Raft
Raft gets several servers to agree on a shared state and keeps working while a majority survives — designed above all to be understandable.
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Merkle Tree
A hierarchy of hashes lets two replicas locate exactly which ranges differ — without sending or comparing every row they hold.
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Quorum
A quorum is how many replicas must respond for an operation to count — and overlapping read and write sets is what lets a read see the last write.
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Eventual Consistency
Replicas converge if updates stop — with no bound on how long, and no statement about what a reader sees in the meantime. Both gaps matter in design.
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Replication and Consistency: Living With Copies That Disagree
Once data has two copies, they can disagree. See what each replication scheme costs, what the models between strong and eventual promise, and when clocks lie.