[rules-users] Drools KnowledgeBuilderImpl.add(...) not thread safe???

[Grigoriev, Grigoriy]

I have already mentioned this issue previously here (see http://n3.nabble.com/Mvel-and-multithreading-td50133.html)

Maybe I have described the problem not detailed enough.

We are using Drools version 5.0.1 and have a concrete problem with it in our production environment. Our customer looks already interrogative at the developer team and have "questions" about drools... please help! :)

Let us assume a multithreaded application, that tries to initialise & start several processes using a StatefulKnowledgeSession in order to use in these processes rule flow. The flow in all the processes is the same but every process instance must have an own StatefulKnowledgeSession instance because of stateful session.

Actually the problem:
At the initializing phase occurs an org.mvel2.CompileException exception from time to time at KnowledgeBuilder.add(...):
Exception in thread "Thread-60" [Error: incomplete statement:  (possible use of reserved keyword as identifier: )]
[Near : {... globals != empt ....}]
                      ^
[Line: 0, Column: 0]
      at org.mvel2.MVELInterpretedRuntime.parseAndExecuteInterpreted(MVELInterpretedRuntime.java:153)
      at org.mvel2.MVELInterpretedRuntime.parse(MVELInterpretedRuntime.java:44)
      at org.mvel2.MVEL.eval(MVEL.java:514)
      at org.mvel2.templates.res.IfNode.eval(IfNode.java:61)
      at org.mvel2.templates.res.TextNode.eval(TextNode.java:46)
      at org.mvel2.templates.res.TerminalNode.eval(TerminalNode.java:39)
      at org.mvel2.templates.res.ForEachNode.eval(ForEachNode.java:116)
      at org.mvel2.templates.res.TextNode.eval(TextNode.java:46)
      at org.mvel2.templates.res.TerminalNode.eval(TerminalNode.java:39)
      at org.mvel2.templates.res.IfNode.eval(IfNode.java:64)
      at org.mvel2.templates.res.TextNode.eval(TextNode.java:46)
      at org.mvel2.templates.res.ExpressionNode.eval(ExpressionNode.java:53)
      at org.mvel2.templates.res.TextNode.eval(TextNode.java:46)
      at org.mvel2.templates.TemplateRuntime.execute(TemplateRuntime.java:195)
      at org.mvel2.templates.TemplateRuntime.execute(TemplateRuntime.java:190)
      at org.mvel2.templates.TemplateRuntime.execute(TemplateRuntime.java:180)
      at org.mvel2.templates.TemplateRuntime.execute(TemplateRuntime.java:169)
      at org.drools.rule.builder.dialect.java.AbstractJavaRuleBuilder.generatTemplates(AbstractJavaRuleBuilder.java:126)
      at org.drools.rule.builder.dialect.java.JavaConsequenceBuilder.build(JavaConsequenceBuilder.java:128)
      at org.drools.rule.builder.RuleBuilder.build(RuleBuilder.java:86)
      at org.drools.compiler.PackageBuilder.addRule(PackageBuilder.java:1159)
      at org.drools.compiler.PackageBuilder.addPackage(PackageBuilder.java:649)
      at org.drools.compiler.PackageBuilder.addPackageFromDrl(PackageBuilder.java:290)
      at org.drools.compiler.PackageBuilder.addKnowledgeResource(PackageBuilder.java:488)
      at org.drools.builder.impl.KnowledgeBuilderImpl.add(KnowledgeBuilderImpl.java:25)
      at com.myfirm.RuleFlowProcess.init(RuleFlowProcess.java:25)
      at com.myfirm.RuleFlowProcess.clone(RuleFlowProcess.java:53)
      at com.myfirm.ThreadTest$CloneThread.run(ThreadTest.java:35)
Caused by: java.lang.NullPointerException
      at org.mvel2.MVELInterpretedRuntime.parseAndExecuteInterpreted(MVELInterpretedRuntime.java:113)
      ... 27 more
Exception in thread "Thread-72" [Error: incomplete statement:  (possible use of reserved keyword as identifier: )]
[Near : {... declarations != empt ....}]
                           ^
[Line: 0, Column: 0]
      at org.mvel2.MVELInterpretedRuntime.parseAndExecuteInterpreted(MVELInterpretedRuntime.java:153)
      at org.mvel2.MVELInterpretedRuntime.parse(MVELInterpretedRuntime.java:44)
      at org.mvel2.MVEL.eval(MVEL.java:514)
      at org.mvel2.templates.res.IfNode.eval(IfNode.java:61)
      at org.mvel2.templates.res.TextNode.eval(TextNode.java:46)
      at org.mvel2.templates.res.ExpressionNode.eval(ExpressionNode.java:53)
      at org.mvel2.templates.res.TextNode.eval(TextNode.java:46)
      at org.mvel2.templates.res.ExpressionNode.eval(ExpressionNode.java:53)
      at org.mvel2.templates.res.TextNode.eval(TextNode.java:46)
      at org.mvel2.templates.res.TerminalNode.eval(TerminalNode.java:39)
      at org.mvel2.templates.res.ForEachNode.eval(ForEachNode.java:116)
      at org.mvel2.templates.res.TextNode.eval(TextNode.java:46)
      at org.mvel2.templates.res.TerminalNode.eval(TerminalNode.java:39)
      at org.mvel2.templates.res.ForEachNode.eval(ForEachNode.java:116)
      at org.mvel2.templates.res.TextNode.eval(TextNode.java:46)
      at org.mvel2.templates.res.ExpressionNode.eval(ExpressionNode.java:53)
      at org.mvel2.templates.res.TextNode.eval(TextNode.java:46)
      at org.mvel2.templates.res.ExpressionNode.eval(ExpressionNode.java:53)
      at org.mvel2.templates.res.TextNode.eval(TextNode.java:46)
      at org.mvel2.templates.TemplateRuntime.execute(TemplateRuntime.java:195)
      at org.mvel2.templates.TemplateRuntime.execute(TemplateRuntime.java:190)
      at org.mvel2.templates.TemplateRuntime.execute(TemplateRuntime.java:180)
      at org.mvel2.templates.TemplateRuntime.execute(TemplateRuntime.java:169)
      at org.drools.rule.builder.dialect.java.AbstractJavaRuleBuilder.generatTemplates(AbstractJavaRuleBuilder.java:135)
      at org.drools.rule.builder.dialect.java.JavaConsequenceBuilder.build(JavaConsequenceBuilder.java:128)
      at org.drools.rule.builder.RuleBuilder.build(RuleBuilder.java:86)
      at org.drools.compiler.PackageBuilder.addRule(PackageBuilder.java:1159)
      at org.drools.compiler.PackageBuilder.addPackage(PackageBuilder.java:649)
      at org.drools.compiler.PackageBuilder.addPackageFromDrl(PackageBuilder.java:290)
      at org.drools.compiler.PackageBuilder.addKnowledgeResource(PackageBuilder.java:488)
      at org.drools.builder.impl.KnowledgeBuilderImpl.add(KnowledgeBuilderImpl.java:25)
      at com.myfirm.RuleFlowProcess.init(RuleFlowProcess.java:25)
      at com.myfirm.RuleFlowProcess.clone(RuleFlowProcess.java:53)
      at com.myfirm.ThreadTest$CloneThread.run(ThreadTest.java:35)
Caused by: java.lang.NullPointerException
      at org.mvel2.MVELInterpretedRuntime.parseAndExecuteInterpreted(MVELInterpretedRuntime.java:113)
      ... 33 more

