It's true, AOLServer does have a kind of abstraction layer. But through calls like ns_db dml you can still write code that's specific to the database you're using in the tcl pages. My suggestion is that somehow this could be prevented. Staying within tcl, all database calls would be in functions rather than in pages, simplifying the process of porting. In fact when you do this sort of thing you end up implementing something like ODBC/JDBC functionality in Tcl. But at least you have one place to concentrate on to port to another database, rather than code scattered across script files everywhere.
The Java idea was only so that the scripting and webserver layers would then be (to some extent) language neutral. Most scripting languages can call Java these days. Plus you get an object model (which ACS 4.0 seems to want) without necessarily having to rely on the database to provide it.
