Lukas Beeler's IT Blog

Application redundancy is the best, and most expensive way to make your infrastructure resilient against problems. Application redundancy requires at least two machines, which then both serve the same application. Application redundancy is also called clustering, replication, multi master replication, etc.

Basically, there are two different architectures to achieve application redundancy:

• Shared Storage
  Shared Storage means that the storage is shared between the two machines. Note that a shared storage does not prevent an Active/Active configuration, with both machines active – however, care must be taken by the programmers of the application to support this mode of operation. The good side of Shared Storage is that you can make Active/Passive configurations with any software – the downside of Shared Storage is that if the storage is down, nothing works.
• Shared Nothing
  Shared Nothing means that there are no shared components between the two or more machines. There is no longer a single point of failure. Shared Nothing is almost always implemented in an Active/Active configuration, ensuring that you don’t waste energy on a machine doing mostly nothing.

Shared Nothing is usually the more elegant approach, but it’s not always supported in the application itself. Or it might require special licenses. But so much for theory – how do these things look in practice?

Here’s a list of services than can be made to support redundancy in a simple fashion.

• Active Directory
  Active Directory is designed for multi master operation. Install a second domain controller, and you’re set. This is easy. Except if you’re using Microsoft’s Small Business Server product. Upgrade SBS to a full blown windows server. SBS is designed for really, really small companies which can’t afford more than one server. Multiple DCs are allowed by all Windows Server Editions, except SBS and Web. Active Directories Multi Master Replication gives you a Shared Mostly Nothing configuration – read up on FSMO roles
• Exchange
  Exchange 2007 offers a feature called Cluster Continuous Replication. CCR requires the enterprise edition of Exchange 2007. This allows you to do you to create a Shared Nothing configuration easily.
• File Serving
  DFS is the way to go. Upgrade to R2 if you’re thinking about implementing DFS. DFS allows true multi master replication with binary diffing (for WAN connections). DFS also allows you to implement a unified naming convention for your shares. You should implement DFS even if you’re not using the replication contained. DFS is supported since Windows 2000, but not on Web and SBS editions.
• SQL Server
  I never worked with SQL server, and don’t know much about it. Our ERP software uses the System i (where they focus on hardware redundancy, and make application redundancy impossible to pay for smaller companies). If you know about SQL Server, write a comment.