Oracle Event Processing's high availability (HA) differs from other kinds of systems in that the data involved (events) is usually very dynamic, changing constantly. In a typical system, such as a database, the data is relatively static and HA systems, for example, both improve the reliability of the stored data and the availability of querying against that data. Since the Oracle Event Processing data changes so fast, storing it reliably can become problematic from a performance standpoint, or may even be pointless if the only relevant data is the latest data.
In a similar vein, Oracle Event Processing is often highly stateful, building up a historically influenced view of incoming event streams, and HA must take account of this statefulness. Of course, the state of the Oracle Event Processing is likely to be changing as rapidly as the incoming events are arriving and so preserving this state reliably and accurately can also be quite problematic...