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From Chris Burroughs <chris.burrou...@gmail.com>
Subject CL.ONE reads / RR / badness_threshold interaction
Date Wed, 06 Apr 2011 20:00:55 GMT
My understanding for For CL.ONE.  For the node that receives the request:

(A) If RR is enabled and this node contains the needed row --> return
immediately and do RR to remaining replicas in background.
(B) If RR is off and this node contains the needed row --> return the
needed data immediately.
(C) If this node does not have the needed row --> regardless of RR ask
all replicas and return the first result.


However case (C) as I have described it does not allow for any notion of
'pinning' as mentioned for dynamic_snitch_badness_threshold:

# if set greater than zero and read_repair_chance is < 1.0, this will allow
# 'pinning' of replicas to hosts in order to increase cache capacity.
# The badness threshold will control how much worse the pinned host has
to be
# before the dynamic snitch will prefer other replicas over it.  This is
# expressed as a double which represents a percentage.  Thus, a value of
# 0.2 means Cassandra would continue to prefer the static snitch values
# until the pinned host was 20% worse than the fastest.


The wiki states CL.ONE "Will return the record returned by the first
replica to respond" [1] implying that the request goes to multiple
replicas, but datastax's docs state that only one node will receive the
request ("Returns the response from *the* closest replica, as determined
by the snitch configured for the cluster" [2]).

Could someone clarify how CL.ONE reads with RR off work?


[1] http://wiki.apache.org/cassandra/API
[2]
http://www.datastax.com/docs/0.7/consistency/index#choosing-consistency-levels
 emphasis added

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