RE: [jbosscache-dev] Problems with node locking semantics in JBC1.4.1.GA

I played around with one way to deal with this. Speaking in db isolation terms, let's say a non-repeatable-read means a tx reads a node's *data map* twice and gets different results. A phantom read means a tx reads a node's *child map* twice and gets different results. Conceptually, a JBC node represents both a "row" (data map) and a query result set (children map -- set of rows that share a common characteristic that the parent represents). (I know this analogy is inexact, but, ...). In 1.4.0.SP1, REPEATABLE_READ meant you wouldn't get non-repeatable-reads, but could get phantom reads. With 1.4.1, you also don't get phantom reads (at least with respect to a node's immediate children). But, in many cases this behavior is unneeded and undesirable, and JBC no longer offers the old behavior. I attempted to remedy this with a new IsolationLevel REPEATABLE_DATA_READ, which basically provides the 1.4.0 semantics. This was actually very easy to do. 1) LockStrategyFactory creates the same LockStrategy for REPEATABLE_DATA_READ as for REPEATABLE_READ -- hence lock behavior at the node level is unaffected. 2) PessimisticLockInterceptor uses the new level when deciding whether to WL a parent before adding/removing a node. Manik nicely encapsulated that logic, so it was simple to change. Here's a diff to the head of Branch_JBossCache_1_4_0: ### Eclipse Workspace Patch 1.0 #P JBossCache Index: src/org/jboss/cache/interceptors/PessimisticLockInterceptor.java =================================================================== RCS file: /cvsroot/jboss/JBossCache/src/org/jboss/cache/interceptors/PessimisticLo ckInterceptor.java,v retrieving revision 1.20.2.8 diff -u -r1.20.2.8 PessimisticLockInterceptor.java --- src/org/jboss/cache/interceptors/PessimisticLockInterceptor.java 14 Dec 2006 12:51:48 -0000 1.20.2.8 +++ src/org/jboss/cache/interceptors/PessimisticLockInterceptor.java 28 Jan 2007 21:08:55 -0000 @@ -41,6 +41,8 @@ public class PessimisticLockInterceptor extends Interceptor { TransactionTable tx_table = null; + + boolean writeLockOnChildInsertRemove = true; /** * Map<Object, java.util.List>. Keys = threads, values = lists of locks held by that thread @@ -55,6 +57,7 @@ tx_table = cache.getTransactionTable(); lock_table = cache.getLockTable(); lock_acquisition_timeout = cache.getLockAcquisitionTimeout(); + writeLockOnChildInsertRemove = (cache.getIsolationLevelClass() != IsolationLevel.REPEATABLE_DATA_READ); } @@ -323,12 +326,14 @@ private boolean writeLockNeeded(int lock_type, int currentNodeIndex, int treeNodeSize, boolean isRemoveOperation, boolean createIfNotExists, Fqn targetFqn, Fqn currentFqn) { - if (isRemoveOperation && currentNodeIndex == treeNodeSize - 2) - return true; // we're doing a remove and we've reached the PARENT node of the target to be removed. - - if (!isTargetNode(currentNodeIndex, treeNodeSize) && !cache.exists(new Fqn(currentFqn, targetFqn.get(currentNodeIndex + 1)))) - return createIfNotExists; // we're at a node in the tree, not yet at the target node, and we need to create the next node. So we need a WL here. - + if (writeLockOnChildInsertRemove) + { + if (isRemoveOperation && currentNodeIndex == treeNodeSize - 2) + return true; // we're doing a remove and we've reached the PARENT node of the target to be removed. + + if (!isTargetNode(currentNodeIndex, treeNodeSize) && !cache.exists(new Fqn(currentFqn, targetFqn.get(currentNodeIndex + 1)))) + return createIfNotExists; // we're at a node in the tree, not yet at the target node, and we need to create the next node. So we need a WL here. + } return lock_type == DataNode.LOCK_TYPE_WRITE && isTargetNode(currentNodeIndex, treeNodeSize) && (createIfNotExists || isRemoveOperation); //normal operation, write lock explicitly requested and this is the target to be written to. } jbosscache-dev-bounces(a)lists.jboss.org wrote: