3 ms·

But the chain itself can be considered as one component and it becomes a single point of failure. Or if we go in the opposite direction, then we can breakdown l

But the chain itself can be considered as one component and it becomes a single point of failure. Or if we go in the opposite direction, then we can breakdown links and say "Every single grain boundary is a single point of failure".

ISL 5y ago
Partially-agreed on the former, but not the latter. So long as the chain is not loaded near its tensile limit, then the grain boundaries that support the load within each link do so in parallel and are therefore redundant. The selection of the allegory of the chain was intentional -- each link must be properly formed, or the entire chain will fail. If it breaks, it is surely correct to say, "the chain broke", but in truth, it was actually link-86. For JWST, the remarkable/audacious thing is that many links in the chain from launch to observation are potential single-point failures. Furthermore, many of them haven't ever been tested independently in space... ever. It is a hell of a triumph that JWST has gotten this far already. If even a small fraction of the instrumentation works at this point, we are going to learn a ton about the universe, simply due to JWST's position, collecting-area, and mirror-diameter.
- systemvoltage 5y ago
  You're right. I think I was searching for a better analogy. Abstractly: Sub-system SPOF conditions can be bundled up as a single System SPOF condition by multiplying the probabilities. Also, I don't want to underplay JWST's success or its challenges. But, when saying 300+ SPOF conditions, one has to specify at what abstraction level. Otherwise, it can be misleading.
earleybird 5y ago
Each link in the chain is itself manufactured/created/constructed. If the weld is bad, the link may fail. "every grain boundary" is not formed in the same way each link is. The analogy is solid - actually, it's very good in that it conveys clearly and succinctly the concept the parent was asking about without introducing extraneous concepts. Very much in the flavour of EWD: "The purpose of abstraction is not to be vague, but to create a new semantic level in which one can be absolutely precise."
lovecg 5y ago
Another example is a small single engine airplane. The engine can be considered a single point of failure but if you zoom in it’s a complex system with many redundant components (multiple cylinders, multiple spark plugs per cylinder, etc.)

ISL 5y ago

Partially-agreed on the former, but not the latter. So long as the chain is not loaded near its tensile limit, then the grain boundaries that support the load within each link do so in parallel and are therefore redundant. The selection of the allegory of the chain was intentional -- each link must be properly formed, or the entire chain will fail. If it breaks, it is surely correct to say, "the chain broke", but in truth, it was actually link-86. For JWST, the remarkable/audacious thing is that many links in the chain from launch to observation are potential single-point failures. Furthermore, many of them haven't ever been tested independently in space... ever. It is a hell of a triumph that JWST has gotten this far already. If even a small fraction of the instrumentation works at this point, we are going to learn a ton about the universe, simply due to JWST's position, collecting-area, and mirror-diameter.

systemvoltage 5y ago
You're right. I think I was searching for a better analogy. Abstractly: Sub-system SPOF conditions can be bundled up as a single System SPOF condition by multiplying the probabilities. Also, I don't want to underplay JWST's success or its challenges. But, when saying 300+ SPOF conditions, one has to specify at what abstraction level. Otherwise, it can be misleading.

earleybird 5y ago

Each link in the chain is itself manufactured/created/constructed. If the weld is bad, the link may fail. "every grain boundary" is not formed in the same way each link is. The analogy is solid - actually, it's very good in that it conveys clearly and succinctly the concept the parent was asking about without introducing extraneous concepts. Very much in the flavour of EWD: "The purpose of abstraction is not to be vague, but to create a new semantic level in which one can be absolutely precise."

lovecg 5y ago

Another example is a small single engine airplane. The engine can be considered a single point of failure but if you zoom in it’s a complex system with many redundant components (multiple cylinders, multiple spark plugs per cylinder, etc.)