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From Robert Wille <rwi...@fold3.com>
Subject Re: Duplicate records returned
Date Sat, 03 Oct 2015 16:54:58 GMT
I don’t think its an application problem. The following simple snippets produce different
totals:

ResultSet rs = cassandraUtil.executeNamedQuery(Q_LIST_IMAGES, rollId);

int total = 0;
for (Row row : rs)
{
total++;
}

---------------------

ResultSet rs = cassandraUtil.executeNamedQuery(Q_LIST_IMAGES, rollId);

int total = 0;
long lastImageId = -1;
for (Row row : rs)
{
long imageId = row.getLong(PARAM_IMAGE_ID);

total++;
lastImageId = imageId;
}

This doesn’t happen for all partitions. In fact most don’t have this problem (maybe 20%
do this). But the ones that do repeat records, do so deterministically. I see this problem
in multiple tables.

I’m only retrieving the clustering key, so it has nothing to do with the data field.

I suspect this is a paging problem, and might be a driver issue.

Robert

On Oct 3, 2015, at 9:29 AM, Eric Stevens <mightye@gmail.com<mailto:mightye@gmail.com>>
wrote:

Can you give us an example of the duplicate records that comes back?  How reliable is it (i.e.
is it every record, is it one record per read, etc)?  By any chance is it just the `data`
field that duplicates while the other fields change per row?

> I don’t see duplicates in cqlsh.

I've never seen this, and I can't think of a failure mode which would cause it to happen.
 Not to say it's impossible, but Cassandra's standard read path involves collapsing duplicate
or otherwise overlapping answers from multiple replicas; such a thing would be a pretty substantial
deviation.  Especially since you don't see the duplicates in cqlsh, I have a hunch this is
an application bug.


On Fri, Oct 2, 2015 at 4:58 PM Robert Wille <rwille@fold3.com<mailto:rwille@fold3.com>>
wrote:
When I run the query "SELECT image FROM roll WHERE roll = :roll“ against this table

CREATE TABLE roll (
roll INT,
image BIGINT,
data VARCHAR static,
mid VARCHAR,
imp_st VARCHAR,
PRIMARY KEY ((roll), image)
) WITH gc_grace_seconds = 3456000 AND compaction = { 'class' : 'LeveledCompactionStrategy',
'sstable_size_in_mb' : 160 };

I often get duplicate records back. Seems like a very simple query to botch. I’m running
2.0.16 with RF=3 and CL=QUORUM and Java client 2.0.10.1. I don’t see duplicates in cqlsh.
Any thoughts?

Thanks

Robert



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