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Traceability Woes

For safety-critical systems deployed in aerospace and defense applications where people’s lives may at be at stake, traceability is often talked about but seldom done in a non-superficial manner. Usually, after talking up a storm about how diligent the team will be in tracing concrete components like source code classes/functions/lines and hardware circuits/boards/assemblies up the spec tree to the highest level of abstract system requirements, the trace structure that ends up being put in place is often “whatever we think we can get by with for certification by internal and external regulatory authorities“.

I don’t think companies and teams willfully and maliciously screw up their traceability efforts. It’s just that the pragmatics of diligently maintaining a scrutable traceability structure from ground zero back up into the abstract requirements cloud gets out of hand rather quickly for any system of appreciable size and complexity. The number of parts, types of parts, interconnections between parts, and types of interconnections grows insanely large in the blink of an eye. Manually creating, and more importantly, maintaining, full bottom-to-top traceability evidence in the face of the inevitable change onslaught that’s sure to arise during development becomes a huge problem that nobody seems to openly acknowledge. Thus, “games” are played by both regulators and developers to dance around the reality and pass audits. D’oh!

To illustrate the difficulty of the traceability challenge, observe the specification tree below. During the development, the tree (even when agile and iterative feedback loops are included in the process) grows downward and the number of parts that comprise the system explodes.

In “theory” as the tree expands, multiple traceability tables are rigorously created in pay-as-you-go fashion while the info and knowledge and understanding is still fresh in the minds of the developers. When “done” (<- LOL!), an inverse traceability tree with explicit trace tables connecting levels like the example below is supposed to be in place so that any stakeholder can be 100% certain that the hundreds of requirements at the top have been satisfied by the thousands of “cleanly” interconnected parts on the bottom. Uh yeah, right.

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