Let's not address the question of raising an exception on rollback() within that issue.
The question of rollback raising an exception is and I think this will be addressed by OGM-465.

They would still use transaction.begin() / commit(), as that's what we use for controlling the flush cycle. In that IRC chat linked above we discussed some ideas how that Transaction implementation could look like. Basically it would call some SPI in ORM for triggering the flush.

Probably it'd make sense to raise a meaningful exception when trying to invoke rollback as that's not possible with the non-transactional backends. The transactional backends we have (ISPN, Neo4j) work with JTA; I don't think there is any need atm. for making them work with RESOURCE_LOCAL. It might make sense for some theoretical store in the future which has some kind of "local" TX but does not work with JTA.

What does that imply for the user? Does he in his user code still begin and commit a transaction? Or can he leave transaction handling out?

Related IRC discussion with : http://transcripts.jboss.org/channel/irc.freenode.org/%23hibernate-dev/2015/%23hibernate-dev.2015-01-23.log.html#t2015-01-23T16:09:27

I changed the title to be more specific. Super generic titles I found tend to be either ignored, require to go into the detail page or if the detail page is not filled, we simply don't remember what the issue was about.