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From "Igor Seliverstov (JIRA)" <j...@apache.org>
Subject [jira] [Created] (IGNITE-11173) MVCC TX: Acquire write version if coordinator changed in the middle of tx.
Date Fri, 01 Feb 2019 12:06:00 GMT
Igor Seliverstov created IGNITE-11173:

             Summary: MVCC TX: Acquire write version if coordinator changed in the middle
of tx.
                 Key: IGNITE-11173
                 URL: https://issues.apache.org/jira/browse/IGNITE-11173
             Project: Ignite
          Issue Type: New Feature
          Components: mvcc
            Reporter: Igor Seliverstov

Current implementation suppose all transactions on previous coordinator were finished before
new coordinator has been initialized, but there is a case when it isn't true. 

A tx which hasn't obtained a topology lock (hasn't done any updates) doesn't prevent a topology
change, this way nothing prevent new transactions start.

All newly created txs suppose all updates on previous coordinator are finished - this mean
a possible update of mapped on previous coordinator txs is considered as stale regardless
the tx is still active. We make such txs readonly to not break the visibility checking logic
(see IGNITE-8841).

There are two possible ways to allow writes after coordinator is changed:
* collect active txs and their dependencies and populate the active txs list with them during
a new coordinator initialization - this approach is quite complex and have obvious issues
like necessity of using full tx versions in a snapshot
* acquire a new version right after topology locked for writes using previously acquired "read"
version for possible write conflicts detecting and visibility checking.

I would prefer the second option as fully understandable and relatively easy to implement.

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