Control data caching in .NET with the Caching API

by Tony Patton | Jun 30, 2006 1:34:00 PM

Tags: .NET, Middleware, Microsoft development tools, Microsoft .NET, Tony Patton, API, Control Data, specific item, Caching API, .NET Newsletter

Takeaway: Decreasing page load time is imperative when dealing with Web site performance. Tony Patton explains how you can make this happen by using the .NET Caching API.

Caching frequently accessed or expensive data in memory boosts application performance by reducing the number of database calls to retrieve the data. ASP.NET provides two basic caching techniques: page (Page Output and Page Fragment) and programmatic caching. In my previous article, I covered the basics of Page Output caching. This article's focus is on caching data via your favorite .NET language and the Caching API, which allows you to add and remove items from the cache as necessary.

Caching API

The Caching API is available via the System.Web.Caching namespace. It contains the Cache object, which contains methods for manipulating items in the cache. The following methods are available:

• Add: allows you to add a specific item to the cache.
• Get: retrieves a specific item from the cache using a key value.
• Insert: adds a specific item to the cache.
• Remove: removes a specific item from the cache designated by key value.

The Add and Insert methods accept parameters to define the following properties:

• A string value for the item's key (used to access the item).
• The actual object to store in the cache.
• Set a dependency (via the CacheDependency object) that defines a condition that clears the cache when a particular dependency (file, directory, keys, or other objects) changes.
• Define an absolute expiration date that sets when the object will expire and be removed from the cache. It accepts a DateTime object.
• Define a sliding expiration date that specifies the amount of time (after an object is accessed last) to wait before an item is removed from the cache. It accepts a TimeSpan object.
• Assign a priority to the item via a CacheItemPriority object. The legal values include NotRemovable, High, AboveNormal, Normal, BelowNormal, and Low.
• Specify a delegate for a callback function to be notified when an item is removed.

Both the Add and Insert methods accept all of these parameters, but the Insert method includes two overloaded signatures that accept a subset of the list. The first signature accepts the key value and actual object, while the other version accepts everything except the priority and delegate. The Insert method is often used since the other parameters are not used as often.

Working with the Cache

If you use the methods provided by the Cache class, it makes it easy to utilize data caching in an application. The C# code in Listing A uses the cache to store a DataSet object. The DataSet object is used to populate a DataGrid control on the ASP.NET page. If the DataSet object is not found in the cache, the database is queried. The Get method is used to check for its existence in the cache before it is added. Listing B contains the VB.NET equivalent.

Another detail you may notice is the need to convert the object to its appropriate type when it is retrieved from the cache. This code adds the object to the cache indefinitely. You may use the Insert or Add methods to assign an expiration date to the item.

You could replace the Cache.Insert("TestDS", ds) from the previous code listings with the line in Listing C to set it to expire in 30 minutes. Listing D contains the VB.NET version. On the other hand, you could use a sliding expiration window with the line in Listing E. The VB.NET version is in Listing F. You may notice this second line uses a normal priority, but it is no different since normal and default are the same.

Cache dependencies

One of the more interesting features is tying the cache to an external source like a file. Cache dependencies allow you to make a cached item dependent on another resource so that when the resource changes the cached item is removed automatically. The dependent item may be other cache items, a system file, or folder.

Using a cache dependency requires an instance of the CacheDependency object that points to the item used. The VB.NET snippet in Listing G creates such an object that points to a local file.

A great feature of ASP.NET 2.0 is the addition of a SQL Server query as a cache dependency. Prior to this enhancement, developers often utilize files and database triggers with the trigger writing to the file when a data source changes.

Using the Response object

Accessing the Response.Cache object returns an instance of the HttpCachePolicy class. This class contains methods for setting cache-specific HTTP headers and controlling the ASP.NET page output cache. The following list contains some of the available methods:

• SetCacheability: sets the Cache-Control HTTP header. The Cache-Control HTTP header controls how documents are to be cached on the network.
• SetNoServerCaching: stops all origin server caching for the current response.
• SetVaryByCustom: specifies a custom text string to vary cached output responses by.
• SetSlidingExpiration: sets cache expiration to sliding. When cache expiration is set to sliding, the Cache-Control HTTP header will be renewed with each response. This expiration mode is identical to the IIS configuration option to add an expiration header to all output set relative to the current time.

Data reuse via caching

Any Web application with a sizable group of users should utilize caching to reduce the number of calls for data and, therefore, decrease response time. ASP.NET provides programmatic access to the caching functionality via the Caching API. It allows you to easily add items to the cache, as well as manually remove items or automatically remove items using expiration dates. Another feature allows you to tie an object's lifetime to an external resource like a system file.

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Tony Patton began his professional career as an application developer earning Java, VB, Lotus, and XML certifications to bolster his knowledge.

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