A lagging replica, even one that just acknowledged a semi-sync write, will return stale results if you route a `SELECT` to it.

First and foremost, the extra copies of the data are for fault tolerance. In specific circumstances they may offer some slack capacity that you can use to serve (potentially stale) reads but they're never going to offer read-your-writes consistency.

The docs you quote are a bit obtuse but the "acknowledgement" is the row event being written to the binary log. "Fully executed and committed" is when it makes its way into InnoDB and becomes available for future reads.

Ah yeah that makes sense. I guess if you are only using replica for fault tolerance and not as a read-replica then it seems semi-synchronous replication is strictly better than synchronous replication. I suppose in semi-sync the failover will take a little longer but it's probably inconsequential.

Thanks!