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From "Walsh, Stephen" <>
Subject Consistency Issues
Date Wed, 30 Sep 2015 14:16:35 GMT
Hi there,

We are having some issues with consistency. I'll try my best to explain.

We have an application that was able to
Write ~1000 p/s
Read ~300 p/s
Total CF created: 400
Total Keyspaces created : 80

On a 4 node Cassandra Cluster with
Version 2.1.6
Replication : 3
Consistency  (Read & Write) : LOCAL_QUORUM
Cores : 4
Ram : 15 GB
Heap Size 8GB

This was fine and worked, but was pushing our application to the max.


Next we added a load balancer (HaProxy) to our application.
So now we have 3 of our nodes talking to 4 Cassandra Nodes with a load of
Write ~1250 p/s
Read 0p/s
Total CF created: 450
Total Keyspaces created : 100

On our application we now see
Cassandra timeout during write query at consistency LOCAL_QUORUM (2 replica were required
but only 1 acknowledged the write)
(we are using java Cassandra driver 2.1.6)

So we increased the number of Cassandra nodes
To 5, then 6  and each time got the same replication error.

So then we double the spec of every node to
8 cores
Heap size 15GB

And we still get this replication error (2 replica were required but only 1 acknowledged the

We know that when we introduce HaProxy Load balancer with 3 of our nodes that its hits Cassandra
3 times quicker.
But we've now increased the Cassandra spec nearly 3 fold, and only for an extra 250 writes
p/s and it still doesn't work.

We're having a hard time finding out why replication is an issue with the size of a cluster.

We tried to get OpsCenter working to monitor the nodes, but due to the amount of CF's in Cassandra
the datastax-agent takes 90% of the CPU on every node.

Any suggestion / recommendation would be very welcome.

Stephen Walsh

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