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From "Aleksey Yeschenko (JIRA)" <j...@apache.org>
Subject [jira] [Updated] (CASSANDRA-13747) Fix short read protection
Date Thu, 17 Aug 2017 12:31:02 GMT

     [ https://issues.apache.org/jira/browse/CASSANDRA-13747?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Aleksey Yeschenko updated CASSANDRA-13747:
------------------------------------------
    Status: Open  (was: Patch Available)

> Fix short read protection
> -------------------------
>
>                 Key: CASSANDRA-13747
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-13747
>             Project: Cassandra
>          Issue Type: Bug
>          Components: Coordination
>            Reporter: Aleksey Yeschenko
>            Assignee: Aleksey Yeschenko
>             Fix For: 3.0.x, 3.11.x
>
>
> {{ShortReadRowProtection.moreContents()}} expects that by the time we get to that method,
the global post-reconciliation counter was already applied to the current partition. However,
sometimes it won’t get applied, and the global counter continues counting with {{rowInCurrentPartition}}
value not reset from previous partition, which in the most obvious case would trigger the
assertion we are observing - {{assert !postReconciliationCounter.isDoneForPartition();}}.
In other cases it’s possible because of this lack of reset to query a node for too few extra
rows, causing unnecessary SRP data requests.
> Why is the counter not always applied to the current partition?
> The merged {{PartitionIterator}} returned from {{DataResolver.resolve()}} has two transformations
applied to it, in the following order:
> {{Filter}} - to purge non-live data from partitions, and to discard empty partitions
altogether (except for Thrift)
> {{Counter}}, to count and stop iteration
> Problem is, {{Filter}} ’s {{applyToPartition()}} code that discards empty partitions
({{closeIfEmpty()}} method) would sometimes consume the iterator, triggering short read protection
*before* {{Counter}} ’s {{applyToPartition()}} gets called and resets its {{rowInCurrentPartition}}
sub-counter.
> We should not be consuming iterators until all transformations are applied to them. For
transformations it means that they cannot consume iterators unless they are the last transformation
on the stack.
> The linked branch fixes the problem by splitting {{Filter}} into two transformations.
The original - {{Filter}} - that does filtering within partitions - and a separate {{EmptyPartitionsDiscarder}},
that discards empty partitions from {{PartitionIterators}}. Thus {{DataResolve.resolve()}},
when constructing its {{PartitionIterator}}, now does merge first, then applies {{Filter}},
then {{Counter}}, and only then, as its last (third) transformation - the {{EmptyPartitionsDiscarder}}.
Being the last one applied, it’s legal for it to consume the iterator, and triggering {{moreContents()}}
is now no longer a problem.
> Fixes: [3.0|https://github.com/iamaleksey/cassandra/commits/13747-3.0], [3.11|https://github.com/iamaleksey/cassandra/commits/13747-3.11],
[4.0|https://github.com/iamaleksey/cassandra/commits/13747-4.0]. dtest [here|https://github.com/iamaleksey/cassandra-dtest/commits/13747].



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