I was confused by the fact that we can allow to use JTA transactions with a non-XA-datasource. Link to the documentation. So what is the difference between XA/non-XA datasources? Why should we use XA-datasources at all?
An XA transaction, in the most general terms, is a "global transaction" that may span multiple resources. A non-XA transaction always involves just one resource.
An XA transaction involves a coordinating transaction manager, with one or more databases (or other resources, like JMS) all involved in a single global transaction. Non-XA transactions have no transaction coordinator, and a single resource is doing all its transaction work itself (this is sometimes called local transactions).
jta="true", Transaction commit automatically.