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Dysfunctional Interactions

In “Engineering A Safer World“, Nancy Leveson states that dysfunctional interactions between system parts play a bigger role in accidents than individual part failures. Relative to yesterday’s systems, today’s systems contain many more parts. But because of manufacturing advances, each part is much more reliable than it used to be.

A consequence of adding more parts to a system is that the numbers of potential connections and interactions between parts starts exploding fast. Hence, there’s a greater chance of one dysfunctional interaction crashing the whole system – even whilst the individual parts and communication links continue to operate reliably.

Even with a “simple” two part system, if its designed-in purpose requires many rich and interdependent interactions to be performed over the single interface, watch out. A single dysfunctional interaction can cause the system to seize up and stop producing the emergent behavior it was designed to provide:

So, what’s the lesson here for system designers? It’s two-fold. Minimize the number of interfaces in your design and, more importantly, limit the number, types, and exchanges over each interface to only those that are required to fulfill the system’s purpose. Of course, if no one knows what’s required (which is the number one cause of unsuccessful systems), then you’re hosed no matter what. D’oh!

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