App Engine recently announced support for Cross Group (XG) transactions; one would expect these transactions to incur additional overhead when used across multiple entity groups, but if I specify I want an XG transaction at the start of a transaction, but then in fact only access one entity group, will I see a significant performance impact?
I ask because JDO requires that the XG-ness be specified at the persistence manager level, and it would be much simpler to just turn it on for all transactions (I only need it for a few, rare cases)…
It’s not documented or defined if this is the case, but it’s safest to assume that the overhead will exist for all transactions.
If you’re concerned about overhead, you probably shouldn’t be using JDO, since creating a PersistenceManager has a lot of overhead in and of itself.