FREE LESSON · Networks & distributed systems · 3 OF 4
Replication, Consistency and Consensus
Copies must agree enough — Replication, consistency, ordering, and consensus
Replication creates availability and durability options, not free agreement.
Copies receive updates at different times and may be partitioned. A consistency model defines which values and orderings clients may observe. Leader-based replication can serialize writes through one authority; quorum systems overlap read and write sets under specific assumptions; consensus lets a changing group agree on an ordered log despite some failures.
State the client-visible guarantee before selecting a replication mechanism.
Linearizability is a real-time illusion with a cost
A linearizable operation appears to take effect atomically between call and response, respecting real-time order. Achieving it across replicas requires coordination with an authoritative quorum or leader, which adds latency and can reject work during partitions. Eventual consistency permits more availability but moves conflict and staleness into the application contract.
Consistency is what clients may conclude; replication is one mechanism used to provide it.