool that can explicitly set request headers. Setting headers explicitly is preferred for testing caching. For more information, see Troubleshooting.

HTTP-based response caching

RFC 9111: HTTP Caching describes how Internet caches should behave. The primary HTTP header used for caching is Cache-Control, which is used to specify cache directives. The directives control caching behavior as requests make their way from clients to servers and as responses make their way from servers back to clients. Requests and responses move through proxy servers, and proxy servers must also conform to the HTTP 1.1 Caching specification.

Common Cache-Control directives are shown in the following table.

Directive	Action
public	A cache may store the response.
private	The response must not be stored by a shared cache. A private cache may store and reuse the response.
max-age	The client doesn't accept a response whose age is greater than the specified number of seconds. Examples: max-age=60 (60 seconds), max-age=2592000 (1 month)
no-cache	On requests: A cache must not use a stored response to satisfy the request. The origin server regenerates the response for the client, and the middleware updates the stored response in its cache. On responses: The response must not be used for a subsequent request without validation on the origin server.
no-store	On requests: A cache must not store the request. On responses: A cache must not store any part of the response.

Other cache headers that play a role in caching are shown in the following table.

Header	Function
Age	An estimate of the amount of time in seconds since the response was generated or successfully validated at the origin server.
Expires	The time after which the response is considered stale.
Pragma	Exists for backwards compatibility with HTTP/1.0 caches for setting no-cache behavior. If the Cache-Control header is present, the Pragma header is ignored.
Vary	Specifies that a cached response must not be sent unless all of the Vary header fields match in both the cached response's original request and the new request.

HTTP-based caching respects request Cache-Control directives

RFC 9111: HTTP Caching (Section 5.2. Cache-Control) requires a cache to honor a valid Cache-Control header sent by the client. A client can make requests with a no-cache header value and force the server to generate a new response for every request.

Always honoring client Cache-Control request headers makes sense if you consider the goal of HTTP caching. Under the official specification, caching is meant to reduce the latency and network overhead of satisfying requests across a network of clients, proxies, and servers. It isn't necessarily a way to control the load on an origin server.

There's no developer control over this caching behavior when using the Response Caching Middleware because the middleware adheres to the official caching specification. Support for output caching to better control server load was added in .NET 7. For more information, see Output caching.

ResponseCache attribute

The ResponseCacheAttribute specifies the parameters necessary for setting appropriate headers in response caching.

Warning

Disable caching for content that contains information for authenticated clients. Caching should only be enabled for content that doesn't change based on a user's identity or whether a user is signed in.

VaryByQueryKeys varies the stored response by the values of the given list of query keys. When a single value of * is provided, the middleware varies responses by all request query string parameters.

Response Caching Middleware must be enabled to set the VaryByQueryKeys property. Otherwise, a runtime exception is thrown. There isn't a corresponding HTTP header for the VaryByQueryKeys property. The property is an HTTP feature handled by Response Caching Middleware. For the middleware to serve a cached response, the query string and query string value must match a previous request. For example, consider the sequence of requests and results shown in the following table:

Request	Returned from
http://example.com?key1=value1	Server
http://example.com?key1=value1	Middleware
http://example.com?key1=NewValue	Server

The first request is returned by the server and cached in middleware. The second request is returned by middleware because the query string matches the previous request. The third request isn't in the middleware cache because the query string value doesn't match a previous request.

The ResponseCacheAttribute is used to configure and create (via IFilterFactory) a Microsoft.AspNetCore.Mvc.Internal.ResponseCacheFilter. The ResponseCacheFilter performs the work of updating the appropriate HTTP headers and features of the response. The filter:

• Removes any existing headers for Vary, Cache-Control, and Pragma.
• Writes out the appropriate headers based on the properties set in the ResponseCacheAttribute.
• Updates the response caching HTTP feature if VaryByQueryKeys is set.

Vary

This header is only written when the VaryByHeader property is set. The property set to the Vary property's value. The following sample uses the VaryByHeader property:

[ApiController]
public class TimeController : ControllerBase
{
    [Route("api/[controller]")]
    [HttpGet]
    [ResponseCache(VaryByHeader = "User-Agent", Duration = 30)]
    public ContentResult GetTime() => Content(
                      DateTime.Now.Millisecond.ToString());

View the response headers with Fiddler or another tool. The response headers include:

Cache-Control: public,max-age=30
Vary: User-Agent

The preceding code requires adding the Response Caching Middleware services AddResponseCaching to the service collection and configures the app to use the middleware with the UseResponseCaching extension method.

var builder = WebApplication.CreateBuilder(args);

builder.Services.AddControllers();
builder.Services.AddResponseCaching();

var app = builder.Build();

app.UseHttpsRedirection();

// UseCors must be called before UseResponseCaching
//app.UseCors();

app.UseResponseCaching();

app.UseAuthorization();

app.MapControllers();

app.Run();

NoStore and Location.None

NoStore overrides most of the other properties. When this property is set to true, the Cache-Control header is set to no-store. If Location is set to None:

• Cache-Control is set to no-store,no-cache.
• Pragma is set to no-cache.

If NoStore is false and Location is None, Cache-Control, and Pragma are set to no-cache.

NoStore is typically set to true for error pages. The following produces response headers that instruct the client not to store the response.

