There's a concept Operator in k8s which will could provide a CRD and do some operations when watching ADD/UPDATE/DELETE events of the CR.
It's a common method to deploy two services for the purpose of high availability, one as the master to respond to requests and the other as a standby to do failover. I have heard that in k8s, controllers could use this method to do HA and APIServer offers the functionality to select master between the two services.
I have searched a lot but couldn't find an example as to how to use this feature in Operator.
Any suggestions on how to do HA for Operator are appreciated.
Seems you are referring to procedure called leader election. There is nice blog post about leader election in kubernetes. Some internal kubernetes services requires this too, for example kube-scheduler, since at moment only one scheduler can work.
Also, you can use other ways for leader election, maybe your language have library for leader election, via consul, etcd, zookeeper, etc.