I don't buy it.
I don't think that software development has to be such a quality disaster. Software development could be far more disciplined, manageable, and credible than it is today.
I'm not saying this from the viewpoint of some remote, ivory tower. In my current job I write and maintain code for both Windows desktop GUIs and embedded systems using the 8051, 68HC08, Z180, ColdFire, MIPS, and Pentium processor families under home-grown RTOS code, Linux, and Windows XP Embedded--and that's just for our currently shipping products. I've worked professionally in various small teams designing, managing, training, and mostly coding for 22 years. I've seen and done lots of software development, and I know what's hard about it.
Working with a variety of environments and tool chains may have contributed to my tendency to question why development hassles occur, as they are occurring. Is it a shortcoming in this particular platform? Or of these particular tools? The language? The design? Some element of our process? Some shortcoming in my experience, training, or understanding? How can I make it better? And if I had unlimited resources with which to tackle the problem in some ideal world, what might I do about it?
We've somehow become conditioned to accept failures of computer code as some inevitable element of modern life. We talk about the "essential complexity" of software as if it were some immutable universal law. But recently I have come to suspect that these limitations apply, not to all possible software development, but solely to the software development paradigm that we've followed, unchallenged, for decades. I suspect there are ways out of this mess, but they all involve first stepping out of our familiar habits and finding new ones without these limitations.
The limited worldview
I first started questioning our paradigm after reading an article last fall on Embedded.com, "Margin," by Jack Ganssle. The article started by recounting the September 22, 2005 emergency landing of an Airbus 320 jetliner with a broken nose wheel. The landing was accomplished with the wheel sideways, friction grinding half of the wheel completely away and stressing a wheel strut that was never designed for this sort of abuse. The plane landed safely because that strut survived the stress, having been specified to a greater strength than its intended application required. The article pointed out that in mechanical engineering, it's possible to design in margin to accommodate greater demands than expected, while in software, we can never have a margin. We can only try to reduce the number of imperfections.
This article bothered me. The comparison felt true and correct but when I thought about it, I began to feel that the concepts being compared were mismatched.
There were two variables to the success of the strut on that airplane. The first variable was the characteristics specified for that strut: did it meet or exceed the requirements? The second was whether the design was properly implemented: was that particular airplane built with the strut that had been designed? The first variable is an engineering choice where margins are possible (and expected): the strength of the part must exceed some known worst-case expectation. The second is a human accountability question, where the likelihood that the correct strut was used, that the strut itself had been made correctly, and so on, can approach but never exceed perfection. The human accountability questions are addressed by processes and quality control that seek to minimize defects, while the engineering questions are addressed by careful analysis building upon a body of known and tested quality metrics and then adding safety margins.
In theory, software--particularly embedded software--should be no different. The specifications variable looks at whether the system has the resources to do the job. Is the CPU fast enough? Is the guaranteed interrupt latency low enough? Does the algorithm cover the expected range of input values? These are engineering questions, all of which allow for margins. The human accountability variable looks at whether the design was properly implemented. Was the correct algorithm used? Was it coded correctly? Are the code and operating system bug-free? Was a particular unit manufactured correctly? These questions are addressed by processes and quality control.
So, for both mechanical and software systems we have the same interplay of engineering choices, for which we can build margin into our choices, and accountability issues, for which we can only strive to improve, but never perfect, our success rate. It would seem, from this perspective, that both mechanical and software engineering should be able to yield the same high levels of success and the same low levels of catastrophic failure. Yet a complex software system turns out to be a lot less reliable. Every line of code represents a possible point of failure if done wrong, and failures tend to be catastrophic, halting the full function of the larger system.
Consider an everyday mechanical product we largely take for granted: the automobile. Every component--down to each screw, washer, or strand of wire--represents a possible point of failure, where the total product either got it right or fell short. The car sitting in my garage represents a larger number of points of failure than many quirky embedded systems projects, which begs the question: why doesn't the car just fall apart where it sits? There must be millions of points of possible failure in the manufacture of my car, so why does it work at all?
