7. High-Water Mark

Background

Distributed systems keep multiple copies of data for fault tolerance and higher availability. To achieve strong consistency, one of the options is to use a leader-follower setup, where the leader is responsible for entertaining all the writes, and the followers replicate data from the leader.

Each transaction on the leader is committed to a write-ahead log (WAL), so that the leader can recover from crashes or failures. A write request is considered successful as soon as it is committed to the WAL on the leader