I'm running a Google Cloud SQL instance ("mysql-master") and it has one replica. ("mysql-replica") Web apps on another servers read and write data on the Cloud SQL instance.
While mysql-master is restarting, can app servers read and write data?
I want to run gcloud sql instances patch mysql-master, but it sometimes requires the instance to restart.
I wonder if web apps can read/write data on the instance while it is restarting.
No, your read replica is a read-only, so it cannot take writes during downtime of the master. There's no good solutions to avoiding the downtime, but if you have a small database, it should come back in order seconds (most come back within 5 seconds or so).