In my use I am planing to use read replica. I have read all docs but can not find some thing on what happen if my read replica goes down or not responding. Does google sql service do anything or is it my responsibility to fix the issue.Any suggestion to achieve read replica higher availability.
In most case there is no reason for read replicas to go down as google is implementing all sorts of fail safe (MySQL,Postgre) insuring that the replication can go on.
The only case I saw a read replica go down was when we changed some databases option allowing unsafe requests that will break the read-replicas. Google can't prevent or act in these case as you allowed the failing requests.
In case the replicas goes down you can try to make it recover or destroy and recreate it. In the case I saw I could only destroy recreate the replicas, it takes a good 10 minutes to recreate the replicas, for it to be available again.