3 ms·

For mission-critical software like avionics or a nuclear power plant? Absolutely! At design time you better figure out what all the error cases are, and blindly

• harry8 7y ago
  Uncaught exception and die. For a banking application, yes, die, hard, as loudly as possible. Downtime is worth it. Do not continue, do nothing until it is understood and fixed. When you're flying. Hmmm. Not so much. Dying is a really bad idea. Ok you should have it tested thoroughly so it isn't going to happen but if it does and "anything could happen" well "anything" is better than killing all the passengers, crew and anyone you might crash on, right? Literally anything is better than that. Idealy, you log it, you refuse to take off again, all the fleet is grounded etc etc but deciding to fall out of the sky because something unexpected happened is probably not the right response. Is graceful shutdown possible? No? Ok, so take your chances and log noisily is a less bad option.
  • MayeulC 7y ago
    Well, for an unmanned rocket, have it automatically self-destruct is likely the saner choice, as you don't want it to fall back somewhere inhabited, and there is usually little use for something on the wrong orbit.
  • Mikhail_Edoshin 7y ago
    The default out-of-box runtime can only handle one general scenario. The runtime designers choose to print the error message and terminate, and this seems to be the only sound option for default behavior. It's up to the developer to replace this default handler when the scenario differs that much.
    • mumblemumble 7y ago
      > this seems to be the only sound option for default behavior I think that is the crux of this debate. In my background, yeah, you fail hard when anything unexpected happens. It's the most straightforward way to fail safe. But my background maps better to the financial case; I've never worked on avionics or anything like that. I can see the point, though, that, in that kind of situation, failing hard doesn't fail safe at all. It's conceivable to me that different problem domains require different default behavior. (And perhaps, by extension, different programming languages.)
      • Const-me 7y ago
        That particular set of requirements is not unique to avionics. It’s pretty close in game development. A glitch is way better than a game which crashes in the middle of gameplay. Especially because many glitches only last a single frame i.e. barely visible. > It's conceivable to me that different problem domains require different default behavior. (And perhaps, by extension, different programming languages) Not sure about problem domains, e.g. 3D graphics is the main problem domain in videogames people play for fun, and CT scanners which save lives. Different projects indeed have different requirements. Every time you see a programming-related holy war on the internets (exceptions/status codes, static/dynamic typing, unit testing or not, FP/OOP, etc.), you can bet the participants have different types of projects on their background, and these projects needed different tradeoffs. More often than not, what’s good for a web startup aint’t good for Nasa or MS, and vice versa.
  • wycy 7y ago
    Whether die-hard or steamroll-thru is the better option depends more on what exactly goes wrong rather than what the application is. For a flight control computer, for example, if the ordered pitch suddenly becomes 3.402823466e+38 degrees it might be better to die hard and restart rather than try to execute the order.
    • ff317 7y ago
      I imagine (just thinking out loud randomly, I'm sure there's other issues to consider!) one way to mitigate this would be to have a second set of much simpler software (written in a different language, perhaps running on a different platform) whose only runtime job is to sit in the communications path from the primary software to the avionics hardware and monitor the outputs of the primary software and ensure the values and/or rate-of-change in certain values are within some physics-based sanity limits before they hit the hardware. It could be responsible for rebooting the primary software and holding the last-known-good output values (for the consuming avionics) while the reboot happens (hopefully quickly!). Of course then who watches the watcher, and you've added more things in the critical path which can have their own failure modes...
      • xaedes 7y ago
        "whose only runtime job is to sit in the communications path from the primary software to the avionics hardware and monitor the outputs of the primary software and ensure the values and/or rate-of-change in certain values are within some physics-based sanity limits before they hit the hardware" Brings to my mind: Erlang
        • deathtrader666 7y ago
          Was waiting for someone to mention Erlang.
