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From Jonathan Ellis <jbel...@gmail.com>
Subject Re: Confused about consistency
Date Sat, 04 Dec 2010 05:03:23 GMT
I think you are running into
https://issues.apache.org/jira/browse/CASSANDRA-1316, where when an
inconsistency on QUORUM/ALL is discovered it always peformed the
repair at QUORUM instead of the original CL.  Thus, reading at ALL you
would see the correct answer on the 2nd read but you weren't
guaranteed to see it on the first.

This was fixed in 0.6.4 but apparently I botched the merge to the 0.7
branch.  I corrected that just now, so when you update, you should be
good to go.

On Fri, Dec 3, 2010 at 9:19 PM, Dan Hendry <dan.hendry.junk@gmail.com> wrote:
> I am seeing fairly strange, behavior in my Cassandra cluster.
> Setup
>  - 3 nodes (lets call them nodes 1 2 and 3)
>  - RF=2
>  - A set of servers (producers) which which write data to the cluster at
> consistency level ONE
>  - A set of servers (consumers/processors) which read data from the cluster
> at consistency level ALL
>  - Cassandra 0.7 (recent out of the svn branch, post beta 3)
>  - Clients use the pelops library
> Situation:
>  - Everything is humming along nicely
>  - A Cassandra node (say 3) goes down (even with 24 GB of ram, OOM errors
> are the bain of my existence)
>  - Producers continue to happily write to the cluster but consumers start
> complaining by throwing TimeOutExceptions and UnavailableExceptions.
>  - I stagger out of bed in the middle of the night and restart Cassandra on
> node 3.
>  - The consumers stop complaining and get back to business but generate
> garbage data for the period node 3 was down. Its almost like half the data
> is missing half the time. (Again, I am reading at consistency level ALL).
>  - I force the consumers to reprocess data for the period node 3 was down.
> They generate accurate output which is different from the first time round.
> To be explicit, what seems to be happening is first read at consistency ALL
> gives "A,C,E" (for example) and the second read at consistency level ALL
> gives "A,B,C,D,E". Is this a Cassandra bug? Is my knowledge of consistency
> levels flawed? My understanding is that you could achieve strongly
> consistent behavior by writing at ONE and reading at ALL.
> After this experience, my theory (uneducated, untested, and
> under-researched) is that "strong consistency" applies only to column
> values, not the set of columns (or super-columns in this case) which make up
> a row. Any thoughts?



-- 
Jonathan Ellis
Project Chair, Apache Cassandra
co-founder of Riptano, the source for professional Cassandra support
http://riptano.com

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