2th of 4 Questions.
How does Redis Sentinel work, and what are its failure scenarios?
Sentinel monitors master/replicas, performs leader election (Raft-like quorum), and promotes a replica on failover. Failure scenarios: split-brain if quorum is wrong (use odd number of sentinels, minimum 3), stale reads during failover window, data loss if min-replicas-to-write is not set.