Quorum Control with Weighted Votes

This page is a deep-dive into the advanced feature of weighted quorum. For a general overview of Quorum, its role in monitoring, and basic recovery, see Understanding Quorum, Monitoring, and Recovery.

MariaDB Galera Cluster supports a weighted quorum, where each node can be assigned a weight in the range of 0 to 255, with which it will participate in quorum calculations. This provides fine-grained control over which nodes are most critical for forming a Primary Component, especially in complex or geographically distributed topologies.

By default, every node has a weight of 1. You can customize a node's weight during runtime by setting the pc.weight provider option:

SET GLOBAL wsrep_provider_options="pc.weight=3";

Quorum Calculation with Weights

The quorum is preserved if, and only if, the sum of the weights of the nodes in a new component is strictly more than half the total weight of the preceding Primary Component (minus any nodes that left gracefully).

The formal calculation is:

$$2∑pi​×wi​−∑li​×wi​​<∑mi​×wi​$$

Where:

• $p_i$: Members of the last seen Primary Component.

• $l_i$: Members that are known to have left gracefully.

• $m_i$: Members of the current component being evaluated.

• $w_i$: The weight of each member (pc.weight).

Changing a node's weight is a cluster-wide membership event. If a network partition occurs at the exact moment a weight-change message is being delivered, it can lead to a corner case where the entire cluster becomes non-primary.

Practical Examples of Weighted Quorum

Prioritizing a Primary Node

In a three-node cluster, to make node1 the most critical for maintaining the Primary Component:

• node1: pc.weight = 2

• node2: pc.weight = 1

• node3: pc.weight = 0

If node2 and node3 fail, node1 remains primary. If node1 fails, the other two nodes become non-primary.

Simple Primary/Replica Failover

In a two-node cluster, to ensure node1 is always the primary in case of a network split:

• node1 (Primary): pc.weight = 1

• node2 (Replica): pc.weight = 0

Primary and Secondary Site Scenario

For a disaster recovery setup with two nodes at a primary site and two at a secondary site:

Primary Site:

• node1: pc.weight = 2

• node2: pc.weight = 2

Secondary Site:

• node3: pc.weight = 1

• node4: pc.weight = 1

If the secondary site or the WAN link fails, the primary site maintains quorum. Additionally, one node at the primary site can fail without causing an outage.

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