Are there *application*-driven reasons to prefer multi-primary topologies over clustering, or vice-versa?

Question

I have an application that currently uses a single primary and I'm looking to do multi-primary by either setting up a reciprocal multi-primary (just two primaries with auto-increment-increment and auto-increment-offset set appropriately) or Clustering-with-a-capital-C. The database is currently MariaDB 10.3, so the clustering would be Galera.

My understanding of multi-primary is that the application would likely require no changes: the application would connect to a single database (doesn't matter which one), and any transaction that needed to obtain any locks would do so locally, any auto-increment values necessary would be generated, and once a COMMIT occurs, that engine would complete the commit and the likelihood of failure-to-replicate to the other node would be very low.

But for Clustering , a COMMIT actually requires that the other node(s) are updated to ensure success, the likelihood of failure during COMMIT (as opposed to during some INSERT/UPDATE/DELETE ) is much higher, and therefore the application would really require some automated retry logic to be built into it.

Is the above accurate, or am I overestimating the likelihood of COMMIT -failure in a Clustered deployment, or perhaps even underestimating the likelihood of COMMIT -failure in a multi-primary environment?

From what I've read, it seems that the Galera Cluster is a little more graceful about handling nodes leaving the re-joining the Cluster and adding new nodes. Is Galera Cluster really just multi-master with the database engine handling all the finicky setup and management, or is there some major difference between the two?

Honestly, I'm more looking for reassurance that moving to Galera Cluster isn't going to end up being an enormous headache relative to the seemingly "easier" and "safer" move to multi-primary.

Answer 1

By "multi-primary", do you mean that each of the Galera nodes would be accepting writes? (In other contexts, "multi-primary" has a different meaning -- and only one Replica.)

One thing to be aware of: "Critical read".

For example, when a user posts something and it writes to one node, and then that user reads from a different node, he expects his post to show up. See wsrep_sync_wait .

(Elaborating on Bill's comment.) The COMMIT on the original write waits for each other node to say "yes, I can and will store that data", but a read on the other nodes may not immediately "see" the value. Using wsrep_sync_wait just before a SELECT makes sure the write is actually visible to the read.