Thursday, May 28, 2009

Isolation / Immutability / Synchronization

If there's any one recipe for software runtime scalability, it's this: (IIS)^2. It's no secret I'm a fan of Internet Information Services, but in today's post it plays the role of host to our application, and my focus instead is on the underlying application's design.

Prevent state from being shared, allowing us to read and write to our hearts' content. Shared Nothing (SN) architectures are the ultimate realization of isolation. The definition of the isolation boundaries within an application might become contentious, and someone will need to be responsible for owning each isolated domain.

Prevent state from being modified - we can have as many readers as we want. In reality though, immutability by code contract is not as straightforward as simply not providing setter methods. In distributed environments especially, the .NET runtime must (de)serialize objects, which requires default constructors and public get/set accessors. We could choose to pass restrictive interface references instead, but this would force the (un)boxing of structs. Possibly, the application of custom attributes to state modifying methods/properties could be inspected statically by a tool like FxCop to prevent accidental object mutation. Objects do require mutation at times to be useful.

Mark critical sections where locks must be acquired before reading or writing. This is the typical style of programming that I've seen in Java and C# and doesn't easily lend itself to distributed programming. Also, while waiting, valuable resources are often wasted. It tends not to scale as well.

The best solution to a large distributed computing problem will undoubtably be a combination of the three above problems. Isolated tasks are likely candidates for physical distribution as they will scale horizontally. Immutability is hard to enforce completely and synchronization implies necessary waiting for other tasks. For these reasons, I give top priority to isolation, and let the other two duke it out as I see fit on the day.

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