Yes, it likely does mean there is inconsistency in the replicas. However,
you shouldn’t be too freaked out about it - Cassandra is design to allow
for this inconsistency to occur and the consistency levels allow you to
achieve consistent results despite replicas not being consistent. To keep
you replicas as consistent as possible (which is still a good thing), you
do need to regularly run repairs (once a week is the standard
recommendation for full repairs). Inconsistency can result from a whole
range of conditions from nodes being down the cluster being overloaded to
On Tue, 6 Mar 2018 at 22:18 Faraz Mateen <fmat...@an10.io> wrote:
> Thanks a lot for the response.
> Setting consistency to ALL/TWO started giving me consistent count results
> on both cqlsh and spark. As expected, my query time has increased by 1.5x (
> Before, it was taking ~1.6 hours but with consistency level ALL, same query
> is taking ~2.4 hours to complete.)
> Does this mean my replicas are out of sync? When I first started pushing
> data to cassandra, I had a single node setup. Then I added two more nodes,
> changed replication factor to 2 and ran nodetool repair to distribute data
> to all the nodes. So, according to my understanding the nodes should have
> passively replicated data among themselves to remain in sync.
> Do I need to run repairs repeatedly to keep data in sync?
> How can I further debug why my replicas were not in sync before?
> On Sun, Mar 4, 2018 at 9:46 AM, Ben Slater <ben.sla...@instaclustr.com>
>> Both CQLSH and the Spark Cassandra query at consistent level ONE
>> (LOCAL_ONE for Spark connector) by default so if there is any inconsistency
>> in your replicas this can resulting in inconsistent query results.
>> See http://cassandra.apache.org/doc/latest/tools/cqlsh.html and
>> info on how to chance consistency. If you are unsure of how consistent the
>> on-disk replicas are (eg if you have been writing at CL One or haven’t run
>> repaires) that using consistency level all should give you the most
>> consistent results but requires all replicas to be available for the query
>> to succeed. If you are using QUORUM for your writes then querying at QUORUM
>> or LOCAL_QUORUM as appropriate should give you consistent results.
>> On Sun, 4 Mar 2018 at 00:59 Kant Kodali <k...@peernova.com> wrote:
>>> The fact that cqlsh itself gives different results tells me that this
>>> has nothing to do with spark. Moreover, spark results are monotonically
>>> increasing which seem to be more consistent than cqlsh. so I believe
>>> spark can be taken out of the equation.
>>> Now, while you are running these queries is there another process or
>>> thread that is writing also at the same time ? If yes then your results are
>>> fine but If it's not, you may want to try nodetool flush first and then run
>>> these iterations again?
>>> On Fri, Mar 2, 2018 at 11:17 PM, Faraz Mateen <fmat...@an10.io> wrote:
>>>> Hi everyone,
>>>> I am trying to use spark to process a large cassandra table (~402
>>>> million entries and 84 columns) but I am getting inconsistent results.
>>>> Initially the requirement was to copy some columns from this table to
>>>> another table. After copying the data, I noticed that some entries in the
>>>> new table were missing. To verify that I took count of the large source
>>>> table but I am getting different values each time. I tried the queries on a
>>>> smaller table (~7 million records) and the results were fine.
>>>> Initially, I attempted to take count using pyspark. Here is my pyspark
>>>> spark = SparkSession.builder.appName("Datacopy App").getOrCreate()
>>>> df =
>>>> query = ("select count(1) from data " )
>>>> vgDF = spark.sql(query)
>>>> Spark submit command is as follows:
>>>> ~/spark-2.1.0-bin-hadoop2.7/bin/spark-submit --master
>>>> spark://10.128.0.18:7077 --packages
>>>> datastax:spark-cassandra-connector:2.0.1-s_2.11 --conf
>>>> spark.cassandra.connection.host="10.128.1.1,10.128.1.2,10.128.1.3" --conf
>>>> "spark.storage.memoryFraction=1" --conf spark.local.dir=/media/db/
>>>> --executor-memory 10G --num-executors=6 --executor-cores=2
>>>> --total-executor-cores 18 pyspark_script.py
>>>> The above spark submit process takes ~90 minutes to complete. I ran it
>>>> three times and here are the counts I got:
>>>> Spark iteration 1: 402273852
>>>> Spark iteration 2: 402273884
>>>> Spark iteration 3: 402274209
>>>> Spark does not show any error or exception during the entire process. I
>>>> ran the same query in cqlsh thrice and got different results again:
>>>> Cqlsh iteration 1: 402273598
>>>> Cqlsh iteration 2: 402273499
>>>> Cqlsh iteration 3: 402273515
>>>> I am unable to find out why I am getting different outcomes from the
>>>> same query. Cassandra system logs (*/var/log/cassandra/system.log*)
>>>> has shown the following error message just once:
>>>> ERROR [SSTableBatchOpen:3] 2018-02-27 09:48:23,592
>>>> CassandraDaemon.java:226 - Exception in thread
>>>> java.lang.AssertionError: Stats component is missing for sstable
>>>> at java.util.concurrent.FutureTask.run(FutureTask.java:266)
>>>> at java.lang.Thread.run(Thread.java:748) [na:1.8.0_131]
>>>> - Cassandra 3.9
>>>> - Spark 2.1.0
>>>> - Datastax's spark-cassandra-connector 2.0.1
>>>> - Scala version 2.11
>>>> - Spark setup with 3 workers and 1 master node.
>>>> - 3 worker nodes also have a cassandra cluster installed.
>>>> - Each worker node has 8 CPU cores and 40 GB RAM.
>>>> Any help will be greatly appreciated.
>> *Ben Slater*
>> *Chief Product Officer <https://www.instaclustr.com/>*
>> Read our latest technical blog posts here
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*Chief Product Officer <https://www.instaclustr.com/>*
Read our latest technical blog posts here
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