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From Denis Magda <dma...@apache.org>
Subject Re: Forbid mixed cache groups with both atomic and transactional caches
Date Tue, 04 Feb 2020 19:42:49 GMT
+1 from my end. It doesn't sound like a big deal if Ignite users need to
define separate groups for atomic and transactional caches.

-
Denis


On Tue, Feb 4, 2020 at 3:28 AM Ivan Rakov <ivan.glukos@gmail.com> wrote:

> Igniters,
>
> Apparently it's possible in Ignite to configure a cache group with both
> ATOMIC and TRANSACTIONAL caches.
> Proof: IgniteCacheGroupsTest#testContinuousQueriesMultipleGroups* tests.
> In my opinion, it would be better to remove such possibility from the
> product. There are several reasons:
>
> 1) The original idea of grouping caches was optimizing storage overhead and
> PME time by joining data of similar caches into the same partitions. ATOMIC
> and TRANSACTIONAL caches provide different guarantees and are designed for
> different use cases, thus they can hardly be called "similar".
>
> 2) Diving deeper: synchronization protocols and possible reasons for
> primary-backup divergences are conceptually different for ATOMIC and
> TRANSACTIONAL cases. In TRANSACTIONAL case, transactions recovery protocol
> allows to recover consistency if any participating node will fail, but for
> ATOMIC caches there's possible scenario with failure of primary node where
> neither of backups will contain the most recent state of the data. Example:
> one backup have received updates 1, 3, 5 while another have received 2, 4
> (which is possible due to message reordering), and even tracking counters
> [1] won't restore the consistency. The problem is that we can't distinguish
> what kind of conflict we have faced in case update counters have diverged
> in a mixed group.
>
> 3) Mixed groups are poorly tested. I can't find any tests except a couple
> of smoke tests in IgniteCacheGroupsTest. We can't be sure that different
> synchronization protocols will work correctly for such configurations,
> especially under load and with a variety of dependent configuration
> parameters.
>
> 4) I have never heard of any feedback on mixed groups. I have asked
> different people on this and no one recalled any attempts to configure such
> groups. I believe that in fact no one has ever tried to do it.
>
> Please let me know if you are aware of any cases where mixed groups are
> used or reasons to keep them. Otherwise I'll create a ticket to prohibit
> mixed configurations.
>
> [1]: https://issues.apache.org/jira/browse/IGNITE-11797
>
> --
> Best Regards,
> Ivan Rakov
>

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