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From Anuj Wadehra <>
Subject Re: Replication of data over 2 Datacentre's, when one node fails we get replica issues
Date Wed, 18 Nov 2015 16:41:07 GMT
Hi Walsh,

My comments:

1. Keeping RF at 2 and CL at LOCAL_QUORUM would not give you any additional fault tolerance.
You wont be able to afford a single node failure with RF=2. I would suggest keeping it at
3 so that you can tolerate a single node failure.

Your query failed because RF=2 and not because the query went to other DC.

2. If you fire a query at LOCAL_QUORUM, it will only read data from local dc ( dc where it
was fired). It will never go to other DC unless there is a digest mismatch in local dc and
read_repair_chance >0.

For completely preventing queries to go in other dc ( even in case of digest mismatch), you
can set read_repair_chance=0 and instead set dc_local_read_repair_chance>0 to make sure
blocking  read repair happens in local dc.



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From:"Walsh, Stephen" <>
Date:Wed, 18 Nov, 2015 at 9:52 pm
Subject:Replication of data over 2 Datacentre's, when one node fails we get replica issues

Hey all,


We’re testing Cassandra failover over 2 Datacentre’s.


There are 3 nodes on each.

All CF’s have a Replication of 2 on both Datacentre’s (DC1:2, DC2:2)


When one Datacentre goes down then all queries go to the other.

This works fine for LOCAL_QUOURM queries. As 2 replicas of the data exist in this Datacentre.


However in the scenario where the 2 Datacentre’s are up and one node goes down, all queries
to that Datacentre will fail for LOCAL_QUOURM

This is because the node that failed had the replica data, and there is only 1 remaining node
with the data. So LOCAL_QUOURM which requires 2 nodes with replica data will fail


Is there a way to not send these queries to the incomplete Datacentre.

What the best way the handle this?




Stephen Walsh

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