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From "Myron A. Semack" <>
Subject RE: Re[6]: Modify keyspace replication strategy and rebalance the nodes
Date Mon, 18 Sep 2017 20:57:24 GMT
How would setting the consistency to ALL help?  Wouldn’t that just cause EVERY read/write
to fail after the ALTER until the repair is complete?

Myron A. Semack

From: Jeff Jirsa []
Sent: Monday, September 18, 2017 2:42 PM
Subject: Re: Re[6]: Modify keyspace replication strategy and rebalance the nodes

The hard part here is nobody's going to be able to tell you exactly what's involved in fixing
this because nobody sees your ring

And since you're using vnodes and have a nontrivial number of instances, sharing that ring
(and doing anything actionable with it) is nontrivial.

If you weren't using vnodes, you could just fix the distribution and decom extra nodes afterward.

I thought - but don't have time or energy to check - that the ec2snitch would be rack aware
even when using simple strategy - if that's not the case (as you seem to indicate), then you're
in a weird spot - you can't go to NTS trivially because doing so will reassign your replicas
to be rack/as aware, certainly violating your consistency guarantees.

If you can change your app to temporarily write with ALL and read with ALL, and then run repair,
then immediately ALTER the keyspace, then run repair again, then drop back to whatever consistency
you're using, you can probably get through it. The challenge is that ALL gets painful if you
lose any instance.

But please test in a lab, and note that this is inherently dangerous, I'm not advising you
to do it, though I do believe it can be made to work.

Jeff Jirsa

On Sep 18, 2017, at 11:18 AM, Dominik Petrovic <<>>
@jeff what do you think is the best approach here to fix this problem?
Thank you all for helping me.

Thursday, September 14, 2017 3:28 PM -07:00 from kurt greaves <<>>:
Sorry that only applies our you're using NTS. You're right that simple strategy won't work
very well in this case. To migrate you'll likely need to do a DC migration to ensuite no downtime,
as replica placement will change even if RF stays the same.

On 15 Sep. 2017 08:26, "kurt greaves" <<>>
If you have racks configured and lose nodes you should replace the node with one from the
same rack. You then need to repair, and definitely don't decommission until you do.

Also 40 nodes with 256 vnodes is not a fun time for repair.

On 15 Sep. 2017 03:36, "Dominik Petrovic" <<>.invalid>
I'm using 3 availability zones, during the life of the cluster we lost nodes, retired others
and we end up having some of the data written/replicated on a single availability zone. We
saw it with nodetool getendpoints.

Thursday, September 14, 2017 9:23 AM -07:00 from Jeff Jirsa <<>>:
With one datacenter/region, what did you discover in an outage you think you'll solve with
network topology strategy? It should be equivalent for a single D.C.

Jeff Jirsa

On Sep 14, 2017, at 8:47 AM, Dominik Petrovic <<>>
Thank you for the replies!

@jeff my current cluster details are:
1 datacenter
40 nodes, with vnodes=256
What is your advice? is it a production cluster, so I need to be very careful about it.

Thu, 14 Sep 2017 -2:47:52 -0700 from Jeff Jirsa <<>>:
The token distribution isn't going to change - the way Cassandra maps replicas will change.

How many data centers/regions will you have when you're done? What's your RF now? You definitely
need to run repair before you ALTER, but you've got a bit of a race here between the repairs
and the ALTER, which you MAY be able to work around if we know more about your cluster.

How many nodes
How many regions
How many replicas per region when you're done?

Jeff Jirsa

On Sep 13, 2017, at 2:04 PM, Dominik Petrovic <<>>
Dear community,
I'd like to receive additional info on how to modify a keyspace replication strategy.

My Cassandra cluster is on AWS, Cassandra 2.1.15 using vnodes, the cluster's snitch is configured
to Ec2Snitch, but the keyspace the developers created has replication class SimpleStrategy
= 3.

During an outage last week we realized the discrepancy between the configuration and we would
now fix the issue using NetworkTopologyStrategy.

What are the suggested steps to perform?
For Cassandra 2.1 I found only this doc:
that does not mention anything about repairing the cluster

For Cassandra 3 I found this other doc:
That involves also the cluster repair operation.

On a test cluster I tried the steps for Cassandra 2.1 but the token distribution in the ring
didn't change so I'm assuming that wasn't the right think to do.
I also perform a nodetool repair -pr but nothing changed as well.
Some advice?

Dominik Petrovic

Dominik Petrovic

Dominik Petrovic

Dominik Petrovic
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