In my car, a failure at one of those millions of points is usually not able to do much harm. Let's say the structural integrity of one copper strand of wire in my engine fails because it was made wrong or stressed too much during assembly. So what? The other strands in the wire still carry electricity, and the loss of the current-carrying capability does not drop below the requirement for that cable. Or say that one machine bolt in a mounting was made from impure metal, can't hold its load, and snaps. I'll guess that if that bolt were fairly important (holding the frame together), other bolts are there too and the car survives; margin comes from redundancy.
Somehow we accept that in software, a bad strand or a bad bolt can cause the whole thing to disintegrate. Why do we accept it? Because our understanding of software technology, our software development worldview, says this is Just The Way It Is.
The universal bolt
Imagine for a minute that I've invented the Universal Bolt. This is a metal object for joining threaded holes that can extend or collapse to fit a variety of lengths. It can expand or contract to fit holes of different diameters. The really cool feature is that I have replaced the bolt's spiral ridge with a series of extendable probes that can accommodate different thread pitches. What a marvelous product! You no longer need to stock a variety of bolts of different sizes and lengths and thread spacings because my Universal Bolt can be used in place of any of them. Of course, with all this flexibility this bolt is admittedly far less solid than conventional bolts, but I have addressed this concern with super-high-strength metal and a clever micro-machined mechanism for locking it to the desired length, thickness, and threading. This introduces a lot of complications, and my Universal Bolt turns out to be more expensive than a conventional bolt. But no problem!
Because it's able to change configurations extremely quickly, a single Universal Bolt can take the place of many conventional bolts simultaneously. What we do is rig up a clever and very fast dispatcher device that quickly moves the bolt from hole to hole. If the dispatcher is fast enough, my Universal Bolt can spend a moment in each hole in turn and get the whole way through your product so fast that it returns to each hole before the joint has had a chance to separate. Sure, the bigger the project, the more complex the dispatcher becomes, but if we keep boosting the dispatcher speed and the bolt reconfiguration speed, we can keep the whole thing together.
Absolutely absurd, right? You'd have to be crazy to get into a car built along these lines. If anything caused the dispatcher to derail, the entire product would collapse in a second.
Well, yes, it is absurd. But it also pretty accurately describes the function of several embedded systems I've worked on. A fast and complex thread dispatcher keeps moving one simple and stupid integer-computation unit all over a big system tending to tasks rapidly enough that they all get done. And if that dispatcher ever once leads the CPU into an invalid memory address the whole thing crashes to a halt. Does this sound familiar to any software developers out there?
The present worldview
Software development started with programming. At one time a computer's primary purpose was to compute: that is, to be a programmable machine for the purpose of processing mathematical algorithms. A fairly simplistic but flexible arithmetic engine was given the ability to step through computations according to instructions that could be changed to handle the next problem. Want to grind out trigonometry tables? A computer is just the tool for you.
Then some of the programs became useful in their own right; something you would run over and over again with different inputs. Suddenly we had software applications. Add some long-term storage and the computer became suited for data processing. The computer was a big and expensive solution for applications, so some clever people came up with time-sharing (or multitasking or, if you prefer, the dispatcher for the Universal Bolt) to get more jobs done with the same hardware.
Sometimes these applications were just better tools for helping people do computations (for example spreadsheets such as VisiCalc). Other times applications had absolutely nothing to do with computing (word processors such as WordStar) but were implemented as computer applications simply because it was less expensive to build them in software than to build dedicated hardware alternatives. The same approach holds true today: Excel, Word, Access, PowerPoint, and so forth are still applications implemented on a computer because to do the same work in a dedicated appliance would be more expensive and less useful. But underneath, a very simple arithmetic engine is racing around a hideously complex and fragile maze of instructions to do the work. Embedded systems are not much different. Just like most big-name software applications, the very concept of the computer has grown and evolved into something unwieldy that tries to do too much.
Our desire to do powerful and flexible things with computing technology has stayed rooted for too long in the concept of executing a single algorithm; the result has simply become absurd. Somehow the tool we've had--creating expanded algorithms for ever-more powerful processors--has become the only way we know how to look at software needs.
We need a different model for software development.
Code reuse to the rescue?
Actually, it's worse than all that. Because not only do we try to create really complex applications by coding really complex algorithms, we tend to do it as a custom job every time.
