Re: [jbosscache-dev] Locking parents for insertion and removal with MVCC

Manik Surtani wrote: > Now with MVCC - since there are no read locks - we have a problem. > Consider: > 1. Tx begns. > 2. Tx reads /a (no locks) > 3. Tx removes /a/b (locks /a/b) > 4. Tx counts children of /a (will still see that /a/b is in /a's > child map). > he approaches I have considered are: > 1. Removing /a/b from /a's child map. > This will provide a consistent view to the tx removing /a/b but > will mean that other txs will also see /a/b disappear when > counting /a's children, but not when querying the cache for /a/b! > Worse, rollbacks would mean re-adding /a/b to /a's child map, > providing weird views on other readers (/a/b disappears for a > while, then reappears) > 2. Make a copy of /a for the tx to work off, when removing /a/b. > This is the same as having LockParentForChildInsertRemove semantics. > Copy the parent as well and work off it, but to prevent problems > with concurrent child removes and adds, we'd have to lock the parent. > My vote is for approach 2. In fact, LockParentForChildInsertRemove > would always need to be enabled when using MVCC. Perhaps this > should be a deprecated property that just supports additional > consistency for OL and PL, and be removed when OL/PL eventually get > removed? > Thoughts? Why not treat a delete like a modify (as I think it has been done in the past)? When a node is deleted, it is locked, and either a sentinel or just an fqn is stored in the TX context that issued the delete. Then the parent still conains the original node, allowing concurrent readers, while the calling tx knows to filter deleted nodes from child operations.