Comments - Mariadb Galera Cluster | Table Update Issue
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Q1)
its recommended that innodb tables always have a primary key, especially with galera. I can't say it will definately fix this problem you are encountering, however it will help a lot of operations be able to be quicker in galera.
Q2a)
Galera limitations are in https://mariadb.com/kb/en/mariadb-galera-cluster-known-limitations/
Q2b)
More information as the the "high impact" is needed to assess this. Provided the known limitations are addressed a migration should be possible. Like all major change test in a development environment.
Q3)
" 3641448 row lock(s)" is likely a symptom of no primary key.
Q4)
sequential and loadbalance are the only options that should be used - https://mariadb.com/kb/en/about-mariadb-connector-j/#failover-and-load-balancing-modes . loadbalance will fail badly until you have a primary key as galera is opportunistic locking. If (initiator,report_date) isn't the primary key I'd stick to sequential.
I see Rick James gave you largely the same answer