I wonder why they went with something like this rather than a cluster like Percona's XtraDB Cluster that uses Galera replication? You could hang some read-only replicas off of the masters and achieve a similar system?

IMHO, it depends, I assume they mention about NDB Cluster not InnoDB Cluster. If their query using very less normalization, me myself also would choose MySQL Cluster instead.. their concept of auto partition + auto sharding did improve compare to other RDBMS. Plus I believe they should really understood the benefit of Index Condition Pushdown concept which really help a lot on complex query. Just remark, im not from MySQL but already tested several POC comparison on both DB, still MySQL Cluster did stand out a bit yet like i said depends on your transaction design back

Jepsen suggests that XtraDB and Galera don't even come close to working as advertised.

https://aphyr.com/posts/328-jepsen-percona-xtradb-cluster

Interesting post.

You mentioned issues with HAProxy (needing to patch / reconfigure, etc)-

wondering why you stuck with this rather than something else?

e.g. other tcp proxy or an SQL aware one

or did the original issues resolve themselves?

Ah.. just saw the later discussion of ProxySQL..

but still would be curious

The feature is very new and marked experimental but Envoy Proxy just gained support for MySQL proxying. The documentation [0] has an example of using in combination with Envoy's RBAC model to restrict access to particular tables.

0. https://www.envoyproxy.io/docs/envoy/latest/configuration/ne...