Hi, In our application we have different rulesets which we load them on demand and cache them (knowledge base) for the sub-sequent requests. For the first time before caching if we hit the server concurrently we are getting the following exception which is not very clear. I assume knowledge bases are thread safe and we can create multiple stateful sessions using the same knowledege base. we are using drools 5.1 final release. java.lang.IllegalArgumentException: org.drools.process.instance.impl at java.lang.ClassLoader.definePackage(ClassLoader.java:1452) at java.net.URLClassLoader.definePackage(URLClassLoader.java:348) at org.apache.catalina.loader.WebappClassLoader.findClassInternal(WebappClassLoader.java:1781) at org.apache.catalina.loader.WebappClassLoader.findClass(WebappClassLoader.java:866) at org.apache.catalina.loader.WebappClassLoader.loadClass(WebappClassLoader.java:1319) at org.apache.catalina.loader.WebappClassLoader.loadClass(WebappClassLoader.java:1198) at org.drools.SessionConfiguration.initProcessInstanceManagerFactory(SessionConfiguration.java:302) at org.drools.SessionConfiguration.getProcessInstanceManagerFactory(SessionConfiguration.java:291) at org.drools.common.AbstractWorkingMemory.<init>(AbstractWorkingMemory.java:367) at org.drools.common.AbstractWorkingMemory.<init>(AbstractWorkingMemory.java:295) at org.drools.common.AbstractWorkingMemory.<init>(AbstractWorkingMemory.java:259) at org.drools.reteoo.ReteooWorkingMemory.<init>(ReteooWorkingMemory.java:96) at org.drools.reteoo.ReteooStatefulSession.<init>(ReteooStatefulSession.java:80) at org.drools.reteoo.ReteooRuleBase.newStatefulSession(ReteooRuleBase.java:386) at org.drools.reteoo.ReteooRuleBase.newStatefulSession(ReteooRuleBase.java:371) at org.drools.impl.KnowledgeBaseImpl.newStatefulKnowledgeSession(KnowledgeBaseImpl.java:172) at org.drools.impl.KnowledgeBaseImpl.newStatefulKnowledgeSession(KnowledgeBaseImpl.java:155) Please help us in this regard. Thanks sudhir.