Aggregate Event Streams can be accessed and read by multiple threads simultaneously. This opens up the real potential for concurrency conflicts that, if left unchecked, could result in a random number of invalid Aggregate states. Consider a scenario when two threads attempt to modify the Event Stream at the same time, as shown in Figure A.10.
The simplest resolution to this situation is to use
EventStoreConcurrencyException at step 4, allowing it to propagate all the way up to the ultimate client: