Quorum Considerations for SQL Server Availability Groups
- Microsoft SQL Server 2017 CU1 and newer
There are architectural considerations for SQL Server availability group deployments if automatic failover is desired. These considerations relate to how SQL Server manages its own AG quorum.
For any availability group where cluster type is External, such as one managed by DxEnterprise, there are two types of quorum and arbitration at work. The first type is for the cluster, which is maintained by DxEnterprise and its optional witness(es). The second is the SQL Server quorum and arbitration for the availability groups(s), which happens at the SQL Server level. Per Microsoft requirements, for an availability group to maintain quorum and enable automatic failovers with a cluster type of External, it either must:
Have three synchronous replicas; or
Have two replicas (primary and secondary) as well as a configuration only replica.
A configuration-only replica can be hosted on any edition of SQL Server, including SQL Server Express. Please refer to Microsoft documentation for additional details on AG quorum requirements and SQL Server edition features.