# Quorum Control with Weighted Votes

{% hint style="info" %}
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](https://mariadb.com/docs/galera-cluster/high-availability/understanding-quorum-monitoring-and-recovery).
{% endhint %}

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](#quorum-calculation-with-weights), especially in complex or [geographically distributed topologies](https://mariadb.com/docs/galera-cluster/galera-cluster-deployment-variants#id-3.-wide-area-network-wan-cluster-multi-data-center).

By default, every [node](https://mariadb.com/docs/galera-cluster/high-availability/monitoring-mariadb-galera-cluster#understanding-galera-node-states) has a weight of `1`. You can customize a node's weight during runtime by setting the `pc.weight` [provider option](https://mariadb.com/docs/galera-cluster/galera-management/configuration/configuring-mariadb-galera-cluster#mandatory-options):

```sql
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`).

{% hint style="danger" %}
Changing a node's weight is a cluster-wide membership event. If a [network partition](https://mariadb.com/docs/galera-cluster/high-availability/understanding-quorum-monitoring-and-recovery#understanding-and-recovering-from-a-split-brain) 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](https://mariadb.com/docs/galera-cluster/high-availability/understanding-quorum-monitoring-and-recovery).
{% endhint %}

## 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](https://mariadb.com/docs/galera-cluster/high-availability/understanding-quorum-monitoring-and-recovery#the-galera-arbitrator-garbd):

* `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](https://mariadb.com/docs/galera-cluster/high-availability/using-mariadb-replication-with-mariadb-galera-cluster/using-mariadb-replication-with-mariadb-galera-cluster-using-mariadb-replica) 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](https://mariadb.com/docs/galera-cluster/galera-cluster-deployment-variants#id-3.-wide-area-network-wan-cluster-multi-data-center) fails, the primary site maintains quorum. Additionally, one node at the primary site can fail without causing an outage.

<sub>*This page is licensed: CC BY-SA / Gnu FDL*</sub>


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