How deeply does Elasticsearch in Action, Second Edition cover quorum?

Explains quorum membership, acknowledgment-based commits, the majority formula, and split-brain mitigation.

Quorum

As explained in Elasticsearch in Action, Second Edition

A quorum is the minimum number of eligible participants whose acknowledgments are required to authorize an operation in a distributed system.

A quorum is a carefully selected majority of master-eligible nodes whose acknowledgments allow a cluster to reach agreement. Elasticsearch uses this group when electing a master and committing cluster-state changes. section 14.6.4 section 14.6.3

During a cluster-state update:

The master computes and publishes the proposed changes. section 14.6.3
Master-eligible nodes acknowledge that they received the update. section 14.6.3
The master commits the change after receiving quorum acknowledgments; it does not need to wait for every node. section 14.6.3

Eight master-eligible nodes

In a 20-node cluster with eight master-eligible nodes, the quorum is five: 8 / 2 + 1 = 5. A cluster therefore needs at least five eligible nodes for quorum-based decisions in this example. As a practical rule, configure at least three master-eligible nodes to help avoid split-brain behavior. section 14.6.4 section 14.7

Minimum number of master-eligible nodes = (number of master-eligible nodes / 2) + 1

Use the quorum formula to estimate the minimum number of master-eligible nodes required for consensus. section 14.6.4

How Elasticsearch in Action, Second Edition covers it

Cover of Elasticsearch in Action, Second Edition

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Also explained in

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Kafka for ArchitectsA quorum architecture commits replicated state only after a majority of participating voters acknowledge the update.
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ScyllaDB in ActionThe QUORUM consistency level requires a majority of replicas for a key to acknowledge an operation.
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LatencyA quorum is a majority subset of nodes whose agreement is sufficient for a distributed operation to complete.
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Think Distributed SystemsA quorum is a sufficient subset of distributed participants whose agreement or participation allows a decision to be made safely.

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Related concepts

consensusused by · Consensus is sufficient agreement among participants or processes on a shared decision, value, state, assessment, or outcome.
master-eligible nodehas part · A master-eligible node is an Elasticsearch node assigned the master role and therefore able to vote in master elections and become the elected master.
consistency levelkind of · A selectable database setting that determines how many replicas or nodes must participate in an operation and what consistency it provides.
data lossaddresses · Data loss is the destruction, corruption, deletion, or unavailability of information that is expected to be retained or remain usable.
leader electionused by · Leader election is a distributed coordination process that selects one eligible participant to act as leader or coordinator.

This explainer was generated by Manning's knowledge graph from the text of Elasticsearch in Action, Second Edition. The section links point to the parts of the book it draws on.