[rules-users] Performance with 'eval'

[Diego Manilla Suárez]

Hi. I need to implement some sort of 'adviser' or 'recommender' system, 
and I'm considering JBoss Rules to do the task. It would be an adviser 
for choosing which car to buy, for example. Imagine: 'I want a car which 
is all-wheel-drive, and I would like it in green, but I don't mind if 
it's in another color'. The user can choose how important is each 
feature for him, or if it's mandatory (in the previous example, the 
'all-wheel-drive' requirement is mandatory, while the color it's not).

So, I would have all the car catalog in the working memory permanently 
(class Car), and I would assert a buyer (class Buyer), and I would like 
to get the cars that best match the user requirements and wishes. I had 
outlined something like this (pseudocode):

################
rule "CarRecommendation"
    when
        buyer : Buyer()
        car : Car (feature1=mandatoryRequirement1, 
feature2=mandatoryRequirement2, ...)
        eval (carUserCompatibilityLevel(car, buyer) > 
COMPATIBILITY_THRESHOLD)
    then
        buyer.getRecommendedCars().add(car);
end

function int carUserCompatibilityLevel(Car car, Buyer buyer) {
    return [compatibility level based on car suitableness to user 
preferences];
}
################
   
My main question is if I'm forced to use an 'eval' element in order to 
get what I want. I'm afraid I need to use it, and I'm concerned about 
the performance since the WorkingMemory will hold up to 10 million 
'cars' (well, they aren't really cars, the car-buyer thing it's just an 
example for the sake of simplicity), and the docs clearly state that 
evals are not nearly as efficient as field constraints (memory usage is 
not a concern in this moment).

Since the 'car' catalog is so large, I wish I could get the cars with 
the highest compatibility levels first and stop when I reach, for 
example, 50 recommended cars (I would add something like 'if 
buyer.getRecommendedCars().size() > CAR_LIMIT retract(buyer)' to the 
consequence), and so I would't have the need for a 
COMPATIBILITY_THRESHOLD. I don't know if that's even possible.

I'm new to this kind of programming and any tip would be appreciated, 
even if it involves to totally change the approach (i. e., not using a 
rule engine for the task). Just any clue that points me in the right 
direction.

Thanks in advance