Quorum, in simple terms, refers to the minimum number of nodes required for a cluster to be considered operational. It ensures that the cluster can continue to function, even in the event of node failures or network partitions.

In a SQL Galera Cluster, the quorum is usually based on the number of voting nodes. Each node in the cluster is assigned a vote, and the cluster can only continue to function if the number of votes that remain active is greater than or equal to the quorum value.

When a node fails or becomes unreachable, the remaining nodes in the cluster need to establish a new quorum to ensure that the cluster can continue operating correctly. This process is known as the quorum calculation.

Let’s say we have a SQL Galera Cluster with five nodes, each having one vote. In this case, the quorum is defined as (number of nodes / 2) + 1. So, in our example, the quorum would be (5 / 2) + 1 = 3. With this quorum value, the cluster requires at least three active nodes to continue functioning.

If a node fails in this five-node cluster, leaving only four active nodes, the cluster will recalculate the quorum and adjust accordingly. In this case, the new quorum would be (4 / 2) + 1 = 3, which means that the cluster can still operate with three active nodes.

However, if two nodes fail simultaneously, leaving only three active nodes, the new quorum calculation would be (3 / 2) + 1 = 2. In this scenario, the cluster would no longer be able to operate since it requires three active nodes to satisfy the quorum.

It’s important to note that non-voting nodes, usually used for load balancing or backup purposes, do not contribute to the quorum calculation. Only nodes with voting rights are taken into consideration.

Understanding the concept of quorum in SQL Galera Cluster is crucial for ensuring the availability and stability of your database infrastructure. By carefully configuring quorum values and monitoring the number of active nodes, you can maintain a robust and fault-tolerant cluster.

#SQLGaleraCluster #Quorum