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Version: v26.0-RC

Quorum Considerations for SQL Server Availability Groups

Applies to:

  • Microsoft SQL Server 2017 CU1 and newer

Summary​

To ensure automatic failover in SQL Server availability group deployments, it's essential to consider architectural requirements related to quorum management. This document outlines the necessary configurations for the SQL Server Availability Group, as well as AG membership quorum and arbitration.

Configuration Requirements​

For availability groups with an External cluster type, such as those managed by DxEnterprise, two types of quorum and arbitration mechanisms are in effect:

Automatic Failover for High Availability with SQL Server​

To meet Microsoft's requirements for automatic failover with an External cluster type, an availability group must have one of the following configurations:

  • Three synchronous replicas
  • Two synchronous replicas plus a configuration-only replica
note

A configuration-only replica can be hosted on any edition of SQL Server, including SQL Server Express. For more information on AG quorum requirements and SQL Server edition features, refer to Microsoft's documentation.

AG Membership Quorum​

In the event of network isolation, a DxEnterprise cluster may split into multiple independent sub-groups, each unaware of the others. In this situation, it is possible for multiple primary replicas of the same AG to be promoted simultaneously, resulting in a split-brain condition. To prevent this, some type of arbitration must occur at the AG membership level. With DxEnterprise AG Membership Quorum, arbitration occurs at the Availability Group level, delivering guaranteed split-brain prevention and enterprise-grade data protection.

AG Membership Quorum determines quorum based on the members that are participating in a specific Availability Group. This ensures that quorum decisions for promotion and continued operation of a primary replica are made using only the replica set associated with that AG.

By default, DxEnterprise enforces AG Membership Quorum for SQL Server Availability Groups. During a network isolation event, only the subgroup that contains a majority of the synchronous replicas is permitted to promote or continue hosting the primary replica. Subgroups that do not meet AG Membership Quorum requirements will have SQL Server resources for that AG shut down and demoted to prevent split-brain conditions.

AG Membership Quorum vs. Cluster-Level Quorum

AG Membership Quorum evaluates only the replicas that participate in a specific Availability Group, rather than all members of the DxEnterprise cluster.

This distinction is especially important in environments where:

  • Not all cluster members participate in every AG
  • exVhost or multi-host replica models are used
  • Multiple AGs share the same cluster with asymmetric node participation

In these scenarios, a cluster-wide node majority does not accurately represent quorum for a specific AG. AG Membership Quorum ensures that only the isolation group containing a majority of synchronous replicas for a given Availability Group is allowed to promote or maintain the primary replica.

References​