[TAP5-834] BaseOptimizedSessionPersistedObject does not work correctly with Tomcat & Jetty

• Type: Bug
• Status: Closed
• Priority: Critical
• Resolution: Fixed
• Affects Version/s: 5.1.0.0, 5.1.0.1, 5.1.0.2, 5.1.0.3, 5.1.0.4, 5.1.0.5, 5.0.18
• Fix Version/s: 5.2.0
• Component/s: tapestry-core
• Assignee: Howard Lewis Ship
• Reporter: Andy Blower
• Created: 03/Sep/09 13:55
• Updated: 25/Nov/10 04:32
• Resolved: 25/Nov/10 03:56

Description

OptimizedSessionPersistedObject's suggestion for implementing isSessionPersistedObjectDirty(), as used by BaseOptimizedSessionPersistedObject, does not work correctly with Tomcat & Jetty. (and quite possibly other servlet containers too, but we only use Jetty & Tomcat so have only confirmed it with them)

OptimizedSessionPersistedObject model relies on the servlet container session object triggering a HttpSessionBindingEvent when an object is re-stored in the session to reset the dirty flag. I've only looked at the source of Tomcat 5.5 and 6 but when an object is replaced in the session using setAttribute() the new object and the existing object are compared by reference only, if they both refer to the same object then no HttpSessionBindingEvent is triggered.

From Tomcat StandardSession.java:

// Call the valueBound() method if necessary
if (notify && value instanceof HttpSessionBindingListener) {
// Don't call any notification if replacing with the same value
Object oldValue = attributes.get(name);
if (value != oldValue) {
event = new HttpSessionBindingEvent(getSession(), name, value);
try

{ ((HttpSessionBindingListener) value).valueBound(event); }

catch (Throwable t)

{ manager.getContainer().getLogger().error (sm.getString("standardSession.bindingEvent"), t); }

}
}

So, using OptimizedSessionPersistedObject, there is currently no way of setting the dirty flag to false after the object has been saved in the session - hence we are propagating all of the SSOs across the cluster all of the time because the dirty flag stays set to true.

I think there are two possible solutions to this issue - I prefer the first by a large margin, but both modify the SessionImpl.restoreDirtyObjects() method.

1) Add a new method to OptimizedSessionPersistedObject interface to reset the dirty flag, and a corresponding method in SessionPersistedObjectAnalyzer - implementing them as appropriate, then call the new reset method after setting the session attribute in SessionImpl.restoreDirtyObjects().

2) Remove the session attribute before adding it in SessionImpl.restoreDirtyObjects(). Although I have a worry that this may potentially cause hard to find concurrency problems.

Issue Links

• breaks: TAP5-1355 Threading issue with SessionStateObjects