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From Sergey Naumov <sknau...@gmail.com>
Subject update does not apply to any replica if consistency = ALL and one replica is down
Date Fri, 17 May 2013 08:48:00 GMT
As described here (
http://maxgrinev.com/2010/07/12/update-idempotency-why-it-is-important-in-cassandra-applications-2/),
if consistency level couldn't be met, updates are applied anyway on
functional replicas, and they could be propagated later to other replicas
using repair mechanisms or by issuing the same request later, as update
operations are idempotent in Cassandra.

But... on my configuration (Cassandra 1.2.4, python CQL 1.0.4, DC1 - 3
nodes, DC2 - 3 nodes, DC3 - 1 node, RF={DC1:3, DC2:2, DC3:1}, Random
Partitioner, GossipingPropertyFileSnitch, one node in DC1 is deliberately
down - and, as RF for DC1 is 3, this down node is a replica node for 100%
of records),  when I try to insert one record with consistency level of
ALL, this insert does not appear on any replica (-s30 - is a serial of
UUID1: 0000001e-0000-1000-0000-xxxxxxxxxxxxx (30 is 1e in hex), -n1 mean
that we will insert/update a single record with first id from this series -
0000001e-0000-1000-0000-000000000000):
*write with consistency ALL:*
cassandra@host11:~/Cassandra$ ./insert.sh -s30 -n1 -cALL
Traceback (most recent call last):
  File "./aux/fastinsert.py", line 54, in insert
    curs.execute(cmd, consistency_level=p.conlvl)
OperationalError: Unable to complete request: one or more nodes were
unavailable.
Last record UUID is 0000001e-0000-1000-0000-000000000000*

*
about 10 seconds passed...
*
read with consistency ONE:*
cassandra@host11:~/Cassandra$ ./select.sh -s30 -n1 -cONE
Total records read: *0*
Last record UUID is 0000001e-0000-1000-0000-000000000000
*read with consistency QUORUM:*
cassandra@host11:~/Cassandra$ ./select.sh -s30 -n1 -cQUORUM
Total records read: *0*
Last record UUID is 0000001e-0000-1000-0000-000000000000
*write with consistency QUORUM:*
cassandra@host11:~/Cassandra$ ./insert.sh -s30 -n1 -cQUORUM
Last record UUID is 0000001e-0000-1000-0000-000000000000
*read with consistency QUORUM:*
cassandra@host11:~/Cassandra$ ./select.sh -s30 -n1 -cQUORUM
Total records read: *1*
Last record UUID is 0000001e-0000-1000-0000-000000000000

Is it a new feature of Cassandra that it does not perform a write to any
replica if consistency couldn't be satisfied? If so, then is it true for
all cases, for example when returned error is "OperationalError: Request
did not complete within rpc_timeout"?

Thanks in advance,
Sergey Naumov.

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