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Comment by vacri | Hacker News Reader

The bit that surprised me the most was the lack of killswitch (or a halt or a pause); that a human supervisor couldn't invoke a "holy shit!" button.

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A "killswitch" is not a trivial thing to build.

If you simply run `kill -9 pid`, you might be holding a large position, or worse, you might be holding some naked shorts. (In fact, you almost certainly are.) This is risky. It can result in failures to deliver, make you vulnerable to large market movements, etc.

Another form of "killswitch" is to not open any new positions, but still attempt to close out your old positions with varying degrees of aggressiveness. But if your system is wildly broken, this might not be doing what you think it's doing. As I understand it, this happened to Knight.

I think that's a strange way to approach a killswitch. The point of a killswitch is to stop everything NOW, so the system can be checked. After the problem is understood, then it's safe to restart strategies gradually and unwind positions. A system shouldn't attempt to send more orders when it's in an unpredictable state. I believe many regulators require such a switch.

You know that saying about finding yourself in a hole? A "close out my risk" button is fine for situations like losing money or whatever, but if you have no clue what your orders, trades and risk even are, the only sensible thing to do is stop making it worse.

This probably surprised regulators and the exchanges too, as it is a requirement for automated trading.

A kill switch for a system that holds complex and rapidly changing state (open positions, market data) is quite different from, and much more complicated than, a kill switch for a system that can be simply halted at a given instant.

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