Thursday, August 11, 2005

.Net Tip - Caching and Data Updates

Wow, it's been a while! I promise to update more frequently. To make up for not posting for a while here is a very useful caching tip that I use on my own sites.

We all have user controls that appear on every page (a perfect example of this - a header control or a navigation control). User controls can very easily be cached using the OutputCache directive:

<%@ OutputCache duration="3600"%>

By embedding the above tag into a user control, it will be cached for an hour. But what if the data it displays changes during the course of the hour? How do I ensure that the data will be updated in a timely manner? The simple solution is in the VaryByCustom attribute of the tag.

VaryByCustom calls a special function called GetVaryByCustomString and passes it the value you specify for the VaryByCustom attribute in the OutputCache tag. Essentially you write the code to determine what value gets returned. With each subsequent request for the cached object, the ASP.Net engine first checks the result GetVaryByCustomString function. If the result has changed - regardless of the duration on the Cache, the system will bypass the cache and execute the code in the control.

Therefore, it is very easy to set up, say, an application variable that gets changed only when the data gets changed, then use the GetVaryByCustomString function to check for updates.

So your OutputCache line will now look like this:

<%@ OutputCache duration="3600" VaryByCustom="updateCheck"%>

Then make the following changes to your Global.asax:
  1. Add the following line to your Application_Start method:


  2. Add the following code as the function GetVaryByCustomString:

    public override string GetVaryByCustomString(HttpContext context,string arg)
    return Application["serial"].ToString();
    //We need to return an empty string so that this compiles
    return "";

Now you are almost done. All you still need to do to ensure that your cached pages are updated when data changes is to make the following call:


This will update the application variable, and change the result of the GetVaryByCustomString function, which will force the Cache to update.

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