Change the HADR Cluster Context of Server Instance (SQL Server)

Article
06/14/2017

This topic describes how to switch the HADR cluster context of an instance of SQL Server by using Transact-SQL in SQL Server 2012 SP1 and later versions. The HADR cluster context determines which Windows Server Failover Clustering (WSFC) cluster manages the metadata for availability replicas hosted by the server instance.

Switch the HADR cluster context only during a cross-cluster migration of Always On Availability Groups to an instance of SQL Server 2012 SP1 on a new WSFC cluster. Cross-cluster migration of Always On Availability Groups supports OS upgrade to Windows 8 or Windows Server 2012 with minimal downtime of availability groups. For more information, see Cross-Cluster Migration of AlwaysOn Availability Groups for OS Upgrade.

Before You Begin

Caution

Switch the HADR cluster context only during cross-cluster migration of Always On Availability Groups deployments.

Limitations and Restrictions

You can switch the HADR cluster context only from the local WSFC cluster to a remote cluster and then back from the remote cluster to the local cluster. You cannot switch the HADR cluster context from one remote cluster to another remote cluster.
The HADR cluster context can be switched to a remote cluster only when the instance of SQL Server is not hosting any availability replicas.
A remote HADR cluster context can be switched back to the local cluster at any time. However, the context cannot be switched again as long as the server instance is hosting any availability replicas.

Prerequisites

The server instance on which you change the HADR cluster context must be running SQL Server 2012 SP1 or above (Enterprise edition or above).
The server instance must be enabled for AlwaysOn. For more information, see Enable and Disable AlwaysOn Availability Groups (SQL Server).

To be eligible to be switched from the local cluster context to a remote cluster cluster, a server instance cannot be hosting any availability replicas. The sys.availability_replicas catalog view should not return any rows.

If any availability replicas exist on the server instance, before you can change the HADR cluster context, you must do one of the following:

Replica Role	Action	Link
Primary	Take the availability group offline.	Take an Availability Group Offline (SQL Server)
Secondary	Remove the replica from its availability group	Remove a Secondary Replica from an Availability Group (SQL Server)

Before you can switch from a remote cluster to the local cluster, all synchronous-commit replicas must be SYNCHRONIZED.

Recommendations

We recommend that you specify the full domain name. This is because to find the target IP address of a short name, ALTER SERVER CONFIGURATION uses DNS resolution. Under some situations, depending on the DNS searching order, using a short name could cause confusion. For example, consider the following command, which is executed on a node in the abc domain, (node1.abc.com). The intended destination cluster is the CLUS01 cluster in the xyz domain (clus01.xyz.com). However, the local domain hosts also hosts a cluster named CLUS01 (clus01.abc.com).

If the short name of the target cluster, CLUS01, were specified, DNS name resolution could return the IP address of the wrong cluster, clus01.abc.com. To avoid such confusion, specify the full name of the target cluster, as in the following example:
```
ALTER SERVER CONFIGURATION SET HADR CLUSTER CONTEXT = 'clus01.xyz.com'  
```

Security

Permissions

SQL Server login

Requires CONTROL SERVER permission.
SQL Server service account

The SQL Server service account of the server instance must have:
- Permission to open the destination WSFC cluster.
- Remote WSFC read-write access.

Using Transact-SQL

To change the WSFC cluster context of an availability replica

Connect to the server instance that hosts either the primary replica or a secondary replica of the availability group.
Use the SET HADR CLUSTER CONTEXT clause of the ALTER SERVER CONFIGURATION statement, as follows:

ALTER SERVER CONFIGURATION SET HADR CLUSTER CONTEXT = { 'windows_cluster' | LOCAL }

where,

windows_cluster
The cluster object name (CON) of a WSFC cluster. You can specify either the short name or the full domain name. We recommend that you specify the full domain name. For more information, see Recommendations, earlier in this topic.

LOCAL
The local WSFC cluster.

Examples

The following example changes the HADR cluster context to a different cluster. To identify the destination WSFC cluster, clus01, the example specifies the full cluster object name, clus01.xyz.com.

ALTER SERVER CONFIGURATION SET HADR CLUSTER CONTEXT = 'clus01.xyz.com';

The following example changes the HADR cluster context to the local WSFC cluster.

ALTER SERVER CONFIGURATION SET HADR CLUSTER CONTEXT = LOCAL;

Follow Up: After Switching the Cluster Context of an Availability Replica

The new HADR cluster context takes effect immediately, without restarting the server instance. The HADR cluster context setting is a persistent instance-level setting that remains unchanged if the server instance restarts.

Confirm the new HADR cluster context by querying the sys.dm_hadr_cluster dynamic management view, as follows:

SELECT cluster_name FROM sys.dm_hadr_cluster

This query should return the name of the cluster to which you set the HADR cluster context.

When the HADR cluster context is switched to a new cluster:

The metadata is cleaned up for any availability replicas that are currently hosted by the instance of SQL Server.
All the databases that previously belonged to an availability replica are now in the RESTORING state.

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