    • K0balt 7y ago
      The answer for this is sanity checking and some kind of layered "reflexes". I design for mission critical things on a regular basis, and one of the error modes I must accommodate is random bit flipping by cosmic rays, emf, or other failures. Sometimes, it is possible to push things to a "safe" failure state and reboot (which often takes only 100 ms or so) Sometimes, though, the error must be caught and corrected to a last known good value or something like that. Everything critical is sanity checked : overall boundaries, rate of change, state related boundaries, etc. Layers of "reflexes" are more robust than a single programmed behavior, because an error in one will be resisted by the others. So much the better when there are segregated systems to check each other. Often, I'll have a "brilliant" system that performs to a very high standard and is complex and brittle. If the brilliant system fails, it just shuts down or reboots. Underneath the brilliant system is another layer or two. A "competent" system that is simpler and more robust, but less efficient and with soft failure modes, and a "brainstem" system that takes nothing for granted, even checking basic functions by bit flipping to negate stuck ram bits or broken ALU logic, but only tries to do a pasable job, reducing algorythmic complexity to its bare acceptable minimum. Typically the system will generate a basic range of acceptable parameters at the lowest level (and take corrective action if needed), then refine the range at each subsequent level..rinse and repeat. That way, each lower level checks the ones above. Or, you just fail downwards if errors are suspected. Either way. Designing failure tolerant systems is not impossible, but it requires a different mindset.
• alxlaz 7y ago
  This is a slightly more complicated topic (i.e. it's complicated by whether or not manual recovery is possible and by whether or not carrying on makes it worse), but in general, the worst thing you can do if you encounter an error in the software that supervises a process is to bail out and leave the process unsupervised. A chemical reaction, for example, is happy to carry on whether or not computers are watching. There are plenty of situations when automated or manual recovery isn't possible and/or where carrying on with a potentially damaged system can make things worse. In practice, you solve this sort of problem via other design solutions, like redundancy. If you think that a critical system needs to keep running, but you also think that some errors will have to be handled by shutting it down, then you make it redundant. Airplane FBW systems are a good example (caveat: I don't do airplanes, I do medical stuff -- I might be wrong about the practical details of this but I think it gets the point across). If the ELAC (elevator and aileron computer) runs into a condition it doesn't know how to handle, there's a good chance it will make things worse if you keep going. But you also don't want your mitigation to be "just halt the damn ELAC", you still want to have control over the elevator and the ailerons. That's why there are several ELACs. More to the point: if something has to keep happening, no matter what, then you design the computing system and the firmware around it so that it keeps happening. > At design time you better figure out what all the error cases are Not being able to figure out what all the error cases are has been an unfortunate component of systems engineering for decades now. The Wright brothers would have probably been able to account for all the failure modes in their flight control systems, but today, you are likely to miss some of the failure modes of a CPU that executes nothing but a NOP and a jump back to that NOP. With the exception of simple and special-purpose systems, built without any programmable logic whatsoever, it's unlikely that you'll be able to figure out what all the error cases are. (There's something to be said here about OISC and whatnot...) That's not to say it's OK to build systems that blindly steamroll over errors -- just that you have to build them so that they can deal with errors that you have not foreseen at design. You will run into that sort of error sooner or later, we are all failible. Edit: as for assertions, performance may be a factor, but that's not why you want to turn them off in production builds for embedded systems. (Although, IMHO, this isn't the right approach for embedded systems at all, but I've seen it used). First of all, you turn them off because, presumably, they make your system deviate from the spec (i.e. the system ends up handling some cases differently in the production build vs. the debug build, and hopefully the one in the production build is the one you want). Second, you turn them off because they can introduce undefined behaviour in your system. For example, if a peripheral gives you an incomplete or malformed packet in response to a command, or fails to answer altogether, you may want to abort with a stack trace in a development build. But what you really want to do IRL is probably to check and reset that peripheral, because for all you know it may be stuck and giving haphazard commands to an actuator. IMHO, assertions are only a partial answer to the problem you really want to solve -- obtaining useful data (e.g. stack traces) in response to error conditions. You can generally log the useful data in addition to actually handling the error correctly. Development and production builds should differ as little as possible -- ideally not at all. Handling potentially critical errors in different ways certainly doesn't count as differing as little as possible.