Scenario to automate the promotion of a read replica when the master db is not responding I would like to set a scenario like the following:


Set an RDS event subscription [that the MySQL is unresponsive] 
that will publish to an SNS topic 
that will trigger a Lambda function 
which will run a script that will promote a read replica [in another region] to master


Is such a case feasible?

Which DB metric should I monitor to confirm that the DB is not responding and that there is no false alarm?

Would it be dangerous to automate the promotion of the read replica? Should such actions remain manual and just add alerts?
Beware of Split Brain.

The tough problem in promoting a Slave to be Master happens when the problem is in the network.  With a dead network, most types of monitoring will mistakenly think that the Master is dead and promote the Slave.  Now you would have both servers alive and thinking they are the 'Master'.  This condition is called the dreaded "split brain".  Pretty quickly, clients insert duplicate keys and/or generate duplicate auto_increment ids.  The data corruption can be terrible to unravel.

MHA half solves it by promoting a slave when it is sure that it is safe.  If the problem might be a network glitch, it punts.

Galera fully solves it by requiring a quorum of nodes to vote that they have the majority.  This, however, requires 3 nodes, preferably in 3 geographic locations.  (Think flood, hurricane, earthquake, etc.)  All nodes can be writable (that's another feature of Galera).  Galera is built into MariaDB and PXC.