Imagine, if you will, that you asked me to build you a car. In order to do this I was provided with your description of what a car ought to do and a forge for making the metal. No existing parts, no existing designs, just a description and some very raw materials. I can say with some confidence that what I would create--if indeed, I succeed in making any sort of car at all--would crash more decisively (and more literally) than any software I've seen.
I've heard about the importance of code reuse for most of my career: how silly it is to reinvent the wheel, how important it is for code quality to build upon trusted code. It's all true. I can also say that professionally we've still got a long way to go. The mainstream software development tools don't do much to facilitate code reuse and there seems to be a rule that the closer some development environment comes to allowing good reuse, the more the performance suffers or the more unstable the end result becomes. All I know is that in my career, the number-one explanation for discovered bugs (both mine and others') tends to be the "cut and paste mistake"--that is, an attempt at code reuse that caused problems.
When we as developers try to reuse code, the code we're reusing is rarely a perfect fit. I encounter this all the time: some module I wrote for a previous application is what I need for the new application. So I try to add the module to my new project and things break. In C or C++, the languages I use most, it turns out that the module needs different #include files than my previous project used, or different compiler switches, or different libraries to link against, or things like that. Every module carries with it its own tangle of requirements and dependencies, and I spend a block of time trying to get the project to build again. Almost always I have to make some code changes to the module, break routines out into different source modules, add some #ifdef conditional sections to reconcile the disagreements, change around the order of some headers or the like--and then see if the module still builds in the original project(s). Did I break something along the way? Very hard to tell. And that's ignoring the fact that the module probably isn't exactly what I need, functionally, so I have to tweak things. I can do this by adding run-time flexibility (adding parameters or flags, slowing down the execution, and making more execution paths that may not ever get tested) or by making #ifdef sections compile differently in different cases, which very quickly renders the code a completely unreadable mess that neither I nor my fellow developers can really maintain. This doesn't sound at all like the goal of reuse we were aiming for.
I know, these problems are characteristic of the tools I'm using. But they're not unique to particular tools or languages. Let's face it: even leaving code 100% untouched doesn't guarantee that it doesn't change. When I'm working in 8051 assembly language it's not uncommon that when adding some initialization code, some medium-distance calls (ACALLs) later in the code will no longer build because their targets are no longer in the same memory block as the caller. So I change the offending ACALLs into the less-bounded but slower LCALL instructions, which changes the timing of the program. That grows the code size, which may suddenly cause conditional branches to no longer reach their targets, requiring that the conditions be rewritten to test the opposite condition and branch around an ACALL that gets to where it needs to go, which again changes the timing. So much for leaving the code alone!
At the other end of the scale, in the desktop PC world of Windows and Pentiums, adding a bit of code causes unrelated and untouched code modules to link to different addresses, suddenly changing where in the code the paging breaks for virtual-memory swapping occur. Or worse yet, two important and heavily used routines now hash to the same locations in the processor's cache; you take a sudden performance hit in code you haven't changed for no reason you can detect.
Our code reuse is, in many ways, trying to build our new insanely complex algorithm in part from snippets of other insanely complex algorithms, hoping that they fit and make our situation better than starting from scratch, although they may not. This observation sums it up: as developers, we're told of the virtues of both code reuse and refactoring, when in fact, these are opposite approaches to crafting code.
It occurs to me that the problem is neither too much nor too little code reuse. Instead, the problem is that our code reuse is all about helping to build huge algorithms, not creating components. What we're doing doesn't match what we think we're doing.
Now imagine that I wanted to construct a new office building. I suppose I could hire a single worker with every needed construction skill and ask him or her, alone, to make my building for me. But I wouldn't. I would hire a project leader and a team of overseers working together on various goals, who in turn would hire their teams of contractors and laborers to perform their various specialties to get the necessary tasks done, all working in parallel. This approach is faster and safer than the solo worker because its success doesn't depend nearly as much upon an individual. Sure, some work inefficiencies might occur when various steps can't proceed until other steps are completed by other workers, but overall, the teamwork is superior to the single "super builder" approach.
