Re: how to upgrade database schema in M-M environment

[Rick James]

It depends.

Are the changes incompatible? That is, do the queries have to be changed at the same time as the schema?

If they are compatible (eg just adding a column or index), then...
Set session.binlog=0; ALTER passive master
Wait for replication to catchup.
Failover.
Repeat on other machine.

If incompatible, then build a layer (as Peter suggests) so that the application does not need to change. Instead, the 'layer' will dynamically map the application's requests to one or the other of the schemas. This will take more steps...
Add the layer (without changing the schema yet)
Test
Add new code to the layer to figure out which version it is talking to and run the appropriate queries.
Test (still using only the old schema)
ALTER the passive master (with binlog=0)
Point the layer to the passive master (now active), and tell it to use the new schema.
ALTER the old master (with binlog=0)
Eventually clean out the old code from the layer.

Next month, you will have the layer ready for the next big change :)