Caching types, Cache Dependencies and Cache Expirations
ASP.NET features several different types of caching namely Page Output Caching, Page Fragment Caching and Data Caching. In Page Output Caching, the entire page is cached in memory so all the subsequent requests for the same page are addressed from the cache itself. In Page Fragment Caching, a specific a portion of the page is cached and not the entire page. Page Output or Fragment Caching can be enabled or disabled at the Page, Application or even the Machine levels.
Data Caching allows us to cache frequently used data and then retrieve the same data from the cache as and when it is needed. We can also set dependencies so that the data in the cache gets refreshed whenever there is a change in the external data store. The external data store can be a file or even a database. Accordingly, there are two types to dependencies, namely, file based and Sql Server based. There are also differing cache expiration policies.
New Caching features in ASP.NET 2.0
ASP.NET 2.0 has introduced a lot of new features in Caching. This new version of ASP.NET includes an excellent feature called automatic database server cache invalidation which is in turn based on database triggered cache invalidation technique. This powerful and easy-to-use feature enables us to output cache database-driven page and partial page content within a site and then leave it to ASP.NET invalidate these cache entries automatically and refresh the content whenever the there are any changes in the database. This ensures that the data in the Cache is in sync with that of the database. Note that the Cache API in ASP.NET 1.x does not allow you to invalidate an item in the cache when the data in the database changes. It is possible to invalidate cached data based on pre-defined conditions that relate to any change in the data in an external data store on which the dependency has been set. The data gets deleted from the Cache with any change in the data in the external data storage. However, the cached data cannot be refreshed directly.
ASP.NET 2.0 also features numerous enhancements to the Cache API. The CacheDependency class of ASP.NET 1.x was sealed to prevent further inheritance. Thus preventing extension of this class to provide custom cache dependency features. ASP.NET 2.0 Cache API provides the ability to create custom cache dependency classes that are inherited from the CacheDependency class. I would discuss all these issues in details in the subsequent articles in this series of articles on Caching in ASP.NET applications.
Page Caching in ASP.NET
There are two types of Page caching, namely, Page Output Caching and Page Fragment Caching. Page Fragment Caching is also known Partial Page Caching.
Page Output Caching
According to MSDN, Page Output Caching is “the simplest form of caching, output caching simply keeps a copy of the HTML that was sent in response to a request in memory. Subsequent requests are then sent the cached output until the cache expires, resulting in potentially very large performance gains (depending on how much effort was required to create the original page output—sending cached output is always very fast and fairly constant)”.
The ASP.NET Cache Engine is responsible for providing support for Caching in ASP.NET. But, how does it work? The output of the ASP.NET web pages is cached such that all subsequent requests for the same page are served from the cache instead of any other persistent storage medium. When we say that the output of a web page is cached, we imply that the web page output is stored in the cache memory. Whenever a new page request is made, the ASP.NET Cache Engine is activated. It checks whether there is a corresponding cache entry for this page. If one is found, it is known as a Cache hit, else, we say that a cache miss has occurred. If there is a cache hit, i.e., if the ASP.NET Cache engine finds a corresponding cache entry for this page, the page is rendered from the cache, otherwise, the page being requested is rendered dynamically. Page Output caching can be specified in either of the following two ways: