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 …[Read more...]