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From Jeff <jeff.la...@gmail.com>
Subject Does the "Query Consistency" feature use MVCC?
Date Fri, 07 Jul 2017 18:36:43 GMT
Howdy!

I've read through the docs at https://apacheignite.readme.io but couldn't
find a definitive answer (even started diving into the source, but figured
I'd try here before going further).  The following is listed as a feature on
the Apache Ignite Datagrid website:

"Ignite provides full query consistency. Updates that happened after the
query execution started do not affect the query result."

>From the docs and JIRA ticket
(https://issues.apache.org/jira/browse/IGNITE-3478), it appears that SQL
queries do not yet support MVCC in relation to DataGrid Transactions (e.g.
If writing A and B in a transaction, able to get new A value but old B
value).

So, for getAll(), ScanQuery, etc. how is this supported?  Is it only when
the cache is set to TRANSACTIONAL?  I tried a quick test which modifies an
object of an ATOMIC cache during the SQL cursor iteration (and small page
size), but the previous value was returned.  Though I'm not sure if there is
an optimization/minimum value on the page size.

Fundamentally, my question is under what conditions (atomicity mode, query
type, etc.) is the query consistency guaranteed and how does Ignite ensure
this (e.g. MVCC, local cache for ongoing queries, etc.)

Thanks,
Jeff





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