Sunday, January 29, 2012

Contained Databases feature in SQL Server 2012

A common DBA task is to migrate a database to a different instance – to a newer, more powerful server; to a different phase in the application lifecycle (dev -> test -> QA -> staging -> production); or as part of various failover conditions. A problem that has plagued us for a long time in this process is that a database is never really all that independent – there is a lot of scaffolding that has to come along for the ride to ensure that the database continues to function as a dependable component in the application. This scaffolding includes logins, SQL Server Agent jobs, linked servers, custom messages stored in sys.messages, and even differences between server and database collations (which in turn can cause problems when working with temporary objects).

In SQL Server 2012, we have a new feature called Contained Databases, which intends to reduce or eliminate the dependencies that a database has on the SQL Server instance, making it easier to migrate a database to a new instance with less of the work involved in reproducing and validating these dependencies.

One of the issues solved in this release involves the ability to define a user with a password at the database level (a “contained database user”). By authenticating at the database level, you can move the database to a new server, and applications can change their connection string without having to create new SQL Server logins – a common problem we see today is orphaned users and/or mismatched security identifiers (SIDs).

This release also solves an issue involving databases with a different collation than the server’s – today, if you use any objects in tempdb, you may find that collation conflicts are a significant part of your troubleshooting efforts, especially if you move your database between instances. In SQL Server 2012, tempdb will automatically create objects using the collation of the Contained Database, rather than the server default, making it easier to rely on your code regardless of the server collation (of course this still does not resolve issues where you try to join #temp tables generated from multiple databases with different collations).

You could even say that the new THROW()Transact-SQL command can help make these transitions easier, as we will be able to raise custom errors without needing to define these messages first in sys.messages. But, to be clear, this is not an explicit feature of Contained Databases.

There are DMVs and events that will help identify queries and objects that are not “contained” and that will present a potential risk should the database be moved to a new instance. The only containment option in SQL Server 2012, however, is PARTIAL – because containment is only observed, not enforced.  In future releases, we will see this model extended to help deal with SQL Server Agent jobs and linked servers, as well as actual enforcement.

While it’s not complete just yet, Contained Databases give us a fantastic first step towards database autonomy. In my previous job as a production DBA and architect, this functionality could have saved me dozens and dozens of hours in deployment preparation and subsequent troubleshooting.
Article Ref: Aaron Bertrand

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