Replicated Logs

Quorum Commit

A prefix is committed when a majority holds each leader term in that prefix.

highlighted = computed this step

Quorum commit

An entry commits when a majority of nodes hold the leader's term at that index. The committed marker stops before any leader-only suffix. Note: the marker is computed from the logs.

quorum commit\text{quorum commit}

Majority threshold

The quorum is 2 out of 3 nodes. Note: the leader counts only if its entry is present with its term.

quorum=2,nodes=3\text{quorum}=2,\quad \text{nodes}=3

a simplified leader-based log replication; quorum commit + log repair are exact on these tiny clusters; full Raft safety, leader election (next book), and network partitions/timing are beyond these traces - no product claims.

Quorum commit - quorum 2; committedIndex 2; divergent: F2nodeidx 1idx 2idx 3Lt1:at2:bt2:cF1t1:at2:bF1 repairt1:at2:bt2:cF2t1:at1:xF2 repairt1:at2:bt2:c

Committed marker

The committedIndex is 2 while the leader has 3 entries. Note: the suffix held only by the leader is not committed.

committedIndex=2,leader entries=3\text{committedIndex}=2,\quad \text{leader entries}=3

a simplified leader-based log replication; quorum commit + log repair are exact on these tiny clusters; full Raft safety, leader election (next book), and network partitions/timing are beyond these traces - no product claims.

Quorum commit - quorum 2; committedIndex 2; divergent: F2nodeidx 1idx 2idx 3Lt1:at2:bt2:cF1t1:at2:bF1 repairt1:at2:bt2:cF2t1:at1:xF2 repairt1:at2:bt2:c

Summary

Quorum commit is a prefix rule: every earlier index must also be quorum-replicated. Note: this is simplified leader-based replication; full Raft safety and leader election are the next book, while partitions and timing are beyond these traces.

quorum prefix\text{quorum prefix}