Hello, I have been fighting with this issue for several days, and I just stumbled upon the reason why, but that raised another question for me. I have a rule checking that two events of the same type do not occur in succession without an event of a specific other type in between them: # If two WorkerInWorkEvents exist for a worker, there must be a WorkerOutOfWorkEvent between them rule "ERROR: Consecutive WorkerInWorkEvents" when $w1 : WorkerInWorkEvent( $workerId : workerId ) $w2 : WorkerInWorkEvent( workerId == $workerId, this after $w1 ) not WorkerOutOfWorkEvent( workerId == $workerId, this after $w1, this before $w2 ) then errors.add("Two WorkerInWorkEvents without a WorkerOutOfWorkEvent in between (" + $w2 + ")"); retract( $w2 ); end I could see via the log that this file was activating, however it was never firing. The retraction of $w2 should have cancelled other rules' activations, but it wasn't doing so because this rule was never firing. I just discovered a thread (http://drools.46999.n3.nabble.com/Drools-fusion-and-absence-of-events-td5...) which finally explained to me the reason why: I was running in STREAM mode, and the engine was waiting to see if a WorkerOutOfWorkEvent timestamped between the two WorkerInWorkEvents was going to show up. When I switched to CLOUD mode, the rule fired as expected and my other activations were cancelled as I expected. Now, my new question is: Since I have not specified any intervals, how long will the engine wait for the WorkerOutOfWorkEvent to show up? Forever? Thanks in advance for any light you can shed on this. Mike