Understanding HA


I've learned a lot about high availability (HA) and disaster recovery (DR) over the last two decades. Some of the things I've learned were from studying, some from experimentation and practice, and a few from hard lessons when some part of a system failed. The latter were how I learned the quickest and most effectively, but those were also the most stressful times in my career. Getting called by senior management because a system is down is not the way that I want to improve my skills.

I know many people struggle with HA and DR, which are two different concepts. They are designed to solve different problems for different situations. SQL Server includes a number of options for how you might handle either HA or DR situations (or both), and the list of options continues to grow over time. We have the AlwaysOn suite of technologies, database mirroring, replication, log shipping, and more.

All of the technologies that you might use to solve HA and/or DR problems may provide other benefits. The ability scale out, or spread load are a possibility with a few of these features. What I find, however, is that many accidental, junior, or otherwise inexperienced technology professionals that need to manage SQL Servers get confused. They seem to think HA and DR are the same thing, and one technology is somehow better than others.

Whether you need HR or DR, or some other solution, it's important the you research, understand, and practice working with the technologies that you will implement. Books Online gives you a good starting point, and there are plenty of online resources where you can ask additional questions or debate the particulars of your situation

Disasters rarely occur, but you should take the opportunities to hone your skills at working with the various technologies you may implement before things fail. Schedule failures of systems in your test environment. See if your HA technology keeps the system running if you remove a system, or test that you can recover using your DR techniques. When something actually goes wrong, you'll be glad you prepared.

Steve Jones

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