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

[Jason T. Greene]

Manik Surtani wrote:
> 
> On 3 Jul 2008, at 21:21, Jason T. Greene wrote:
> 
>> 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.
> 
> I guess that approach can still be taken.  Just that then it introduces 
> the potential for phantom reads (nodes appearing in the child set of a 
> parent), but by that stage lockParent can be a cfg option to prevent that.

Right, but phantoms are still allowed by RR, and locking the parent can 
significantly reduce performance. As an example, in POJO Cache, a list 
is mapped as a root node, and its child elements are child nodes. 
Locking the parent effectively serializes concurrent list adds. This is 
especially a problem if the TX is not short-lived. Now it could be 
possible to change the POJO Cache list impl to use hash branches in the 
fqn to stripe the parent locking (this is what the internal area 
currently does), however, this is stll less optimal than having no 
parent locking.

I think the real desire for locking the parent is for doing certain 
atomic operations, like those on ConcurrentMap. However we could provide 
a special API for such things, and in it's absence the user can still do 
a cooperative lock on the parent with an update.

-- 
Jason T. Greene
JBoss, a division of Red Hat