I figured out this is a problem with Event and dynamic salience. Suppose we have the following declarations and rules:

declare Message
    @role(event)
end

rule "rule1"
salience RuleSalience.RULE1
when
    $m : Message()
then
    System.out.println("====================> rule1");
    retract($m);
    RuleSalience.RULE1 = RuleSalience.RULE2 - 1;
end    

rule "rule2"
salience RuleSalience.RULE2
when
    $m : Message()
then
    System.out.println("=====================> rule2");
    retract($m);
    RuleSalience.RULE2 = RuleSalience.RULE1 - 1;
end  

I want when a Message is inserted, either rule1 or rule2 is fired. I also want two rules can fire alternatively. So when one rule is fired, its salience becomes lower. 

The result is, if I don't declare Message as an Event, it works fine. Otherwise, only rule1 is fired. 

The problem seems to be at org.drools.core.reteoo.LeftInputAdapterNode::doInsertSegmentMemory(). Because of stream mode, both rules are not inserted StagedLeftTuples. So rule1 and rule2 are not compared based on salience. Because rule1 has a smaller act#, it is always fired first. 

The attached zip is a test case. If you want mvn clean test, it will fail. However, if you remove the following lines from Sample.drl, the test will succeed. 

"declare Message
    @role(event)
end"

Thanks.

Jinghai