Caching Web Part Content

The problem:

Here is a common scenario. You have a web part whose contents include a gridview, which is populated by going through all (or many) subsites and gathering the information from each of these sites. The gridview has to allow sorting and possibly even filtering.

The problem, though, is that sorting and filtering typically cause postback, which means you need to perform the expensive operation of loading the data again. If the data happens to be static (or relatively static), you wind up loading the same data over and over again, which results in performance deterioration.

The solution:

The solution depends on whether the data displayed the gridview is the same for every user.

a) If it is the same, then it will be worthwhile to use the Cache object and add the datatable (or any other data source object) to the cache if it is not already there. This will speed up the loading time for every user, since the cache will be populated only once. A couple of gotchas, though:

1) Make sure you place a lock around the code that accesses cache. Otherwise, if it takes 15 seconds to load the data table and many users are trying to access this page at the same time, then all of these users will be simultaneously trying to update the same cache object - and this would result in performance problems.

2) If the data occasionally changes, it would be wise to implement cache dependency, so that the cache is invalidated whenever the data changes.

b) If it is not the same, then use Session instead of Cache. But, make sure the session state is enabled. Another gotcha is that there is no expiration of the Session object, nor there is anything equivalent to cache dependency for Session objects - so you'll need to find some other way to clear the session object once the underlying data table changes.