Re: interconnection between multiple databases. synchronization of multiple mysql databases

[Rick James]

Scaling reads -- One Master + many Slaves gives you arbitrary scaling of reads. Each machine is (mostly) independent when it comes to read and write.

Write only to the Master, read from either the Master or Slave.
If you write to a Slave, the change will not be reflected in the Master, and it could conflict with writes replicated from the Master. Example: duplicate AUTO_INCREMENT key.

If, by "cluster", you mean "NDB Cluster", then that is a very different discussion. You probably do not need that.

With a Slave, you have a backup. For further backup (eg, dump to tape), I recommend stopping mysqld on the Slave and copying the disk (or at least mysql's directory).

Replication is the copying of every Write that happens on the Master to the Slave, where it is also performed. These writes will lock tables/rows to preserve the integrity of the table and of the reads, so there is some conflict. But (usually) you will not notice this, because the blockage is (usually) sub-second in duration.