[Route("api/[controller]/ticks")]
[HttpGet]
[ResponseCache(Location = ResponseCacheLocation.None, NoStore = true)]
public ContentResult GetTimeTicks() => Content(
                  DateTime.Now.Ticks.ToString());

The preceding code includes the following headers in the response:

Cache-Control: no-store,no-cache
Pragma: no-cache

To apply the ResponseCacheAttribute to all of the app's MVC controller or Razor Pages page responses, add it with an MVC filter or Razor Pages filter.

In an MVC app:

builder.Services.AddControllersWithViews().AddMvcOptions(options => 
    options.Filters.Add(
        new ResponseCacheAttribute
        {
            NoStore = true, 
            Location = ResponseCacheLocation.None
        }));

For an approach that applies to Razor Pages apps, see Adding ResponseCacheAttribute to MVC global filter list does not apply to Razor Pages (dotnet/aspnetcore #18890). The example provided in the issue comment was written for apps targeting ASP.NET Core prior to the release of Minimal APIs at 6.0. For 6.0 or later apps, change the service registration in the example to builder.Services.AddSingleton... for Program.cs.

Location and Duration

To enable caching, Duration must be set to a positive value and Location must be either Any (the default) or Client. The framework sets the Cache-Control header to the location value followed by the max-age of the response.

Location's options of Any and Client translate into Cache-Control header values of public and private, respectively. As noted in the NoStore and Location.None section, setting Location to None sets both Cache-Control and Pragma headers to no-cache.

Location.Any (Cache-Control set to public) indicates that the client or any intermediate proxy may cache the value, including Response Caching Middleware.

Location.Client (Cache-Control set to private) indicates that only the client may cache the value. No intermediate cache should cache the value, including Response Caching Middleware.

Cache control headers provide guidance to clients and intermediary proxies when and how to cache responses. There's no guarantee that clients and proxies will honor RFC 9111: HTTP Caching. Response Caching Middleware always follows the caching rules laid out by the specification.

The following example shows the headers produced by setting Duration and leaving the default Location value:

[Route("api/[controller]/ms")]
[HttpGet]
[ResponseCache(Duration = 10, Location = ResponseCacheLocation.Any, NoStore = false)]
public ContentResult GetTimeMS() => Content(
                  DateTime.Now.Millisecond.ToString());

The preceding code includes the following headers in the response:

Cache-Control: public,max-age=10

Cache profiles

Instead of duplicating response cache settings on many controller action attributes, cache profiles can be configured as options when setting up MVC/Razor Pages. Values found in a referenced cache profile are used as the defaults by the ResponseCacheAttribute and are overridden by any properties specified on the attribute.

The following example shows a 30 second cache profile:

using Microsoft.AspNetCore.Mvc;

var builder = WebApplication.CreateBuilder(args);

builder.Services.AddResponseCaching();
builder.Services.AddControllers(options =>
{
    options.CacheProfiles.Add("Default30",
        new CacheProfile()
        {
            Duration = 30
        });
});

var app = builder.Build();

app.UseHttpsRedirection();

// UseCors must be called before UseResponseCaching
//app.UseCors();

app.UseResponseCaching();

app.UseAuthorization();

app.MapControllers();

app.Run();

The following code references the Default30 cache profile:

[ApiController]
[ResponseCache(CacheProfileName = "Default30")]
public class Time2Controller : ControllerBase
{
    [Route("api/[controller]")]
    [HttpGet]
    public ContentResult GetTime() => Content(
                      DateTime.Now.Millisecond.ToString());

    [Route("api/[controller]/ticks")]
    [HttpGet]
    public ContentResult GetTimeTicks() => Content(
                      DateTime.Now.Ticks.ToString());
}

The resulting header response by the Default30 cache profile includes:

Cache-Control: public,max-age=30

The [ResponseCache] attribute can be applied to:

• Razor Pages: Attributes can't be applied to handler methods. Browsers used with UI apps prevent response caching.
• MVC controllers.
• MVC action methods: Method-level attributes override the settings specified in class-level attributes.

The following code applies the [ResponseCache] attribute at the controller level and method level:

[ApiController]
[ResponseCache(VaryByHeader = "User-Agent", Duration = 30)]
public class Time4Controller : ControllerBase
{
    [Route("api/[controller]")]
    [HttpGet]
    public ContentResult GetTime() => Content(
                      DateTime.Now.Millisecond.ToString());

    [Route("api/[controller]/ticks")]
    [HttpGet]
    public ContentResult GetTimeTicks() => Content(
                  DateTime.Now.Ticks.ToString());

    [Route("api/[controller]/ms")]
    [HttpGet]
    [ResponseCache(Duration = 10, Location = ResponseCacheLocation.Any, NoStore = false)]
    public ContentResult GetTimeMS() => Content(
                      DateTime.Now.Millisecond.ToString());
}

Additional resources

• Storing Responses in Caches
• Cache-Control
• Cache in-memory in ASP.NET Core
• Distributed caching in ASP.NET Core
• Detect changes with change tokens in ASP.NET Core
• Response Caching Middleware in ASP.NET Core
• Cache Tag Helper in ASP.NET Core MVC
• Distributed Cache Tag Helper in ASP.NET Core