To demonstrate this problem I have created a simple test maven project that reproduces this error (s. attach):
I have a class that creates & inits stateful knowledge session in a init method (RuleFlowProcess):

/**
 * inits the rule flow
 */
public class RuleFlowProcess {
      private StatefulKnowledgeSession ksession;
      public void init() {
            KnowledgeBuilder kbuilder = KnowledgeBuilderFactory.newKnowledgeBuilder();
            kbuilder.add(ResourceFactory.newClassPathResource("threadSafeTest.rf"), ResourceType.DRF);
            kbuilder.add(ResourceFactory.newClassPathResource("testRules1.drl"), ResourceType.DRL);
            kbuilder.add(ResourceFactory.newClassPathResource("testRules2.drl"), ResourceType.DRL);
            kbuilder.add(ResourceFactory.newClassPathResource("testRules3.drl"), ResourceType.DRL);
            kbuilder.add(ResourceFactory.newClassPathResource("testRules4.drl"), ResourceType.DRL);

            KnowledgeBuilderErrors errors = kbuilder.getErrors();
            if (errors.size() > 0) {
                  StringBuilder errorString = new StringBuilder();
                  for (KnowledgeBuilderError error : errors) {
                        errorString.append(error);
                  }
                  throw new RuntimeException(errorString.toString());
            }

            KnowledgeBase kbase = KnowledgeBaseFactory.newKnowledgeBase();
            kbase.addKnowledgePackages(kbuilder.getKnowledgePackages());
            ksession = kbase.newStatefulKnowledgeSession();
      }
...
}

I create & init concurrently in several (200) threads this RuleFlowProcess and some threads crash with this exceptions while adding the resources.

I have run this project on two PCs:
core 2 duo @ 2.6 ghz, win xp prof sp3, sun jdk 1.6.0_17
Pentium Dual CPU E2160 @ 1.8GHz, XP prof SP3, sun jdk 1.6.0_16.

Could you please help us to clear this problem? Are we doing here something wrong?
If it should be an drools error, could you please create an issue for it?

Thanks!

Wbr,
Grigoriy Grigoriev
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