Mysql – Is it possible to join MySQL 8.0 to a cluster of MySQL 5.7

We were able to resolve this using mysqldump instead of mydumper/myloader. Apparently there is a bug in myloader or in Galera itself which prevents replication to flow normally.

I think you are not alone with this issue. We stopped the multi-master replication and instead beefed up RAID and XtraBackup and no issues with 5.5 or 5.6.

I have read that once you're using the wsrep and clustering, however, these issues creep in. Unsure if they have a solution yet but your best bet may be to follow this thread:

• https://www.percona.com/forums/questions-discussions/percona-xtradb-cluster/11056-too-many-connections-brings-down-cluster

I hope this helps and that article likely has best "from Percona engineers' mouth" commands to use to diagnose your issue.

To summarize Percona engineer tips, check error logs of course and:

show status like 'wsrep%';

Also some tools mentioned in article. One thing we have in our Ansible playbooks for all servers running DB software is to increase max open file limits (ulimit -n), etc.

I'm not sure if it will help but Peter from Percona recommends explicitly setting max_connections when granting privileges. I've not tried that but this might be useful as well to try and diagnose where the issue is, or if indeed it's related to the cluster.

mysql> GRANT USAGE ON *.* TO 'batchjob1'@'localhost'
    ->     WITH MAX_USER_CONNECTIONS 10;

• https://www.percona.com/blog/2014/07/29/prevent-mysql-downtime-set-max_user_connections/