[jbosscache-dev] Re: Fixing MVCC(RC) + UnversionedNode for PCACHE-82

[Manik Surtani]

Is this something you also see outside of POJO Cache?  E.g., by  
following a similar sequence of calls, like:

1.  put(/a/b/c, k, p1)
2.  remove(/a/b/c)
3.  put(/a/b/c, k, p2)
4.  remove(/a/b/c)

?


On 24 Jun 2009, at 16:32, Galder Zamarreno wrote:

> Hi all,
>
> Re: https://jira.jboss.org/jira/browse/PCACHE-82
>
> Even though this is not urgent, it's a good exercise for me to get a  
> better understanding of MVCC and hence why I decided to finally  
> tackle it.
>
> I've been trying to figure out what exactly is the bug in this JIRA.  
> I've created a test case that reproduces the bug (attached). The  
> issue is clearly related to MVCC(ReadCommitted) and UnversionedNode  
> instances but I'm failing to understand what the fix exactly is.
>
> Let me sum up what happens internally (note that a lot of the  
> information below is from logging statements I added):
>
> 1. Create a Person, Galder, and attach it under /person/ 
> testDoubleAttachAndDetach
> 2. Create a Person, Asier, and attach it in the same fqn.
> 3. In the 2nd attachment, ReadCommittedNode.markForUpdate() backs up  
> the NodeReference for /person/testDoubleAttachAndDetach creates a  
> copy of the UnversionedNode representing underneath.
> 4. More importantly, because calling attach the 2nd time means  
> detach needs to happen before to remove the old value, so  
> MVCCNodeHelper.wrapNodesRecursivelyForRemoval() executes the  
> addChild() below:
>
> // update child ref on parent to point to child as this is now a copy.
> if (parentLockNeeded && (needToCopyNode || needToCopyParent)) {
>  if (parent == null) throw new NodeNotExistsException("Parent node "  
> + parentFqn + " does not exist!");  
> parent.getDelegationTarget().addChild(node.getDelegationTarget());
> }
>
> This leads to adding the UnversionedNode version of  
> testDoubleAttachAndDetach to /person node's children.
>
> 5. At the end 2nd attachment, ReadCommittedNode.updateNode() method  
> takes the backup (NodeReference) and the node (UnversionedNode) and  
> swaps them again but the parent's children, /person, is still  
> pointing to an UnversionedNode which leads to the ClassCastException  
> that you see when you execute the test.
>
> I'm not sure what really should be happening here since I don't  
> understand the rationaly of that addChild() call. Should it be  
> happening at all in this case? Or maybe a corrective action of some  
> sort needs to be done somewhere?
>
> Thanks for the help in advance!
> -- 
> Galder Zamarreño
> Sr. Software Engineer
> Infinispan, JBoss Cache
> /*
> * JBoss, Home of Professional Open Source.
> * Copyright 2009, Red Hat Middleware LLC, and individual contributors
> * as indicated by the @author tags. See the copyright.txt file in the
> * distribution for a full listing of individual contributors.
> *
> * This is free software; you can redistribute it and/or modify it
> * under the terms of the GNU Lesser General Public License as
> * published by the Free Software Foundation; either version 2.1 of
> * the License, or (at your option) any later version.
> *
> * This software is distributed in the hope that it will be useful,
> * but WITHOUT ANY WARRANTY; without even the implied warranty of
> * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
> * Lesser General Public License for more details.
> *
> * You should have received a copy of the GNU Lesser General Public
> * License along with this software; if not, write to the Free
> * Software Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA
> * 02110-1301 USA, or see the FSF site: http://www.fsf.org.
> */
> package org.jboss.cache.pojo;
>
> import static org.testng.AssertJUnit.assertNull;
> import static org.testng.AssertJUnit.assertEquals;
>
> import org.apache.commons.logging.Log;
> import org.apache.commons.logging.LogFactory;
> import org.jboss.cache.CacheSPI;
> import org.jboss.cache.Fqn;
> import org.jboss.cache.pojo.PojoCache;
> import org.jboss.cache.pojo.PojoCacheFactory;
> import org.jboss.cache.pojo.test.Address;
> import org.jboss.cache.pojo.test.Person;
> import org.testng.annotations.Test;
>
> /**
> * ClassCastExceptionTest.
> *
> * @author Galder Zamarreño
> */
> @Test(groups = {"functional"})
> public class AttachDetachTest
> {
>   private static final Log log =  
> LogFactory.getLog(AttachDetachTest.class);
>   private PojoCache cache;
>
>   public void testDoubleAttachAndDetach() throws Exception
>   {
>      log.info("testDoubleAttachAndDetach() ....");
>
>      String configFile = "META-INF/local-readcommitted.xml";
>      cache = PojoCacheFactory.createCache(configFile);
>
>      try
>      {
> //         String id = "person/testLoadAttachDetach";
> //         Person galder = createPerson("Galder", 20);
> //         log.info("Attaching Galder...");
> //         cache.attach(id, galder);
> //         cache.stop();
> //
> //         cache = PojoCacheFactory.createCache(configFile);
> //         Person p = (Person) cache.find(id);
> //         log.info("Person found: " + p);
> //         // assertEquals("age ", p.getAge(), galder.getAge());
> //
> //         Person asier = createPerson("Asier", 34);
> //         log.info("Attaching Asier...");
> //         cache.attach(id, asier);
> //
> //         cache.detach(id);
>
>         String id = "person/testDoubleAttachAndDetach";
>         Person galder = createPerson("Galder", 20);
>         log.info("Attaching Galder...");
>         cache.attach(id, galder);
> //         cache.stop();
> //
> //         cache = PojoCacheFactory.createCache(configFile);
> //         Person p = (Person) cache.find(id);
> //         log.info("Person found: " + p);
>         // assertEquals("age ", p.getAge(), galder.getAge());
>
>         Person asier = createPerson("Asier", 34);
>         log.info("Attaching Asier...");
>         cache.attach(id, asier);
>
>         cache.detach(id);
>      }
>      catch (Exception e)
>      {
>         log.error("Unexpected exception", e);
>      }
>      finally
>      {
>         cache.getCache().removeNode(Fqn.ROOT);
> //         ((CacheSPI<Object, Object>)  
> cache 
> .getCache 
> ()).getCacheLoaderManager().getCacheLoader().remove(Fqn.ROOT);
>         cache.stop();
>      }
>
> //      Thread.sleep(9100);// default is 3 seconds so joe should  
> have been passivated.
> //
> //      assertNull("Node should be evicted ", ((CacheSPI<Object,  
> Object>) cache.getCache()).peek(new Fqn<String>(id), false));
>
> //      Person p = (Person) cache.find(id);
> //
> //      log.info("Person found: " + p);
> //
> //      Person galder = createPerson("Galder Zamarreno", 29);
> //      cache.attach(id, galder);
>
> //      assertEquals("age ", 20, joe.getAge());
> //      joe.setAge(30);
> //      assertEquals("age ", 30, joe.getAge());
> //      assertEquals("age ", p.getAge(), joe.getAge());
> //      assertFalse("Instance not equal (this is known side effect)  
> ", joe == p);
>
> //      cache.detach(id);
>
>   }
>
>   private Person createPerson(String name, int age)
>   {
>      Person p = new Person();
>      p.setName(name);
>      p.setAge(age);
>      Address add = new Address();
>      add.setZip(2000);
>      add.setCity("Neuchatel");
>      p.setAddress(add);
>      return p;
>   }
> }

--
Manik Surtani
manik at jboss.org
Lead, Infinispan
Lead, JBoss Cache
http://www.infinispan.org
http://www.jbosscache.org