Add link between bpmn transaction and db transaction layers

Currently, bpmn transaction consistency is layered on top of the general activity consistency model (db transactions + optimistic locking + retry).

In addition, bpmn transactions can only be rolled back using the cancel end event.

The bpmn specification requires that there is a "link" between bpmn transactions and the underlying transaction protocol (the bpmn specification has WS-AT./WS-BA./WS-Coord. very much in mind).

Activiti is an embeddable java engine, which means:

• we share transactions with services
• we use java (Jdbc/Jta) transactions as the "underlying transaction protocol"

AT:

• when a jdbc/jta transaction "inside" a bpmn transaction is rolled back, the bpmn transaction is cancelled.

Note:

• this issues requires some investigation / research
• this issue is more of a "reminder". We might not want to actually do this.