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reading about it online, it appears we can use both Galera replication and native MariaDB/MySQL replication together. Can anyone confirm that this is correct?

I'm thinking of a scenario where Data Center A has 3 Galera nodes synchronously replicating with each other, and Data Center B also has 3 Galera nodes synchronously replicating with each other. Then between one node in Data Center A and one node in Data Center B we have MariaDB/MySQL asyncoronous replication, in master-master configuration so going both ways.

As I understand it we now have:

  • Nice fast clusters for reads and writes in both data centers, which have redundancy within themselves if one node goes down.

  • Redundancy between data centers, so even if all of A or all of B goes down the other data center will stay functioning.

  • Under normal circumstances all nodes in both data centers have consistent data, or will within milliseconds of each other.

Have I missed anything - any potential problems? Thanks in advance for your advice.

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  • AUTO_INCREMENT is probably not usable.
  • UNIQUE need care to avoid accidental dups.

How far apart (in ms) are the datacenters?

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  • AUTO_INCREMENT is already a concern for simple MySQL replication between two servers, and is solved using auto_increment_increment and auto_increment_offset. In my example above auto_increment_increment would be 6, and each server would have a different auto_increment_offset. Would there be any reason that wouldn't work here? Let's say in this hypothetical scenario the data centers are 20ms apart. What implications might this have? Commented Oct 19, 2020 at 4:41
  • @DavidCunningham -- I expect Galera would insist on auto_increment_increment being 3. That is, if you set it to 6, it will change it to 3. Furthermore, if you take a node out of a cluster (or it crashes), the incr will be changed to 2? Commented Oct 19, 2020 at 4:44
  • Regardless, you should set up a testbed with, say 6 VMs on one machine -- to see what happens. Commented Oct 19, 2020 at 4:45
  • If Galera automatically changes the auto_increment_increment that could be a problem, since the cluster will be bigger than it realises. We'll have to test it, and testing is certainly something we'll do. I was checking to see if anyone else had used the above scenario and had experience to share. Commented Oct 19, 2020 at 22:30
  • I note Galera has a wsrep_auto_increment_control option we can disable to control the auto_increment_increment and auto_increment_offset manually. Commented Oct 20, 2020 at 3:42

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