The topic and partition whose leader needs to be elected
The current leader and isr of input partition read from zookeeper
The leader and isr request, with the newly selected leader and isr, and the set of replicas to receive the LeaderAndIsrRequest.
If no replica in the assigned replicas list is alive
New leader = preferred (first assigned) replica (if in isr and alive); New isr = current isr; Replicas to receive LeaderAndIsr request = assigned replicas