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This is the fourteenth in a series of blog posts I’m doing on the upcoming VS 2010 and .NET 4 release.  Today’s post covers some of the output caching extensibility improvements being made to ASP.NET 4.  These can be used by all ASP.NET 4 applications – including those built using both ASP.NET Web Forms and ASP.NET MVC.

Output Caching Today

ASP.NET 1.0 introduced the concept of output caching, which enables developers to capture and store the generated output of pages, controls, controllers, and HTTP responses in an in-memory cache.  On subsequent web requests, ASP.NET can then serve content faster by retrieving and using the generated output from the cache instead of having to execute the page/controller to regenerate the output from scratch.  This can dramatically improve the performance of your application – especially in cases where your content is based on data within a database (since when the content is output cached you end up not having to hit the database on subsequent requests).

ASP.NET’s output caching system is flexible enough to enable you to cache different versions of content depending on the querystring/form-post parameters to a page/controller (for example: Browse.aspx?category=Beverages versus. Browse.aspx?category=Meat).  It also enables you to cache different versions based on the browser type or user-language preference of the client visiting your application. This allows you to cache a mobile version of a page separate from a version optimized for a desktop browser, as well as to vary the cache entries based on whether the user wants to read a page in English or French.  You can configure ASP.NET to cache a particular output cache entry for a specific period of time (for example: 1 minute – after which the next request will regenerate the content and cache it again).  You can also configure ASP.NET to dynamically invalidate a cache entry based on an external event (for example: if the database upon which the cached content depends changes). 

One limitation of output caching with ASP.NET V1->V3.5, though, is that the cache store itself is not extensible – and the cached content always has to be stored in-memory.

ASP.NET 4 Output Cache Extensibility

ASP.NET 4 adds an extensibility point to output caching that now enables developers to configure one or more custom output-cache providers. Output-cache providers can use any storage mechanism to persist output cached content. This makes it possible to easily create output-cache providers that store the cached content using any persistence mechanism – including local or remote disks, databases, cloud storage, and distributed cache engines (like memcached or velocity).

You can create a custom output-cache provider by creating a class that derives from the new System.Web.Caching.OutputCacheProvider class in ASP.NET 4.  Within your derived class you then override 4 public methods that provide implementations for adding/removing/retrieving/updating cached content (a unique key is passed to identify each separate cached entry).  You can then configure ASP.NET 4 to use your custom outputcache provider by registering it using the new <providers> subsection of the <outputCache> element within an application’s web.config file:


Above I’ve added a new output cache provider (which I’ve named “SampleCache”) that is implemented using the “ScottOutputCache” class within my OutputCacheSample.dll assembly.  I’ve also configured ASP.NET to use my “SampleCache” implementation as the default output cache implementation whenever content is output cached – this is done by setting the “defaultProvider” attribute on the <outputCache> element. 

And now, when I add an OutputCache directive to the top of any of .aspx page the content will be cached and stored using my ScottOutputCache provider:

<%@ OutputCache Duration="60" VaryByParam="None"  %>

Likewise, if I add an [OutputCache] attribute on any action method within an ASP.NET MVC Controller the content will also be cached and stored using my ScottOutputCache provider:


Customizing Which Output Cache Provider is Used

Above I configured ASP.NET to by default always cache content using my “SampleCache” provider whenever output caching is used within the application.

As a slightly more advanced option, developers can also configure ASP.NET to dynamically choose which output cache provider to use on a per-request basis.  This is useful for scenarios where you want to have a slightly richer set of cache semantics. For example, you might want to cache the “Home” page or "Top 10" requested pages of a site using the built-in ASP.NET in-memory provider (which will be super fast because the content will be in-memory), and then cache less frequently requested pages that get lower traffic on disk.

You can dynamically indicate which output cache provider to use on a per-request basis by overriding the GetOutputCacheProviderName() method within the Global.asax file of an application.  Below I’m indicating that for the “Home.aspx” page within my application I want to use the built-in in-memory ASP.NET output cache provider – and that I then want to use whatever is registered within the web.config file for every other request:


This ability to easily switch between providers depending on scenarios ends up enabling a bunch of really powerful scenarios.

Common Output Cache Providers

We’ll be shipping samples that demonstrate how to implement a disk-based output cache provider that stores cached content on the file-system.  We’ll also be shipping samples that demonstrate how to integrate output caching with the new Windows Server AppFabric Caching Service (formerly known as “Velocity”).  AppFabric’s caching service will be a free, fully supported, Microsoft distributed cache system.  It will also be easy to use the new ASP.NET 4 output caching extensibility with memcached – a popular open source distributed caching system.

You can learn more about how to create providers by watching Stefan Schackow’s ASP.NET 4 Core Runtime talk at PDC 2009.  You can also learn more about AppFabric’s Caching service from this PDC 2009 talk.


The addition of output-cache provider extensibility to ASP.NET 4 enables developers to easily pursue more aggressive and more intelligent output-caching strategies for web sites and applications.  These can significantly improve the performance and responsiveness of applications, and improve both end-user experiences as well as reduce the amount of server resources required. 

Hope this helps,