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From Richard Dawe <rich.d...@messagesystems.com>
Subject Re: Should replica placement change after a topology change?
Date Fri, 11 Sep 2015 14:24:20 GMT
Thanks, Nate and Rob. We are going to have to migrate some installations from SimpleSnitch
to Ec2Snitch, others to GossipingPropertyFileSnitch. Your help is much appreciated!

Best regards, Rich

On 10/09/2015 20:33, "Nate McCall" <nate@thelastpickle.com<mailto:nate@thelastpickle.com>>
wrote:


So if you have a topology that would change if you switched from SimpleStrategy to NetworkTopologyStrategy
plus multiple racks, it sounds like a different migration strategy would be needed?

I am imagining:

  1.  Switch to a different snitch, and the keyspace from SimpleStrategy to NTS but keep it
all in one rack. So effectively the same topology, but with a different snitch.
  2.  Set up a new data centre with the desired topology.
  3.  Change the keyspace to have replicas in the new DC.
  4.  Rebuild all the nodes in the new DC.
  5.  Flip all your clients over to the new DC.
  6.  Decommission your original DC.

That would work, yes. I would add :

- 4.5. Repair all nodes.

I can confirm that the above process works (definitely include Rob's repair suggestion, though).
It is really the only way we've found to safely go from SimpleSnitch to rack-aware NTS.

The same process works/is required for SimpleSnitch to Ec2Snitch fwiw.


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