Distributed Systems Pattern: High-Water Mark

Unmesh adds High-Water Mark to his collection of patterns. The
high-water mark is an index into the log file that records the last log
entry that is known to have successfully replicated to a Quorum of
followers. The leader also passes on the high-water mark to its followers
during its replication. All servers in the cluster should only transmit
data to clients that reflects updates that are below the high-water
mark.

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