Hi,
i had the same problem. I have pointed each cluster Server to his own mysql database. The database is replicating with the others mysql databases. Just one Machine has write permissions, the other Member Servers are just reading. But on heavy utilization you need strong servers or you will get replica lags (Hazelcast/Openfire needs also a very fast server). For my needs i don't need to write anything from the Cluster Openfire Member Servers to the DBase(its done by the Senior Member). This could be a possible problem.