An application that runs “in the cloud” is designed fundamentally differently than a traditional enterprise application.  Cloud apps live on fundamentally unreliable, oversubscribed infrastructure; consequently, we must adopt the same mindset that drove the first RAID storage systems to create a Redundant Array of Inexpensive Nodes (RAIN).

The drivers for RAIN are the same as RAID.  It’s more cost effective and much more scalable to put together a set of inexpensive units redundantly than build a single large super-reliable unit.  Each node in the array handles a fraction of the overall workload so application design must partition the workloads into atomic units.

I’ve attempted to generally map RAIN into RAID style levels.  Not a perfect fit, but helpful.

• RAIN 0 – no redundancy.  If one part fails then the whole application dies.  Think of a web server handing off to a backend system that fronts for the database.  You may succeed in subdividing the workload to improve throughput, but a failure in any component breaks the system.
• RAIN 1 – active-passive clustering.   If one part fails then a second steps in to take over the workload.  Simple redundancy yet expensive because half your resources are idle.
• RAIN 2 – active-active clustering.  Both parts of the application perform work so resource utilization is better, but now you’ve got a data synchronization problem.
• RAIN 5 – multiple nodes can process the load. 
• RAIN 6 – multiple nodes with specific dedicated stand-by capacity.  Sometimes called “N+1” deployment, this approach works will with failure-ready designs.
• RAIN 5-1 or 5-2 – multiple front end nodes (“farm”) backed by a redundant database.
• RAIN 5-5 – multiple front end nodes with a distributed database tier.
• RAIN 50 – mixed use nodes where data is stored local to the front end nodes.
• RAIN 551 or 552 – geographical distribution of an application so that nodes are running in multiple data centers with data synchronization
• RAIN 555 – nirvana (no, I’m not going to suggest a 666).

Unlike RAID, there’s an extra hardware dimension to RAIN.  All our careful redundancy goes out the window if the nodes are packed onto the same server and/or network path.  We’ll save that for another post. 

I hope you’ll agree that Clouds create RAINy apps.