Replicated Logs

Quorum and Availability

A majority can commit a shared prefix while a leader-only suffix remains uncommitted.

highlighted = computed this step

Quorum and availability

A majority can still commit even when one node contributes only an old prefix. Entries held by the leader and one follower reach quorum; a leader-only suffix does not. Note: the committed marker shows the boundary.

availability quorum\text{availability quorum}

Two available copies

The quorum is 2, and the committedIndex is 3. Note: the stale node does not prevent the shared prefix from committing.

quorum=2,committedIndex=3\text{quorum}=2,\quad \text{committedIndex}=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 and availability - quorum 2; committedIndex 3; divergent: -nodeidx 1idx 2idx 3idx 4Lt1:at1:bt1:ct1:dF1t1:at1:bt1:cF1 repairt1:at1:bt1:ct1:dF2t1:aF2 repairt1:at1:bt1:ct1:d

Leader-only suffix

The leader has 4 entries, so the suffix after the committed marker is not committed. Note: one copy is not a quorum.

leader entries=4\text{leader entries}=4

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 and availability - quorum 2; committedIndex 3; divergent: -nodeidx 1idx 2idx 3idx 4Lt1:at1:bt1:ct1:dF1t1:at1:bt1:cF1 repairt1:at1:bt1:ct1:dF2t1:aF2 repairt1:at1:bt1:ct1:d

Summary

Availability here means a majority is enough for the shared prefix, not for a leader-only suffix. 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.

majority boundary\text{majority boundary}