Well, after having spent several fruitless train journeys trying to get Terracotta to cluster an existing application I've finally given up, coming to the conclusion that Tim was probably right; clustering really isn't something that you want to implement transparently.

I could probably have managed to achieve clustering of my app's state but it would have involved rewriting portions of my code - it just wasn't designed for this sort of thing. I've endured running out of heap space (why?!) and the JVM terminating whenever it doesn't like something (excusable?), messages instructing me that my loggers are being distributed (ah.) and even a cold-call offering professional services support (in response to a blog entry?!).

Admittedly, Terracotta makes no claims that clustering should be bolted on post-hoc or taken on lightly; it simply provides a familiar programming paradigm for achieving it. While it's transparent to your Java code, it's not transparent to your Java development.

I guess what I've learned is that if you want distribution, you still need to think long and hard about it. The cost of implementation may be reduced by using something like Terracotta, but that doesn't mean you can leave the implementation to "cheaper" developers - the complexity of the problem remains.