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Date: Thu, 21 Jul 2005 14:46:13 -0400
To: jackrabbit-dev@incubator.apache.org
From: Nicolas Belisle <Nicolas.Belisle@bibl.ulaval.ca>
Subject: Re: Problems with concurrent sessions
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Hi Marcel,

If all three tryLock() fail, the event is certainly active. The event is=20
then waiting for an ACTIVE lock to be released (the last tryLock() confirms=
=20
that fact). The event is not cancelled/removed until the lock is obtained.=
=20
Then doInTransaction() is called.

The only way I see to stop the transaction from executing is to stop the=20
server...

Am I missing something ?

Regards,

Nicolas


Le 05:54 2005-07-19, vous avez =E9crit:
>Hi Nicolas,
>
>Looks better now, but there are still cases where the doTransaction()=20
>method will not be called, though very unlikely: when all three tryLock()=
=20
>attempts fail. not very likely but theoretically possible...
>
>regards
>  marcel
>
>Nicolas Belisle wrote:
>>Hi,
>>Thanks again for your comments.
>>Here's the second version of my template class. It should resolves the=20
>>concurrency issues you mentionned :
>>package app;
>>import javax.jcr.Credentials;
>>import javax.jcr.LoginException;
>>import javax.jcr.Node;
>>import javax.jcr.Repository;
>>import javax.jcr.RepositoryException;
>>import javax.jcr.Session;
>>import javax.jcr.lock.LockException;
>>import javax.jcr.observation.Event;
>>import javax.jcr.observation.EventIterator;
>>import javax.jcr.observation.EventListener;
>>public abstract class SerializableTemplate {
>>     private Session session;
>>     private Node scope;
>>     private boolean done =3D false;
>>     private EventListener el;
>>     public SerializableTemplate(Repository repository, Credentials cr,=20
>> String scopePath) throws LoginException, RepositoryException {
>>         session =3D repository.login(cr);
>>         scope =3D session.getRootNode().getNode(scopePath);
>>         //scope =3D session.getNodeByUUID(scope.getUUID());
>>     }
>>     public abstract void doInTransaction(Session session) throws=20
>> RepositoryException;
>>     public void execute() throws RepositoryException {
>>         if (tryLock()) {
>>             return;
>>         }
>>         this.el =3D new EventListener() {
>>             public void onEvent(EventIterator events) {
>>                 try {
>>                     tryLock();
>>                 } catch (RepositoryException e) {
>>                     throw new RuntimeException(e);
>>                 }
>>             }
>>         };
>>
>>session.getWorkspace().getObservationManager().addEventListener(el,=20
>>Event.PROPERTY_REMOVED, scope.getPath(), true, null, null, false);
>>         //Try again, in case the lock is removed before observer could=20
>> be put in place
>>         tryLock();
>>     }
>>     private synchronized boolean tryLock() throws RepositoryException {
>>         try {
>>             if (done) {
>>                 return false;
>>             }
>>             if (!scope.isLocked()) {
>>                 scope.lock(true, true);
>>                 try {
>>                     if (el !=3D null) {
>>
>>session.getWorkspace().getObservationManager().removeEventListener(el);
>>                     }
>>                     doInTransaction(session);
>>                 } finally {
>>                     done =3D true;
>>                     if (session.isLive()) {
>>                         session.logout();
>>                     }
>>                 }
>>                 return true;
>>             }
>>         } catch (LockException e) {
>>             e.printStackTrace();
>>         }
>>         return false;
>>     }
>>}
>>Here's how to use it :
>>SerializableTemplate sTemplate =3D new SerializableTemplate(repository,=
 new=20
>>SimpleCredentials("user", "password".toCharArray()), "node/path") {
>>         //@Override
>>         public void doInTransaction(Session session) throws=20
>> RepositoryException {
>>                 //Do your favorite transaction...
>>         };
>>sTemplate.execute();
>>
>>For the constructor you suggested, I actually came up with a similiar=20
>>design at first, but found a problem with it : since the template class=20
>>might use an EventListener the class should be responsible for closing=20
>>the session (the EventListener can wait a while...). Else, the event=20
>>could be removed by the user before being invoked. That's the reason for=
=20
>>my ugly constructor.
>>I welcome your comments again...
>>Regards,
>>Nicolas