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Using Client-Scoped Variables for Application Scalability
Since session variables can only be stored in server RAM and since application and server variables aren't user-specific, we can't use those scopes to maintain client state in a load-balanced environment. We must therefore turn to client-scoped variables to solve this problem. More specifically, these client-scoped variables need to be stored in a specially designated database as opposed to the server registry (the default option for client variables). In this article I'm going to provide the systematic process for designing or converting a CF application to run on a Web server cluster, using database-stored client-scoped variables.

Configuration Outline
There are three basic areas you'll need to address in order to implement database-stored client-scoped variables within your application. The three steps are as follows:
1. Database configuration: Since all members of your ColdFusion Web cluster can "see" your database server cluster, this is where your client variables will be stored. You'll create a database on your server for use as the client variable repository (CVR).
2. CF server(s) configuration: All the servers in your cluster will need to be configured not to store client variables in the registry, but to use instead the database you'll have configured in Step 1. This involves using the ColdFusion Administrator to create an ODBC data source and the connection to your CVR. I'll also describe in detail how to disable the session-aware load-balancing feature within ClusterCATS.
3. Application design/conversion: If you'll be developing a new application, this step will help you design your application to support server clustering right from the start. If you have an existing application, this step will describe the changes that are required to make your application "clustering friendly."

If your application is already running in some form and you're tasked with planning or implementing a conversion, I would strongly recommend that you verify that all current features of your application are in working order prior to beginning this process. If you know everything worked before you started, you'll have a much easier time determining what problems may have surfaced as a result of the conversion process.