Redeployment issues

[lakr1]

Hello,

I am using JBoss 3.2.5 with a deployed treecache, replSync.xml as MBean, which is accessed from a deployed EJB in a cluster, which works fine until the EJB is redeployed.

After redeployment the EJB I gets ClassCastException when trying to get items from the cache.

The objects that I put into the cache is residing in the deployed ejb.jar, I suppose that after a redeployment those classes are not usable anymore (belonging to a discarded classloader!?). This error is kind of understandable, the question is how this can be avoided.

I was thinking if it is possible to register some EJB deployment listener that can clear the in memory cached items in the treecache. Any ideas on how this can be done? Or any other idea.

Best Regards
Lars Krog-Jensen

[norbert]

this is due to the fact that objects in the cache that is residing in the same JVM as the ejb are accessed by reference without any check whether the previous loaded classes of these objects are accessible to the current ejb's classloader anymore. This isn't easy to solve as long the objects themselve are stored in the cache and not externalized or serialized versions of them.

I'd expect you would get the same error if you deploy two ejbs accessing the same instance of TreeCache from 2 different ears using classes that are both loaded by the different classloaders of the 2 applications.

Flushing the local cache on undeployment of the ejb is not a solution, since this might lead to totally unexpected behavior (as it makes a difference wheter another member in the group is active until the ejb is redeployed which would allow to refetch the state.)

As a workaround you may include the classes in the servers classpath, so the classes do not have to be reloaded. Maybe this is the only suitable solution as long we agree that it's desirable to allow a per-JVM shared cache at all.

[ben.wang]

This classloading issue is also disucssed here:
http://www.jboss.org/index.html?module=bb&op=viewtopic&p=3842379

There is really no good (optimized) solution to it except to have another in-vm cache per ejb app.

I have also put it under the latest FAQ. Check it out.

-Ben

[lakr1]

Hello,

Does the MarshalledValue solution implie that my POJO will be serialized on put and deserialized on get. If that is true, it would seriously degrade the perfomance as I cache quite large objects.


Norbert: As a workaround you may include the classes in the servers classpath, so the classes do not have to be reloaded.

That would mean that the those classes cannot be hotdeployed at all!

Norbert: I'd expect you would get the same error if you deploy two ejbs accessing the same instance of TreeCache from 2 different ears using classes that are both loaded by the different classloaders of the 2 applications.

Yes I get the same error.

// Lars

[norbert]

>Does the MarshalledValue solution implie that my >POJO will be serialized on put and deserialized on >get. If that is true, it would seriously degrade the >perfomance as I cache quite large objects.

correct. That's why storing references to the actuall object itself is choosen for the local cache.

>Norbert: As a workaround you may include the >classes in the servers classpath, so the classes do >not have to be reloaded.

>That would mean that the those classes cannot be >hotdeployed at all!

also correct.

As you might see there is no way to have both:
Store references to object instances and access them later while having the classes (that might have changed in the meantime) redeployed will never work.

[ben.wang]

I have also put a faq on class loading scope regarding to JBossCache (and caching system, in general, AFAIK). Here is the url:
http://docs.jboss.org/jbcache/FAQ.html

-Ben

[tomerbd2]

Hi,

I'm having the same problem described in this thread and using MarshaledValue solved my problem, however as described, this solution is having a performance penalty, so I would prefer not to use it.

Its written in the thread that:

There is really no good (optimized) solution to it except to have another in-vm cache per ejb app.

What is an in-vm cache per ejb app?
How can I implement such a solution?

Thanks

[brian.stansberry]

Have a look at the region-based marshalling feature discussed at http://labs.jboss.com/file-access/default/members/jbosscache/freezone/docs/1.4.0/TreeCache/en/html/treecache_marshaller.html.

Basically allows you to register a classloader with region of the cache. Simplified usage:

On deploy of your app:

1) register a classloader with the region of your cache where you will store objects for that app.
2) activate that region

On undeploy of your app:

3) Inactivate the region
4) Unregister the classloader