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As far as www2 is concerned, I never logged in. Most likely, your application will deal with me just like any other user who's trying to load a page that requires a login: it'll redirect me to a login screen instead of showing me the page. But as a user who already logged in when I was on www1, I won't be at all pleased. To put it mildly, this isn't good. Leave such a problem unresolved and your users will probably want to know where you live! You might ask, "So what's the big deal?" Why not just use session-sensitive load balancing and keep the user on a particular server, with all their session variables stored in that server's memory for the length of their session? Wouldn't that be easiest? Here's where I stand by the term true load balancing and can show it to be a better configuration. Session-sensitive load balancing has one very real pitfall that occurs when a user has been load-balanced to a particular server and has established a session there, but that server then becomes unavailable. Should that happen, the user will inevitably be shifted to another server in the cluster, with the added hassle of losing their session variables in the process. You'll need to change your application code to resolve this problem, so why not use the more robust (true) load-balancing solution as well? As you may have guessed by now, I'm no advocate of session-sensitive load balancing. I think of it this way: the expense of hardware, software and configuration of a server cluster can be justified only by gains in performance and failover protection; session-sensitive load balancing can potentially be an obstacle to both of those goals, while true load balancing actively involves all of the servers in your cluster on a continuing basis, fully achieving both of those goals. Most important, use of session-sensitive load balancing inappropriately encourages the use of session variables, which will quickly detract from the user-transparent failover abilities of the clustering solution. The bottom line? With either method of load balancing, use of session variables is unacceptable. In reality, it doesn't matter which load-balancing solution you choose - as long as you don't use session variables, either one will work. However, it's my belief that if you really want to gain the full benefits of server clustering, you'll want to compose your ColdFusion application to support true load-balancing, while still being able to maintain client state. That's what this article is about.
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