Forum OpenACS Q&A: Response to Postgres, distributed databases, two phase commits, or other replication strategies....

Several app servers work with C++ in addition to Java.  Persistence is one of them.

I'm not sure about whether they implement two-phase commit ontop of databases which don't support it.  I'd have to defer to an app-server wonk.

There are several companies that are rolling out akamai-like distributed caching specifically for dynamic content.  I thought persistence's Dynamai was one of them...  These seemed like a much cheaper way to maintain uptime than managing a global db cluster (yuck),  but it's possible it's not the fault tolerance you're looking for.

It makes a huge difference in cost if some of the data needs to be continuously available, and it's mostly read-only.  What I'm suggesting wouldn't address your read-write needs.