[erlang-questions] let it crash erlang/ada [[was: Time for OTP to be Renamed?]

Miles Fidelman mfidelman@REDACTED
Mon Feb 17 13:29:18 CET 2014

Jesper Louis Andersen wrote:
> On Sun, Feb 16, 2014 at 10:11 PM, Miles Fidelman 
> <mfidelman@REDACTED <mailto:mfidelman@REDACTED>> wrote:
>     Good point.  "Let it crash" does take on a whole different meaning
>     when dealing with aircraft and such.
> This is a different point as well! You have two axis:
> * soft vs hard realtime. Some systems require hard realtime and then 
> your tools are limited to languages where you have explicit memory 
> control, enabling you to avoid allocating memory and triggering 
> garbage collection. In soft realtime systems, you have more leeway, 
> and if built the way of the Erlang runtime system, you get really good 
> soft realtime capability.
> * Proactive vs Reactive error handling. The idea of "let it crash" is 
> definitively reactive, whereas static type systems, proofs, model 
> checking, etc are means of proactive error handling.
> My claim however, is that you need "Let it crash" in Aircrafts as well 
> if you want to have a stable aircraft. The model where you blindly 
> attempt to eradicate every error from a program is bound to fail 
> sooner or later. Usually "let it crash" in those situations is 
> implemented in hardware by having multiple redundant systems. But 
> rarely are systems exempt of failure. Even in a highly controlled 
> environment.

We've really strayed off-topic here, but....

My all-time favorite design for seriously mission-critical systems was 
the flight control system for the Space Shuttle.  I'm not sure this is 
true of the later versions, but originally:
- the flight control software ran on 5 parallel computers, that voted on 
- 4 of the computers came from one contractor (hardware and software)
- the 5th machine, just ran mission-critical code, with a completely 
separate design (both hardware and software)
- I don't remember how the tie-breaking algorithm worked



In theory, there is no difference between theory and practice.
In practice, there is.   .... Yogi Berra

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