But no, in software development, we prefer that one entity (the CPU) do the entire job and then focus on trying to make him/her/it faster. That's a Bad Idea. If we're going to do anything to make software development more manageable, we need to divide up the work into more reasonable pieces.
To a software developer it may sound like I'm suggesting that we divide jobs into multiple threads, hardly a new idea. No, I emphatically do not suggest this. I have plenty of experience with multithreading and know when it helps and when it hurts. Threads always add complication, uncertainty, and difficulty in testing.
The problem with multithreading is easily illustrated when human beings do it. Suppose you have several tasks to perform, but you're constantly putting one down to work on another one, while being interrupted and redirected by phone calls and visitors. You may be more responsive to specific immediate demands but take longer to complete all your tasks and risk botching them because your attention is divided.
Asking a single processor to jump between tasks and to service interrupts introduces virtually the same reductions in efficiency and reliability. The admittedly greater ability of a processor to "concentrate" on its tasks is more than offset by the lack of "common sense" that enables humans to recognize when a neglected task is getting into trouble. Dividing a CPU across threads doesn't improve the overall picture. Rather, it means that the overall resulting "algorithm" is even more convoluted and less deterministic--hardly the goals we are seeking.
All this indicates to me that when our software development becomes unmanageable and untrustworthy we're probably asking the processor to do too much.
Choosing new models
We've looked at the internal workings and fallacies of the software developers' world. Let's look at how other engineering disciplines approach quality and see if we can apply some of these to software development.
In the mechanical world, complex systems are built out of less-complex assemblies built out of still-less-complex components. There are endless variations on the machine bolt because different needs call for different lengths, diameters, materials, and so on. When creating an assembly that needs to join two pieces of metal, an engineer doesn't need to rethink the question of how pieces can be joined; he or she draws from the existing body of work on bolts and other fasteners and selects a "right" choice on the basis of the specification requirements. Components can be characterized by their specific properties: how strong, how heavy, how durable, acceptable ranges of temperature or pressure, or whatever.
In software development we don't have much of this yet. Right now when we talk about creating "components" we mean complex assemblies that try to gain value by solving lots of variants on a problem. We may say a report-generation module or a spreadsheet-style grid control is a "component." Nope, those are complex major assemblies. A software equivalent to the bolt might be, for example, the "pointer to next" in a linked list--a completely different level of thinking.
In the world of building construction, and indeed in most large endeavors, we employ a team of workers with differing skill sets working in an overlapping time domain to accomplish a large task. Beyond the timeframe and risk advantages, this approach reduces the breadth of skill required by any individual worker, improving the reliability of that worker's output. The efficiency and success of these projects involves the engineering of the process (design), the communication among members of the team (management), and the workers themselves (proficiency). More overall energy is expended than in a lone effort but it works out much better. In software development, both object orientation and threading have been proposed as comparable concepts but neither one really comes close. I submit that the nearest software equivalent would be to divide a task among a number of processors, each one handling its piece of the work as a member of a larger team.
So the basics of other engineering and design disciplines don't map very well onto our present software development. But recognizing this, I see a choice before us.
We can simply whine that software development "isn't like that," and resign ourselves to inherently poor quality, striving merely to find the Best Process du Jour that might enable our efforts to suck less.
Or we can start with the requirement that software development should involve trustworthy components, specifications, and margins; that it should allow assemblies of increasing complexity to be built from trustworthy lesser components; it should involve a team approach to performing complex tasks; and it should be something that can be generally dependable and trustworthy. And then--and only then--start building the software development disciplines of the 21st century on these foundations.
Here's a thought. Suppose we pursued the teamwork model with a vengeance and decided that for every small task within a larger computing job, we assign another processor. Suppose that every software "object" or every subroutine call is a processor. Leaders, managers, specialists, laborers would all be implemented as separate processors, each with its own specialized "code." Sure, this was impossible when computing started out, but in today's world simple processors are pretty cheap. (In fact, you can implement a whole bunch of modest processors inside a single CPLD or FPGA chip.) Sure, this doesn't fit today's reality of desktop PCs for a wide range of general use. It's easier to envision this approach in the embedded computing world where the tasks performed by the hardware are more static. But let's think it through.
