Drools: Prioritize fact matching based on a field's value

Consider the following scenario in Drools: We have a rule, matching objects of type A and B against each other.

rule 1
    when
        $a : A()
        $b : B($a.matches($b), flagged == false)
    then
        mofidy($b) { flag($a) }
end

However, A objects have a field called priority, and when a B arrives in the working memory, a matching A with the highest priority should flag it, above all other matching As. How is this possible in Drools? How does it affect performance?

Assuming that B.flag() is setting its flagged attribute to true, you can try something like this:

rule 1
when
    $a : A()
    not A(this != $a, priority > $a.priority)
    $b : B($a.matches($b), flagged == false)
then
    mofidy($b) { flag($a) }
end

One thing to notice in this example is that if an A object with a high priority is inserted, any B that was already flagged with a lower A will not be reflagged.

If you need to reflag yours Bs, then you can try something like this:

rule 1
when
    $a : A()
    not A(this != $a, priority > $a.priority)
    $b : B($a.matches($b), flag != $a)
then
    mofidy($b) { flag($a) }
end

Hope it helps,