A decade ago MySQL folks were in love with the concept of a relay slave for MySQL high availability across data centers. A relay is a single slave in a remote data center that receives replication from the global master and, in turn, replicates to all the other local slaves in that data center. This saved a lot of bandwidth, especially back in the days before memcached when scaling reads meant lots of slaves. Sending 20 copies of your replication stream cross-WAN gets expensive.
In Galera and Percona XtraDB Cluster (PXC), by default when a transaction commits on a given node it is sent to every other node in the cluster from that node. That is, the actual writeset payload (the RBR events) are sent over the network to every other node, so the bandwidth to[Read more...]