You use the logs from the slave to commit the 2nd portion of a two-phase commit, and immediately stop processing new transactions when you only have the slave u

You use the logs from the slave to commit the 2nd portion of a two-phase commit, and immediately stop processing new transactions when you only have the slave up. Your hypothetical API does support two-phase commit correct? Because if it doesn't you have lots of solutions for losing data/creating inconsistent data anyway.