patching SQL Server with Availability Group and Secondary Replica
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Availability Group with One Remote Secondary Replica

If you have deployed an availability group only for disaster recovery, you may need to fail over the availability group to an asynchronous-commit secondary replica. Such configuration is illustrated by the following figure:

Availability Group Upgrade in DR Scenario

In this situation, you must fail over the availability group to the asynchronous-commit secondary replica during the rolling upgrade/update. To prevent data loss, change the commit mode to synchronous commit and wait for the secondary replica to be synchronized before you fail over the availability group. Therefore, the rolling upgrade/update process may look as follows:

1.Upgrade/update the remote server

2.Change the commit mode to synchronous commit

3.Wait until synchronization state is SYNCHRONIZED

4.Fail over the availability group to the remote site

5.Upgrade/update the local (primary site) server

6.Fail over the availability group to the primary site

7.Change the commit mode to asynchronous commit