[jira] [Created] (CURATOR-294) Optimize TreeCache, fix possible concurrency issue

["Scott Blum (JIRA)" <ji...@apache.org>]

Scott Blum created CURATOR-294:
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             Summary: Optimize TreeCache, fix possible concurrency issue
                 Key: CURATOR-294
                 URL: https://issues.apache.org/jira/browse/CURATOR-294
             Project: Apache Curator
          Issue Type: Improvement
          Components: Recipes
            Reporter: Nick Hill
            Assignee: Scott Blum


Hi, I have been looking at the TreeCache impl and have some questions.

It doesn't look right to me that there's separate atomic refs for a node's data and stat. It seems the stat in a ChildData object obtained from getCurrentData() might not correspond to the data that it's with. This could be problematic when doing conditional state changes given assumptions about that data.

An obvious and simple solution to this would be to have a single AtomicReference<ChildData> field instead, which would have the additional significant benefit of eliminating ChildData obj creation on every cache access. PathChildrenCache works this way, but my understanding was that TreeCache is intended to be a (more flexible) replacement.

Furthermore I'd propose that the data field of ChildData be just a final byte[] instead of an AtomicReference. This would avoid needing two volatile reads to get to the data, and mean that "sharing" these (per above) is a bit safer. The ChildData byte[] AtomicReference is only used by PathChildrenCache.clearDataBytes() (not currently used by TreeCache at all), and that capability could be easily maintained by having PathChildrenCache use it's own simple subclass of ChildData containing the atomic reference.
If similar capability were to be added to TreeCache, I'd suggest it would be better to just replace the node's ChildData object with a copy that has the byte[] field nulled out (but same stat ref).

I'm fairly new to the code so apologies if there's something I've missed/misunderstood! But if there is agreement, I'd also be happy to prepare a PR.

Regards,
Nick



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