Let's use as an example the common DVD player. Open one up and you'll see that there is an optical drive, a power supply, some controls, and so on, all managed by one super processor with a big heat sink doing all sorts of hard work. I don't care what DVD player you have, that processor contains a lot of software. I've also never seen a DVD player that is bug-free. From my cheapie Magnavox player that locks up after idling too long and sometimes forgets to acknowledge the remote control, to my beloved but aging Pioneer player that has decoding errors on just two commercial DVDs out of many hundreds tried, they all have software glitches. And that's overlooking widespread shortcuts in the MPEG-2 decoding that can result in subtly wrong visual effects on many different makes and models. It's software and it's imperfect.
Instead, imagine that the DVD player functionality was split across many component processors. One processor might handle seeking on the optical drive; another would handle focusing the laser. Turning the optical data into byte values, reading bytes into buffers of sectors, and presenting streams of information from consecutive sectors (which span tracks) are all tasks that might each warrant a processor. Some processor is keeping track of the progress through a disk (what is being displayed right now, in terms of title, track and time). Some processor is reversing the CSS encoding in the big VOB file representing one title on the disc. Another is breaking the decrypted VOB into the MPEG program stream packs and dispatching pieces to various other processors for video and audio processing. One video processor breaks out the video data into frames, another breaks the frames into slices, another breaks the slices into macroblocks, others crunch the bit compressed data into chunks of meaning, and still others run the DCT math to reconstruct the macroblocks from their encoding. There are processors keeping track of past frame data for handling the motion compensation for the "P" frames, processors precomputing frames in order to build the "B" frames, and processors reassembling the whole mess according to the presentation time stamps built into the data. You've got processors synchronizing the presentation of video with the work that those other audio processors were doing at the same time in breaking down the compressed audio. On top of this all you've got processors adding on-screen display elements (subtitle overlay for example). At a higher level you have a processor handling the interaction of the user with menus, which may involve the infrared remote receiver (another processor handles turning those blinks into commands) or buttons on the front (still another processor watches and de-bounces each button).
That's a lot of processors! But then, it's a lot of work. Today's DVD players are doing all these tasks already, just not putting a lot of workers on the job. Is it any wonder that there are bugs? And, supporting my premise, is it any wonder that the processors in modern DVD players generally have dedicated hardware accelerators for some of the low-level tasks like DCT math, onscreen display generation, or bit-stream unpacking?
But suppose we went crazy and made each distinct task run in its own processor. Setting aside for a moment some obvious complications like the lack of a single memory space holding the frame data required, the result of using lots of processors still looks more complicated than the current approach. And it is. But look inside each processor and you would see much simpler code. Actually you'd see a lot of today's code spread over a lot of different processors. But each processor would be much simpler in itself. The code, doing only one task, would be reasonably straightforward. And maintainable? Better than maintainable: once you got it right, it would never need to be maintained again! Why redesign a "bolt" that works?
In the overall system, you would end up with more overall complexity because in addition to all the various algorithm components in their own processors, you need overseers and a lot of communication among processors to pull it off. If there is only one YUV video data stream coming out of the system (composite or S-Video or component or HDMI or whatever) then there needs to be a processor managing just that stream, being fed information by a lot of different processors at different points, almost certainly with some "middle management" processors gathering data from underlings into fewer high-level elements to combine. The communication and management become the new challenges. So the total amount of code and silicon exceeds the current requirement. Is this progress?
Yes, it is progress because in this model the complexity has been so diluted that the only really complicated part is in putting together the pieces. Most of the pieces, once completed, never need to be touched again, so although the first DVD player built this way would be a major undertaking, the next model would not be. Want to add better on-screen configuration features or progressive scan generation? Most of the system is untouched. And more importantly, when tackling the next generation design (say, a Blu-Ray player, which does all of these things but also adds MPEG-4, H.264 and VC-1 decoding, a Java interpreter for user interface, and new protection systems), you've got components to reuse without fear. Look at it this way. The jump from DVD (480i, at 10 million active pixels per second) to Blu-Ray (1080p, at 124 million pixels per second) is a twelve-fold jump in throughput. Such a jump can be accomplished by radically boosting the performance of a system that already has heat-dissipation concerns or by just adding more low-complexity team members to pitch in on doing the same kinds of work. Which sounds easier?
