Why not simply use something like this:- rule "TaxCalc" when $i : Invoice($st : salesTax, $gst : gstRate, $sa : salesAmount, $id : importDuty, $gr : govtRebate) then $i.setTotalTax(round($st + ($gst * $sa / 100) + $id - $gr, 2)); update($i); end What do you gain by checking whether the Total Tax is different from the value of the calculation? I must be missing something from your simplified example?!? (I think, big disclaimer!!) you could achieve your requirement using eval and a tax value function that uses a cache under the hood, much like the following:- rule "TaxCalc" when $i : Invoice($tt : totalTax, $st : salesTax, $gst : gstRate, $sa : salesAmount, $id : importDuty, $gr : govtRebate) eval(Invoice($tt != taxFunction($st, $gst, $sa, Sid, $gr)) then $i.setTotalTax(taxFunction($st, $gst, $sa, Sid, $gr); update($i); end Evals cannot be indexed so you take a performance hit. With kind regards, Mike ________________________________ From: rules-users-bounces(a)lists.jboss.org [mailto:rules-users-bounces@lists.jboss.org] On Behalf Of Shabbir Dhari Sent: 03 July 2009 01:24 To: rules-users(a)lists.jboss.org Subject: [rules-users] Formula value assginment in LHS Dear all We are developing a financial business application that contains hundreds of business rules for validations and calculations. In the calculation rules we change the value of attribute if values past in the request is incorrect e.g. rule "TaxCalc" when $i : Invoice() Invoice (totalTax != (round(salesTax + (gstRate * salesAmount / 100) + importDuty - govtRebate, 2))) then i.setTotalTax(round(salesTax + (gstRate * salesAmount / 100) + importDuty - govtRebate, 2)); end The above code works perfectly fine. Only problem is the perform calculation twice. What I was looking if possible I can store calculated tax in a variable and simply assign variable to attribute if rule condiation fails. Some thing like: rule "TaxCalc" when $i : Invoice() Invoice (totalTax != calTax : (round(salesTax + (gstRate * salesAmount / 100)+ importDuty - govtRebate, 2))) then i.setTotalTax(calTax); end But this does not work - shows system error at calculated value assignment. Is there any work around? ________________________________ Click here to find out more POP access for Hotmail is here! <http://windowslive.ninemsn.com.au/article.aspx?id=802246>

How are you executing the rules? Are you using web services via the Drools execution server? because, I have seen multiple firings in this scenario. Regards, Prem 2009/7/2 Shabbir Dhari <sdhari(a)hotmail.com&gt;