Hello, I'm seeing this exception in catalina.out of Tomcat 5.5.15 (2 node cluster). This is Tomcat code, so I'm wondering if I should fix something or if the problem is inside Tomcat? Ronald. SEVERE: Unable to serialize delta request for sessionid [F63F9EDF02547D7610359C7A990650A7] java.io.NotSerializableException: org.apache.catalina.realm.GenericPrincipal at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1075) at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:291) at java.util.LinkedList.writeObject(LinkedList.java:755) at sun.reflect.GeneratedMethodAccessor64.invoke(Unknown Source) at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25) at java.lang.reflect.Method.invoke(Method.java:585) at java.io.ObjectStreamClass.invokeWriteObject(ObjectStreamClass.java:890) at java.io.ObjectOutputStream.writeSerialData(ObjectOutputStream.java:1333) at java.io.ObjectOutputStream.writeOrdinaryObject(ObjectOutputStream.java:1284) at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1073) at java.io.ObjectOutputStream.access$100(ObjectOutputStream.java:135) at java.io.ObjectOutputStream$PutFieldImpl.writeFields(ObjectOutputStream.java:1512) at java.io.ObjectOutputStream.writeFields(ObjectOutputStream.java:418) at javax.security.auth.Subject$SecureSet.writeObject(Subject.java:1281) at sun.reflect.GeneratedMethodAccessor63.invoke(Unknown Source) at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25) at java.lang.reflect.Method.invoke(Method.java:585) at java.io.ObjectStreamClass.invokeWriteObject(ObjectStreamClass.java:890) at java.io.ObjectOutputStream.writeSerialData(ObjectOutputStream.java:1333) at java.io.ObjectOutputStream.writeOrdinaryObject(ObjectOutputStream.java:1284) at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1073) at java.io.ObjectOutputStream.defaultWriteFields(ObjectOutputStream.java:1369) at java.io.ObjectOutputStream.defaultWriteObject(ObjectOutputStream.java:380) at java.util.Collections$SynchronizedCollection.writeObject(Collections.java:1606) at sun.reflect.GeneratedMethodAccessor62.invoke(Unknown Source) at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25) at java.lang.reflect.Method.invoke(Method.java:585) at java.io.ObjectStreamClass.invokeWriteObject(ObjectStreamClass.java:890) at java.io.ObjectOutputStream.writeSerialData(ObjectOutputStream.java:1333) at java.io.ObjectOutputStream.writeOrdinaryObject(ObjectOutputStream.java:1284) at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1073) at java.io.ObjectOutputStream.defaultWriteFields(ObjectOutputStream.java:1369) at java.io.ObjectOutputStream.defaultWriteObject(ObjectOutputStream.java:380) at javax.security.auth.Subject.writeObject(Subject.java:919) at sun.reflect.GeneratedMethodAccessor61.invoke(Unknown Source) at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25) at java.lang.reflect.Method.invoke(Method.java:585) at java.io.ObjectStreamClass.invokeWriteObject(ObjectStreamClass.java:890) at java.io.ObjectOutputStream.writeSerialData(ObjectOutputStream.java:1333) at java.io.ObjectOutputStream.writeOrdinaryObject(ObjectOutputStream.java:1284) at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1073) at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:291) at org.apache.catalina.cluster.session.DeltaRequest$AttributeInfo.writeExternal(DeltaRequest.java:346) at org.apache.catalina.cluster.session.DeltaRequest.writeExternal(DeltaRequest.java:263) at org.apache.catalina.cluster.session.DeltaManager.unloadDeltaRequest(DeltaManager.java:695) at org.apache.catalina.cluster.session.DeltaManager.requestCompleted(DeltaManager.java:1249) at org.apache.catalina.cluster.tcp.ReplicationValve.sendSessionReplicationMessage(ReplicationValve.java:328) at org.apache.catalina.cluster.tcp.ReplicationValve.invoke(ReplicationValve.java:271) at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:105) at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:107) at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:148) at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:869) at org.apache.coyote.http11.Http11BaseProtocol$Http11ConnectionHandler.processConnection(Http11BaseProtocol.java:667) at org.apache.tomcat.util.net.PoolTcpEndpoint.processSocket(PoolTcpEndpoint.java:527) at org.apache.tomcat.util.net.LeaderFollowerWorkerThread.runIt(LeaderFollowerWorkerThread.java:80) at org.apache.tomcat.util.threads.ThreadPool$ControlRunnable.run(ThreadPool.java:684) at java.lang.Thread.run(Thread.java:595)