I submit that dividing complex tasks over multiple processors is the way to reign in the mushrooming complexity, reliability, testability, and maintainability concerns.
Now, in this lots-of-processors model, just as in the real world of teams of human workers, communications and management of common resources become big issues. I don't wish to downplay these challenges and I don't have a ready-made solution for how they cooperate. But I know a model to throw away, one we generally use today across multiple processors: the Remote Procedure Call. That is, one processor asks another processor to do a task, waits until it gets the answer, and then moves on. This approach is comfortable to our Big Algorithm worldview but manages to give you the worst of both worlds. It's still a function call in a single algorithm that now has the added weakness of depending upon two processors to not screw up and the communications between them being perfect. No, the right model for multiple processors has to involve dispatching requests and signaling completions. This is a big deal. But it's what we need to focus on to get out of the trap of our present software development paradigm.
But think further. If we can work out the framework for all this communication and management, I'll bet we could work out the ability for the same processor to do different jobs at different times (just as, in constructing that office building, the same electrician who installed the main breaker box could, on another day, install a light switch). And I'll bet we could allow for scaling, where the number of available workers could change and the work still gets done, just at a different rate. And I'll also bet that we could have specialized processors better at certain tasks than others (such as DSP, or floating point, or matrix algebra, or sparsely populated n-dimensional arrays, or what have you), but then allow a more general-purpose processor with nothing to do to stand in for a more specialized processor (with a performance penalty). And so on, modeling the ways we manage projects in the human world.
And if you've made headway with all of this, then, and only then, are you ready to try building processor chips with highly segmented execution such that a smaller number of cores can emulate a larger number of processors without letting anybody jeopardize anybody else. Who knows? We could end up back at the general-purpose computing device that is our modern PC. But instead of one screamer processor (just now starting to offer multiple cores) it would be a crowd of moderate processors that work well together.
I'm calling for a big change in our software development paradigm. And maybe my model of lots of processors isn't the right one to replace it with. Maybe something else works better. But I do believe that the "essential complexity" of software will continue to be true as long as our approach stays the same. Until we treat software development as something other than Big Programming, we're going to be lucky when our code works at all. And that's no way to build anything.
I'm not discouraged. Although the software development industry tends toward very minor evolution of processes and tools, we have had points in our history where a team takes a fresh look at addressing a number of related issues at once. I would submit that Java was one, simultaneously tackling a language, a runtime environment, an object model, and code-security concerns. An even bigger jump was the interrelated creation of the C programming language and the UNIX operating system, which tackled a language for structured programming, an operating environment, a view of data as streams, and a model of code modules and how they communicated with each other. Other people will have different opinions on "milestone" points in software development. But my point is this: we're capable of substantial change. And that's important, because tweaks to specific tools or specific practices or the choice of programming languages or methods will not overcome the problems we face.
We face problems of software quality, reliability, and maintainability because our old tools are no longer adequate for our projects. And until people sit down to take a fresh look at the whole world of how software is developed, there will be no magic fix.
We can do better. The embedded system is the best venue for a radical rethinking of the software development practices and the creation of new approaches, tools, and methods because the world of embedded systems isn't so dependent upon the status quo of processors and operating systems. And successful new software development approaches will gradually work their way into the mainstream of computing, just as elements of Java were absorbed into .Net and C##, and elements of UNIX were absorbed into MS-DOS.
We need to start by setting aside our current worldviews and approaches long enough to think the whole thing through again. Let's stop making excuses for why software development can't be as reliable as mechanical engineering. Rather, let's look at what works in other engineering disciplines, try to imagine software development following those models, and see what we can accomplish. We need reliability, partitioning of tasks, redundancy, design margins, and more, so we need components that are separate, that support each other rather than weakening each other, that can build on other components logically rather than introducing complexity and risk, that have specifications and known interfaces and known communications links. We need to start there and work forward toward our goal.
I'm a good developer. I want to develop good products. Really, is that asking a lot?
