If unconditionally MySql server goes lower,
- Do you know the ways I possibly could still show my page to user? Most likely getting a backup DB server is needed?
- If I have multiple DB servers and something goes lower, I'm able to fetch data from another temporary server? How you can identify that primary server is lower? Who'd choose alternate server? role of reverse proxy server here? Is Varnish able to do this type of switching via VCL (Varnish Configuration Language)?
- Method of maintain copy of primary DB on temporary server? Master slave replica method?
In case your service is dependent on database heavily, yes you must have a backup DB. Otherwise, whether it's only supporting some minor functionality, you can apply the key of "degradable experience" for the reason that situation.
Why don't you write a little service/daemon that functions being an interface for your DB, operates on your server machine and polls your servers for activity? This particular service would then handle all the DB-related queries individually, choose the right server, etc so that your www service would only forward demands towards the daemon and receive results accordingly.