Split Brain - SQL Server

Split Brain is a term used in SQL Server clustering to describe a scenario where the nodes in the cluster lose communication with each other, and each node believes that it is the only active node in the cluster. This can cause multiple issues, including data corruption, duplicate transactions, and other unexpected behaviors. Here's an in-depth detail of split brain in SQL Server clustering with examples:

In a SQL Server clustering environment, there are multiple nodes that are connected to each other via a network. Each node has access to the same shared storage, which contains the SQL Server database files. In a normal situation, the nodes communicate with each other to ensure that only one node is active at any given time. This is achieved through a mechanism called a cluster heartbeat, where the nodes send signals to each other to indicate that they are still alive and functioning.

However, in certain scenarios, the cluster heartbeat may fail, and the nodes may lose communication with each other. This can happen due to a network failure, hardware failure, or other reasons. When this happens, each node in the cluster may believe that it is the only active node in the cluster and may attempt to take control of the SQL Server instance. This can cause a split-brain scenario, where the cluster is effectively divided into multiple independent clusters.

For example, let's consider a two-node SQL Server cluster. If the network connection between the nodes is lost, both nodes may believe that they are the only active node in the cluster. Each node may try to bring up the SQL Server instance, causing duplicate transactions and data corruption. This can lead to serious issues such as data loss, application downtime, and business disruption.

To prevent split brain scenarios in SQL Server clustering, several mechanisms are available. These mechanisms are designed to detect a failure in the cluster heartbeat and take appropriate actions to prevent data corruption. Some of the mechanisms include:

Quorum: In a SQL Server cluster, the quorum is a mechanism that determines which node has the authority to make decisions. The quorum is typically based on a majority of the nodes in the cluster. If a node loses communication with the cluster, it will lose its vote in the quorum, preventing it from taking control of the SQL Server instance.

Node Majority: This mechanism requires that more than half of the nodes in the cluster be active for the cluster to function. This ensures that there is always a majority of nodes that can communicate with each other and prevent a split-brain scenario.

Dynamic Witness: This mechanism allows for a third-party server or disk to act as a tiebreaker in a split-brain scenario. The witness will determine which node should take control of the SQL Server instance, preventing data corruption.

Split brain is a scenario in SQL Server clustering where the nodes lose communication with each other, causing data corruption and other issues. To prevent split brain scenarios, several mechanisms such as quorum, node majority, and dynamic witness are available. These mechanisms are designed to ensure that there is always a majority of active nodes in the cluster, preventing any node from taking control of the SQL Server instance and causing data corruption.