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From Bhaskar Muppana <mgvbhas...@gmail.com>
Subject Re: Bug in paging reads and limit reads (short reads!)
Date Wed, 02 Nov 2016 21:58:05 GMT
Thanks Benjamin! I am also hoping to ask couple of questions on difference
in this code base between 2.0, 2.1 and 3.0. I understand 2.0 is closed from
community point of view. But, we are still using in some cases, so we need
to fix it internally.

With my limited understanding of the code, it looks like to me there is no
protection against short reads in 2.0. But in 3.0, we seem to have some
short read transformation. At least, from comments it feels like that code
should have taken care of cases like this. So this must be a bug in 3.0
code, where as in 2.0 the complete feature for short read protection is
missing. Is my understanding right? Can you please shed more light on this?

I have created a JIRA as you asked -
https://issues.apache.org/jira/browse/CASSANDRA-12872

This is my first JIRA, so I don't know conventions to set priority. As its
data inconsistency issue, I believe this is critical. Feel free to change
the priority according to community conventions.

Thanks,
Bhaskar

On Wed, Nov 2, 2016 at 1:26 AM, Benjamin Lerer <benjamin.lerer@datastax.com>
wrote:

> Hi Bhaskar,
>
> Thanks for reporting that problem. It is a nice catch :-)
>
> Could you open a JIRA ticket with all the information that you provided?
>
> I will try to fix that problem.
>
> Benjamin
>
>
> On Wed, Nov 2, 2016 at 12:00 AM, Bhaskar Muppana <mgvbhaskar@gmail.com>
> wrote:
>
> > Hi Guys,
> >
> > We are seeing an issue with paging reads missing some small number of
> > columns when we do paging/limit reads. We get this on a single DC cluster
> > itself when both reads and writes are happening with QUORUM. Paging/limit
> > reads see this issue. I have attached the ccm based script which
> reproduces
> > the problem.
> >
> > * Keyspace RF - 2
> > * Table (id int, course text, marks int, primary key(id, course))
> > * replicas for partition key 1 - r1, r2 and r3
> > * insert (1, '1', 1) ,  (1, '2', 2),  (1, '3', 3),  (1, '4', 4),  (1,
> '5',
> > 5) - succeeded on all 3 replicas
> > * insert (1, '6', 6) succeeded on r1 and r3, failed on r2
> > * delete (1, '2'), (1, '3'), (1, '4'), (1, '5') succeeded on r1 and r2,
> > failed on r3
> > * insert (1, '7', 7) succeeded on r1 and r2, failed on r3
> >
> > Local data on 3 nodes looks like as below now
> >
> > r1: (1, '1', 1), tombstone(2-5 records), (1, '6', 6), (1, '7', 7)
> > r2: (1, '1', 1), tombstone(2-5 records), (1, '7', 7)
> > r3: (1, '1', 1),  (1, '2', 2),  (1, '3', 3),  (1, '4', 4),  (1, '5',
> > 5), (1, '6', 6)
> >
> > If we do a paging read with page_size 2, and if it gets data from r2 and
> > r3, then it will only get the data (1, '1', 1) and (1, '7', 7) skipping
> > record 6. This problem would happen if the same query is not doing paging
> > but limit set to 2 records.
> >
> > Resolution code for reads works same for paging queries and normal
> > queries. Co-ordinator shouldn't respond back to client with
> records/columns
> > that it didn't have complete visibility on all required replicas (in this
> > case 2 replicas). In above case, it is sending back record (1, '7', 7)
> back
> > to client, but its visibility on r3 is limited up to (1, '2', 2) and it
> is
> > relying on just r2 data to assume (1, '6', 6) doesn't exist, which is
> > wrong. End of the resolution all it can conclusively say any thing about
> is
> > (1, '1', 1), which exists and (1, '2', 2), which is deleted.
> >
> > Ideally we should have different resolution implementation for
> > paging/limit queries.
> >
> > We could reproduce this on 2.0.17, 2.1.16 and 3.0.9.
> >
> > Seems like 3.0.9 we have ShortReadProtection transformation on list
> > queries. I assume that is to protect against the cases like above. But,
> we
> > can reproduce the issue in 3.0.9 as well.
> >
> > Thanks,
> > Bhaskar
> >
> >
> >
> >
> >
> >
> >
>

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