[jboss-jira] [JBoss JIRA] Created: (JBCACHE-955) Restore ability to insert and remove child nodes with only a read lock on the parent

[Brian Stansberry (JIRA)]

Restore ability to insert and remove child nodes with only a read lock on the parent
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                 Key: JBCACHE-955
                 URL: http://jira.jboss.com/jira/browse/JBCACHE-955
             Project: JBoss Cache
          Issue Type: Feature Request
      Security Level: Public (Everyone can see)
    Affects Versions: 1.4.1.GA
            Reporter: Brian Stansberry
         Assigned To: Brian Stansberry
            Priority: Critical
             Fix For: 2.0.0.BETA1, 1.4.1.SP1


In 1.4.1.GA JBC introduced tighter locking semantics wherein the insertion or removal of a node required a write lock on the parent. In recent previous versions (and maybe always) only a read lock was required.

The 1.4.1.GA behavior provides greater correctness, but at a cost of lesser concurrency.  Some applications written assuming the old behavior will not function properly with the new behavior.

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.

This JIRA will restore the old behavior for the pessimistic locking use case. A new cache configuration flag LockParentForChildInsertRemove will be added.  If set to false, the 1.4.0.SP1 behavior of only requiring the RL will be restored.  The default value of the flag will be true.

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