How to design a grid to withstand a partial network failure

Hi,
We are evaluating Coherence for a mission-critical system where we want to test partial network failure scenario. We want to run 4 physical hosts, 8 JVMs with 2 JVM on each host. The evaluation criteria is to connect 2 machines on either side of a router, kill one side during a load test, thereby disconnecting the 2 machines and run with the remaining two. In order to have a fail-safe behavior in this scenario, I guess we must ascertain that the back-ups for the objects on one side of a router are always made on the other side. Can Coherence detect such a network set up and store backups accordingly? Or is there a way to configure this by overriding the default behavior?
Pls advise
Thanks,
Sairam

Hi Sairam
If you use scenario 1) then your test will work. As this scenario only has two machines then the primary node for a piece of data will be on one machine and Coherence will make sure the backup is on the other machine. If you then break the link between the machines or lose a machine you will not have lost data.
If however you have more than 2 machines then you break the link between them you have what is known as a split-brain - which means you have effectively split your cluster in two. Both sides only know they they can no longer see the other part of the cluster and assume they are must be the remaining working part. In this case though you will have lost data from the cluster as some of the backups for each part of the cluster will be on the other part. There is nothing you can do about this, you cannot control which machines backups are allocated to.
Increasing the backup count to 3 does not give you any more reliance than having a backup count of 2. As far as I know Coherence only guarantees that the placement of the first backup is on another machine.
I am not quite sure what you are trying to test as a Coherence cluster cannot automatically survive a network failure that splits the cluster. There are things in 3.6 that you might be able to do with Quorums to mitigate the damage while you recover and there are things you can do to make recovery easier - but you will have to recover lost data.
JK

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