Caching with ASP+ and ASP

BY Salim Naim

When we use the word "cache," we are talking about data that is saved in memory on the web server. Just because you can cache data doesn't mean you should. If you have data that is constantly changing, caching the data isn't going to buy you anything. In fact, you would just be wasting memory. However, if you have data that you will access over and over and that does not change often (in this case, often means every few seconds or minutes), this data is a good candidate for caching.

Output caching sometimes referred to as page caching is the term for saving data that is in its final presentation state, you take the input and transform it into what you are going to present, then cache the transformed data -- you save the data as you would display it. This type of caching gives you the most bang for the performance increase because you only retrieve the data and format it once. It saves work (translated into time) for every request you don't have to reprocess. Output caching works especially well for a Web page that will be shown to many in the same manner.

Data Caching is the term for saving specific data programmatically. This type of caching also gives you a greater throughput gain as well as fine control over what to cache and what not to cache.

With the above terms defined now we’ll move to identifying perfect candidates for caching. I often hear people saying that ASP and IIS are too slow and they always point to sites that take long time to render and leave the user staring at a blank page until the browser has enough HTML to start rendering. ASP and IIS are not slow but what they do suffer is the side effect of there easy to program. ASP generally speaking is a very easy server-side programming framework and with VBScript as the scripting engine the entire process of web development is even easier this ends up leaving the process of performance optimization up to the programmer and that’s where slow sites merge from.

ASP sites are usually very dynamic, list box get generated dynamically, menu options get generated as well as general site content this is not going to end, since people expect more functionality out of sites and always draw comparisons to desktop applications And with newer technologies on the scene like XML and XSLT and cross browser issues remain with us (causing people to use server-side transformation), more load is going to be placed on application servers further adding to ASP/IIS reputation.

Caching is not a simple performance enhancement, to fully take advantage of caching you have to understand your content and place judgment on classifying which is purely dynamic and which is semi dynamic and which is fairly static in order to cache/leave accordingly.

IIS and Caching

IIS 4.0 and 5.0 provides out of the box two kinds of caches

• IIS Template Cache
• IIS Script Engine Cache

To best monitor the performance enhancement these two caches provide refer to the following performance monitor counters

• Active server pages:  Template Cached
• Active server pages: Template Cache Hit Rate
• Active server pages:          Script Engine Cached

These caches provide performance increases however more is need to accommodate site needs

Current Caching Techniques With ASP

ASP out of the box doesn’t provide any caching and relies on IIS caching, this limitation was overcome by many third party vendors such as Post Point Software XCache and other custom components.

Output caching

With XCache one can create output caching as well as partial caching with the use of XCaches’s SDK and Objects.

The introduction of Post points, <$..$> processing blocks and

<!-- #dynamic virtual=”{file..}”à includes allow output caching and provides you with a greater performance increase.

Data Caching

Data caching is fine control over caching, performed programmatically and deals with the cache of any data (result set from a DB query, XSTL transformation, etc..). there are many components on the web that provide such functionality but in my mind the best is one provided by the Microsoft team of Duwamish introduced in the phase 4 of the implementation. What makes this object so good, for starters it’s free and comes with the ATL COM code written in. Secondly this object addresses all main performance and functionality issues. Event handling is provided for the cache, multiple threading model as well as the concurrency with critical section locking.

Example:

Entries in global.asa

Set Application("Cache") = Server.CreateObject("MSDN_DUWAMISH4Lib.Cache") Application("Cache").Initialize 1000000, 1000 Application("Cache").FlushEnabled = False

' Flush Delta time - 1/2 hour expressed in milliseconds 'Application("Cache").FlushDeltaTime = 0.5 * 60 * 60 * 1000 Application("Cache").RaiseErrors = False

once the above entries have been set (a full documentation is available onine at msdn.Microsoft.com), you can use the object to cache any data

  szData = Application("Cache")(szKey)

  If szData = "" Then

    ... build up the data to store, query result XSLT transformation etc..          

    ' add data first time

    Application("Cache").Add szKey, szData

  Else

    ' just use the data cached without executing the process

  End if

With the above techniques and the many similar available, caching in ASP is possible but is not provided as part of the framework.

Caching with ASP+ .NET

With the introduction of ASP+, caching got a whole lot better. The .NET framework now not only relying on IIS caches, it provides Page/Output caching as well as Data caching out of the box.

Page Caching or Output Caching

To Page Cache in ASP+ you can use the OutputCache Directive

<%@ OutputCache Duration="10" %>

or

the programmatic control of the functionality can be provided by the System.Web.HttpCachePolicy class

Response.Cache.SetExpires(DateTime.Now.AddSeconds(60))

Response.Cache.SetCacheability(HttpCacheability.Public)

Output Caching or Data Caching

To cache arbitrary data in ASP+ you can use System.Web.Caching namespace, caching is a joy with  Cache class. To cache data it’s as simple as just using the Cache object.

Private Static cacheSynchronize As String = "Key"

szData = Cache("Key")

If szData <> Null Then

   'use data

Else

  SyncLock(cacheSynchronize)

   ' add information to cache

   Cache("Key") = szData

  End SyncLock

End If

Note: Note extra synchronization has been added to account for concurrency.

Now the great advantage of the above techniques is that no third party tool or software was required to provide you this added functionality.

Finishing Note

Caching is great and it’s something that every programmer should consider, however there are some drawbacks to caching. If implemented at a pre mature stage in the site/Application development, caching can hide a lot of bugs and spring them out when you least expected making things very inconsistent.

Example: the output of a COM object or function that doesn’t free resources can be cached covering the memory leaks that exist.

Caching should be the last thing to implement before production it will increase performance and put a smile on your face as well as give time for bugs to be dealt with.

For extra information you can reach me at

http://salimnaim.webjump.com