Percona XtraDB Cluster 8.0.25 (PXC) has introduced a new option to perform online schema modifications: NBO (Non-Blocking Operation). When using PXC, the cluster relies on the wsrep_OSU_method parameter to define the Online Schema Upgrade (OSU) method the node uses to replicate DDL statements.  Until now, we normally have three options: Use Total Isolation Order (TOI, [&]

https://www.percona.com/blog/percona-xtradb-cluster-non-blocking-operation-for-online-schema-upgrade/