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From Ivan Rakov <ivan.glu...@gmail.com>
Subject Forbid mixed cache groups with both atomic and transactional caches
Date Tue, 04 Feb 2020 11:27:57 GMT
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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