AspNetCore.Docs/aspnetcore/performance/caching/middleware.md

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Response Caching Middleware in ASP.NET Core	guardrex	Configuration and use of Response Caching Middleware in ASP.NET Core applications.	ASP.NET Core,response caching,caching,ResponseCache,ResponseCaching,Cache-Control,VaryByQueryKeys,middleware	riande	wpickett	08/22/2017	article	f9267eab-2762-42ac-1638-4a25d2c9d67c	asp.net-core	performance/caching/middleware

Response Caching Middleware in ASP.NET Core

By Luke Latham and John Luo

View or download sample code

This document provides details on how to configure the Response Caching Middleware in ASP.NET Core apps. The middleware determines when responses are cacheable, stores responses, and serves responses from cache. For an introduction to HTTP caching and the ResponseCache attribute, see Response Caching.

Package

To include the middleware in a project, add a reference to the Microsoft.AspNetCore.ResponseCaching package or use the Microsoft.AspNetCore.All package.

Configuration

In ConfigureServices, add the middleware to the service collection.

ASP.NET Core 2.x

[!code-csharpMain]

ASP.NET Core 1.x

[!code-csharpMain]

---

Configure the app to use the middleware with the UseResponseCaching extension method, which adds the middleware to the request processing pipeline. The sample app adds a Cache-Control header to the response that caches cacheable responses for up to 10 seconds. The sample sends a Vary header to configure the middleware to serve a cached response only if the Accept-Encoding header of subsequent requests matches that of the original request.

ASP.NET Core 2.x

[!code-csharpMain]

ASP.NET Core 1.x

[!code-csharpMain]

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The Response Caching Middleware only caches 200 (OK) server responses. Any other responses, including error pages, are ignored by the middleware.

[!WARNING] Responses containing content for authenticated clients must be marked as not cacheable to prevent the middleware from storing and serving those responses. See Conditions for caching for details on how the middleware determines if a response is cacheable.

Options

The middleware offers three options for controlling response caching.

Option	Default Value
UseCaseSensitivePaths	Determines if responses are cached on case-sensitive paths. The default value is false.
MaximumBodySize	The largest cacheable size for the response body in bytes. The default value is 64 * 1024 * 1024 (64 MB).
SizeLimit	The size limit for the response cache middleware in bytes. The default value is 100 * 1024 * 1024 (100 MB).

The following example configures the middleware to cache responses smaller than or equal to 1,024 bytes using case-sensitive paths, storing the responses to /page1 and /Page1 separately.

services.AddResponseCaching(options =>
{
    options.UseCaseSensitivePaths = true;
    options.MaximumBodySize = 1024;
});

VaryByQueryKeys

When using MVC, the ResponseCache attribute specifies the parameters necessary for setting appropriate headers for response caching. The only parameter of the ResponseCache attribute that strictly requires the middleware is VaryByQueryKeys, which doesn't correspond to an actual HTTP header. For more information, see ResponseCache Attribute.

When not using MVC, you can vary response caching with the VaryByQueryKeys feature. Use the ResponseCachingFeature directly from the IFeatureCollection of the HttpContext:

var responseCachingFeature = context.HttpContext.Features.Get<IResponseCachingFeature>();
if (responseCachingFeature != null)
{
    responseCachingFeature.VaryByQueryKeys = new[] { "MyKey" };
}

HTTP headers used by Response Caching Middleware

Response caching by the middleware is configured via HTTP headers. The relevant headers are listed below with notes on how they affect caching.

Header	Details
Authorization	The response isn't cached if the header exists.
Cache-Control	The middleware only considers caching responses marked with the public cache directive. You can control caching with the following parameters: max-age max-stale† min-fresh must-revalidate no-cache no-store only-if-cached private public s-maxage proxy-revalidate‡ †If no limit is specified to max-stale, the middleware takes no action. ‡proxy-revalidate has the same effect as must-revalidate. For more information, see RFC 7231: Request Cache-Control Directives.
Pragma	A Pragma: no-cache header in the request produces the same effect as Cache-Control: no-cache. This header is overridden by the relevant directives in the Cache-Control header, if present. Considered for backward compatibility with HTTP/1.0.
Set-Cookie	The response isn't cached if the header exists.
Vary	The Vary header is used to vary the cached response by another header. For example, you can cache responses by encoding by including the Vary: Accept-Encoding header, which caches responses for requests with headers Accept-Encoding: gzip and Accept-Encoding: text/plain separately. A response with a header value of * is never stored.
Expires	A response deemed stale by this header isn't stored or retrieved unless overridden by other Cache-Control headers.
If-None-Match	The full response is served from cache if the value isn't * and the ETag of the response doesn't match any of the values provided. Otherwise, a 304 (Not Modified) response is served.
If-Modified-Since	If the If-None-Match header isn't present, a full response is served from cache if the cached response date is newer than the value provided. Otherwise, a 304 (Not Modified) response is served.
Date	When serving from cache, the Date header is set by the middleware if it wasn't provided on the original response.
Content-Length	When serving from cache, the Content-Length header is set by the middleware if it wasn't provided on the original response.
Age	The Age header sent in the original response is ignored. The middleware computes a new value when serving a cached response.

Troubleshooting

If caching behavior isn't as you expect, confirm that responses are cacheable and capable of being served from the cache by examining the request's incoming headers and the response's outgoing headers. Enabling logging can help when debugging. The middleware logs caching behavior and when a response is retrieved from cache.

When testing and troubleshooting caching behavior, a browser may set request headers that affect caching in undesirable ways. For example, a browser may set the Cache-Control header to no-cache when you refresh the page. The following tools can explicitly set request headers, and are preferred for testing caching:

• Fiddler
• Firebug
• Postman

Conditions for caching

• The request must result in a 200 (OK) response from the server.
• The request method must be GET or HEAD.
• Terminal middleware, such as Static File Middleware, must not process the response prior to the Response Caching Middleware.
• The Authorization header must not be present.
• Cache-Control header parameters must be valid, and the response must be marked public and not marked private.
• The Pragma: no-cache header/value must not be present if the Cache-Control header isn't present, as the Cache-Control header overrides the Pragma header when present.
• The Set-Cookie header must not be present.
• Vary header parameters must be valid and not equal to *.
• The Content-Length header value (if set) must match the size of the response body.
• The HttpSendFileFeature isn't used.
• The response must not be stale as specified by the Expires header and the max-age and s-maxage cache directives.
• Response buffering is successful, and the size of the response is smaller than the configured or default SizeLimit.
• The response must be cacheable according to the RFC 7234 specifications. For example, the no-store directive must not exist in request or response header fields. See Section 3: Storing Responses in Caches of RFC 7234 for details.

[!NOTE] The Antiforgery system for generating secure tokens to prevent Cross-Site Request Forgery (CSRF) attacks sets the Cache-Control and Pragma headers to no-cache so that responses aren't cached.

Additional resources

• Application Startup
• Middleware