You can lose communication between the primary and secondary replicas for some reason.
There is a policy on SQL Server to check the data synchronization state of the database replica. The policy is in an unhealthy state when the data synchronization state is NOT SYNCHRONIZING or the state is not SYNCHRONIZED for the synchronous-commit database replica.
Based on SQL Server logs, I observed the following messages:
The target database, ‘YourDatabase’, is participating in an availability group and is currently not accessible for queries. Either data movement is suspended or the availability replica is not enabled for read access. To allow read-only access to this and other databases in the availability group, enable read access to one or more secondary availability replicas in the group. For more information, see the ALTER AVAILABILITY GROUP statement in SQL Server Books Online. (Microsoft SQL Server, Error: 976)
According to Technet, this issue can be caused by the following:
The availability replica might be disconnected.
The data movement might be suspended.
The database might not be accessible.
There might be a temporary delay issue due to network latency or the load on the primary or secondary replica.
Further information: https://technet.microsoft.com/en-us/library/hh245199.aspx
Resolution:
You can fix it, using the following T-SQL command to force the resume synchronization:
ALTER DATABASE YourDatabase SET HADR RESUME;
Or
Using SQL Server Management Studio
In Object Explorer, connect to the server instance that hosts the availability replica on which you want to resume a database, and expand the server tree.
Expand the AlwaysOn High Availability node and the Availability Groups node.
Expand the availability group.
Expand the Availability Databases node, right-click the database, and click Resume Data Movement.
In the Resume Data Movement dialog box, click OK.
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