Good $time_of_day! I'm a Drools newbie trying to use it in production. So I want to share some of my frustration and some of my solutions to this frustration :) First, let's start with insertLogical - it's useless. It can easily mislead user into thinking that changed objects are retracted. Of course, other users bumped into it before me: http://drools-java-rules-engine.46999.n3.nabble.com/insertLogical-and-mod... , but the proposed resolution is ugly. So I wrote a wrapper around the 'insertLogical' function which introduces the notion of 'fact key', so only one fact with the given key can exist in the working memory at a time. To borrow the example in the referenced mail thread: ============ rule "fact = 1"    when        $f : UserFact(fact1 == 1);    then          insertLogical(new CreatedFact($f.getFact2())); end ============ Becomes: ============ global ConsequenceHelper ch; # Code of the ConsequenceHelper is attached rule "fact = 1" language "mvel"    when        $f : UserFact(fact1 == 1);    then          ch.insertGuarded(drools, new CreatedFact($f.getFact2()), {$f.getFact2()}); # Here {$f.getFact2()} is the CreatedFact's key. end ============ So if this rule fires again, the stale object will be retracted and the newly created object will be asserted. It would be nice to see something like this integrated into the Drools core... Next, I'd like to say something about Drools LHS language. Well, it's VERY confusing. I spent two hours debugging internals of MVEL to understand why this doesn't work: ============ rule "Double Shift OT"        dialect "mvel" agenda-group "OT Calculation" when        $at : AcctTransaction()        exists(AcctTransaction(this!=$at, acct.plannedEndTimestamp coincides[2h] $at.plannedStartTimestamp)) then ... end ============ Turns out, I can't use these nice Drools Fusion predicates to for deep properties. It would be nice, if it was at least mentioned somewhere in the documentation. No workaround for this one, I had to write global helper functions. And the final problem (so far). Drools has this nice 'from' feature which allows to use objects not in the working memory. However, it's not possible to do this: ============ rule "Blah" when    $l : Long() from new Long(42L) then ... end ============ I had to write a helper function which just returns its parameter to work around this one. And the final note - DRL is too cut&pasty. There's no _good_ way to reuse predicates used in the LHS (there's rule inheritance, but it's not always applicable) and actions in the RHS.