Where Caching Takes Place

Where caching takes place includes the following primary locations:

• Server cache is used to quickly retrieve web page content, either raw data or the output of previously generated web pages, thus saving server load for many users.
• Proxy cache utilizes specialized network hardware located in-between the client and the server to retrieve entire web pages and referenced files for multiple computers, thus saving both server load and bandwidth.
• Client cache (also known as browser cache) is used to retrieve entire web pages and referenced files at the client, thus saving both server load and bandwidth for a single user.

What Types of Caching Are Available

What types of caching are available depends primarily on the type of data to be stored:

• Data caching stores arbitrary data such as simple strings, custom objects, and complex data objects like array lists, data sets, and hash tables in server memory
• Fragment caching stores the generated output of web page portions and only at the server.
• Output caching stores the generated output of an entire web page at the server or the client.
• Image caching stores images in client-side cache.

When to Use Caching

When to use each caching process is based largely upon limitations. Typically the advantages of all caching processes are desired, but are often ruled out based upon limitations. The following describes when to use each caching process based upon advantages and limitations:

• Data caching should be used to store frequently accessed data in server memory. However, do not store so much data that it overburdens server memory, adversely affecting the entire computer performance. Data caching is the most flexible form of cache because it can be changed on any web request. It offers the slowest overall performance improvement compared to other forms of caching because web page content must be regenerated with every request.
• Output caching is used to store the generated output of an entire web page at the server or the client. Refreshing items in both of these storage locations depends primarily on expiration periods. If data changes before the expiration period ends then the client will view its obsolete cached representation. For this reason it is best to use output caching when data changes can be synchronized with expiration periods. Client and server caching also have differences.Server cache can store several versions of a web page, whereas the client cache can only store a single version. On the other hand, client cache can store more data because it does not tie up server memory. Client output caching is the fastest form of caching because it does not require a round trip to the server. Server output caching is the next fastest form of caching because the web page is not regenerated.
• Fragment caching is used to store the generated output of web page portions and only at the server. Refreshing fragment caching depends primarily on expiration periods which, like output caching, can cause cached data to become obsolete. Fragment caching can store several versions of web page portions. Fragment caching also requires a round trip to the server.
• Image caching is controlled through Internet Information Server (IIS) and not ASP.NET. By default, images that are returned from IIS to the client are cached but have no expiration date, which means that the client will continually issue requests to the server to ascertain whether the original image has been modified. To prevent such numerous requests, images can be placed within a directory marked by IIS as Enable Content Expiration. These images will be given a future expiration date, which means that the client will not issue requests to the server until its cached copy of the image expires.
  Note: IIS Enable Content Expiration is applicable to other files as well as images. However, other files such as .js and .css files are automatically given expiration dates, images are not.

Caching Summary

The following chart summarizes each of these caching processes, locations, advantages, and limitations: