Hazelcast - PartitionGroup + Multiple Backups

Question

Assuming 4 nodes split across 2 data centers (DC1-1, DC1-2, DC2-1, DC2-2).

Using partition groups and the default backup count of 1, the documentation and other questions/articles are pretty clear about how data is distributed assuming well distributed data - 25% per node as primary, all the primary data in DC1-1/DC1-2 will be backed up on either DC2-1/DC2-2 and vice versa.

It is not clear what the expected behavior is under same situation if we were to increase backup count to 2. Assuming entry #1 currently as primary on DC1-1. Would the two entries of backup both be forced to the two DC2 nodes? Is there a way to make it such that there is one backup in each partitiongroup? (i.e. primary DC1-1, backup on DC1-2, backup on either DC2-1 or DC2-2)?

Thanks

Answer 1

First of all we do not recommend to split a single cluster over multiple data centers. There are possible exceptions but keep in mind that latency between data centers is important as you partition the data.

To your question: If you have just two partition groups defined there is no way to create more than one backup. You have to imagine a normal cluster to be one node per partition group, therefore you can have pG-1 backups. If you change the configuration to 2 partition groups that means you can only have one backup.