SQL Server Single-User Mode Deadlock Risks in Log Shipping Scenarios

When configuring a SQL Server database for single-user mode, only one user can access the database at a time. Any additional sessions attempting to connect will be placed in a waiting state, trying to acquire an exclusive lock on the database.

This behavior becomes problematic when operations within the single-user session attempt to acquire exclusive locks while other sessions are waiting. The initial session holds a shared lock after setting single-user mode. If that session then tries to escalate to an exclusive lock while another session is already waiting for exclusive access, a deadlock occurs.

A common scenario involves log shipping with a secondary database in standby mode. The restore job follows this sequence:

1. The log shipping restore job sets the database to single-user mode and acquires an exclusive lock
2. The job transitions the exclusive lock to a shared lock
3. Another session attempts to access the database and waits for an exclusive lock
4. The restore job tries to acquire an exclusive lock for the restore operation
5. The restore job is blocked by the waiting session, creating a deadlock
6. The restore job becomes the deadlock victim and fails with an error

The time between steps 2 and 4 is typically very short, but in environments with frequent database access, this issue becomes more likely.

Despite restore job failures, the system remains consistent. When the job runs again, it will restore both the current and previously failed transaction log backups. However, organizations should evaluate the impact based on their service level requirements.

Best Practices:

• Monitor log shipping restore job success rates
• Consider alternative approaches for high-access environments
• Implement retry logic in automation scripts
• Review service level agreements to determine acceptable failure rates

For more information on SQL Server database modes and log shipping configuration, visit the Microsoft SQL Server documentation.