Connect to an instance using the DAC. in VS .NET

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Connect to an instance using the DAC.
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Estimated lesson time: 10 minutes
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Using the DAC
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The DAC is implemented as a specific TCP endpoint in a SQL Server instance that is always attached to a dedicated UMS. The naming of this feature has already caused a significant amount of unfortunate confusion. The DAC is not intended for use as the connection for all administrative operations. The DAC was created to provide a connection that could always be used by a member of the sysadmin role to access a SQL Server instance, thereby guaranteeing that an administrator could not be locked out of SQL Server due to resource allocation issues. Although it is always running on a dedicated UMS, the DAC has limited resources allocated to it, so the operations that can be performed via the DAC are also limited. Any operation that would spawn multiple threads, such as a backup or restore, are not allowed within the DAC. Only a single connection at a time is allowed. If the connection is already being used, any subsequent connections are refused. Additionally, you create a connection to the DAC in only two ways:
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Monitoring and Troubleshooting SQL Server Performance
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The DAC does not allow connections from SQL Server Profiler, the DTA, Object Explorer in SSMS, third-party applications, or any other application.
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DAC: A Last Resort
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The DAC is intended as a last-resort connection method. If you can create a connection to SQL Server through normal means, you should use that mechanism, which allows access to anything with the capability to issue queries against SQL Server. Only when connection by any other method is unsuccessful should you use the DAC. You should also use it only for simple operations such as querying DMVs/DMFs and terminating processes.
To connect to the DAC, specify ADMIN: and the name of your instance for example, ADMIN:MyMachine\Instance1.
SQL Server and Windows Error Logs
To resolve database errors, you should also review the SQL Server error logs and the Windows Application Event Log for errors. Although a variety of informational messages is logged to each of these locations, any error with a severity level of 16 or higher automatically gets logged to the SQL Server error log and the Windows Application Event Log. Errors with a severity of 16 or higher are critical errors that you need to investigate immediately. Although you can scan each of these logs manually, it is much more common to point an enterprise monitoring product such as Microsoft Operations Manager at these logs and then have the monitoring tool page an operator when it encounters errors.
PRACTICE
Connecting to the DAC
In this practice, you will create a connection to the DAC and determine whether any processes are being blocked.
It would be nice to produce a nonresponsive server to demonstrate the ability to always be able to connect to the DAC, but we will skip that step to avoid undue complexity.
IMPORTANT
1. Launch SSMS. 2. Click Database Engine Query. 3. In the Connect To Server dialog box, insert ADMIN: before the instance name.
Lesson 7: Resolving Database Errors
4. After the query window is open, execute the following query to determine whether there are any blocked processes:
SELECT session_id, sql_handle, plan_handle FROM sys.dm_exec_requests WHERE blocking_process_id > 0
Lesson Summary
When a SQL Server instance is having performance issues but is so busy that an administrator cannot connect to even attempt to fix the issue, the DAC provides a last resort connection method for use by members of the sysadmin role. It is always available and cannot be locked out. To resolve database errors, you should also review the SQL Server error logs and the Windows Application Event Log, especially for critical errors with a severity level of 16 or higher.
Lesson Review
The following questions are intended to reinforce key information presented in this lesson. The questions are also available on the companion CD if you prefer to review them in electronic form.
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