Mark Bereit has been developing software and designing hardware since 1984, including six years as the founder and head of a small business offering contract hardware and software development for other businesses. For the past seven years he has been the director of product development for IRIS Technologies, a manufacturer of video technology products. You can reach him at [email protected].
Brooks, Frederick P., "No Silver Bullet: Essence and Accidents of Software Engineering," Computer, April 1987. www-inst.eecs.berkeley.edu/~maratb/readings/NoSilverBullet.html
Ganssle, Jack, "Margin," Embedded Pulse, September 27, 2005, www.embedded.com/showArticle.jhtml?articleID=171201184
Ganssle, Jack, "Subtract software costs by adding CPUs," Embedded Systems Programming, May 2005, p.16: www.embedded.com/showArticle.jhtml?articleID=161600589
Goering, Richard, "Multiple processors mean multiple challenges," EE Times, September 19, 2005: www.eetimes.com/news/latest/showArticle.jhtml?articleID=170703813
I would think that the number of interactions (and resulting failure modes) to be managed when designing an aircraft wheel strut are a fraction of those required when designing an embedded system. I work in the automotive industry where an automobile has multiple nodes interconnected by a communication bus. A supplier developing a node must account for a huge number of interactions and timing variability of the messages alone. It's very difficult to anticipate and test all the possible interactions. In contrast, I would think the failure modes for a wheel strut could be more effectively anticipated and tested.
I would agree that the complexity in the design of the vehicle bus is responsible for the huge number of failure modes, and perhaps could be simplified, but as a supplier I must work within the complexity already established.
- Matthew Nichelson
Software Engineering Manager Johnson Controls, Inc
Why does mechanical engineering perform better than us?
I think there are three major reasons:
--First one is mechanics rules universalism. I mean, the interactions of the atoms in a piece of steel do obey to universal rules that you can learn once for all. In a software system this is of course all but the case, because we have a whole bunch of different languages, different instruction sets, differents processors, different execution spaces, so at the end even if two pieces (Ie. two software components, whatever they are) look the same (same interfaces, specifications), we cannot guarantee that they will react the same way even when stressed in similar conditions. There might be good reasons for introducing new languages or cores, but this doesn't help us as it keep creating even more diversity and calls for new bugs.
--The second reason is that in a mechanical system you can generally predict which part of the system will be affected by a particular failure. This is because, as you pointed out in your article these systems are made of smaller subsystem, and there are physical barriers between them. I agree that's exactly what we miss. Most often our subsystems are using the same resources, so there are no or few physical barriers.That way you cannot predict to which extent a particular bug will affect the rest of the system.
Is running each individual task on a dedicated processor the solution though? Might help, but it will not solve the problem entirely if other resources are still shared, like the memory or I/O lines. Here also we have to think of creating secured frontiers. MMU helps a bit, but better you would ideally want to execute each task on a physically independant subsystem, with its own bunch of HW resources. We are not there yet ...
--The third reason is inputs diversity.
The specifications of the oil you put in your car are very close, whatever the brand. And people accept that the engine might fall apart if they put in, say, water instead. For a DVD player (to take the same example as you used) things are much less simple. I can tell you, because I've been working in this domain for 5 years, that many discs do all but comply to the DVD-Video specifications. Since, because these are top tens, people would expect their DVD player to playback them correctly. But of course this creates a lot of diversity for the inputs your system has to digest ... And fault tolerancy is not a proper answer, because of course people do expect these discs to play without hickups. So a proper test strategy is crucial too. As you can't pretend to test every possible input you need to understand where you system can break, and try to stress these particular weak points. The issue is to specify the margins the system must be able to cope to.
And ultimately, we have to accept that our systems have to be used in a certain way, and focus on making this set of use cases sufficiently robust, and documented. And explain our customers that the behaviour is not guaranted if the system is used out of this boundaries.
Sounds crazy? Well, look at a plane. Planes are supposed to be quite safe aren't they? But they're only safe in certain conditions. That's why we define flight domains, in which the plane is guaranted to operated safely. And yes, if the pilot forces the plane to leave this safety bubble it may just stall and crash. But that's the pilot (user) responsibility. Nobody in the aerospace industry would claim and guarantee you that the plane flies in ANY possibile condition. Why should we for our systems?
- Francois Audeon