DAC pack up all your troubles
- by Tony Davis
Visual Studio 2010, or perhaps its apparently-forthcoming sister, "SQL Studio", is being geared up to become the natural way for developers to create databases. Central to this drive is the introduction of 'data-tier application components', or DACs. Applications are developed as normal but when it comes to deployment, instead of supplying the DBA with a bunch of scripts to create the required database objects, the developer creates a single DAC Package ("DAC Pack"); a zipped XML file containing all the database objects needed by the application, along with versioning information, policies for deployment, and so on. It's an intriguing prospect. Developers can work on their development database using their existing tools and source control, and then package up the changes into a single DACPAC for deployment and management. DBAs get an "application level view" of how their instances are being used and the ability to collectively, rather than individually, manage the objects. The DBA needing to manage a large number of relatively small databases can use "DAC snapshots" to get a quick overview of what has changed across all the databases they manage. The reason that DAC packs haven't caused more excitement is that they can only be pushed to SQL Server 2008 R2, and they must be developed or inspected using Visual Studio 2010. Furthermore, what we see right now in VS2010 is more of a 'work-in-progress' or 'vision of the future', with serious shortcomings and restrictions that render it unsuitable for anything but small 'non-critical' departmental databases. The first problem is that DAC packs support a limited set of schema objects (corresponding closely to the features available on 'Azure'). This means that Service Broker queues, CLR Objects, and perhaps most critically security (permissions, certificates etc.), are off-limits. Applications that require these objects will need to add them via a post-deployment TSQL script, rather defeating the whole idea. More worrying still is the process for altering a database with a DAC pack. The grand 'collective' philosophy, whereby a single XML file can be used for deploying and managing builds and changes, extends, unfortunately, to database upgrades. Any change to a database object will result in the creation of a new database, copying the data from the old version, nuking the previous one, and then renaming the new one. Simple eh? The problem is that even something as trivial as adding a comment to a stored procedure in a 5GB database will require the server to find at least twice as much space, as well sufficient elbow-room in the transaction log for copying the largest table. Of course, you'll need to take the database offline for the full course of the deployment, which is likely to take a long time if there is a lot of data. This upgrade/rename process breaks the log chain, makes any subsequent full restore operation highly complicated, and will also break log shipping. As with any grand vision, the devil is always in the detail. It's hard to fathom why Microsoft hasn't used a SQL Compare-style approach to the upgrade process, altering a database with a change script, and this will surely be adopted in the near future. Something had to be in place for VS2010, but right now DAC packs only make sense for Azure. For this, they're cute, but hardly compelling. Nevertheless, DBAs would do well to get familiar with VS 2010 and DAC packs. Like it or not, they're both coming. Cheers, Tony.
