Developing software for safety-critical systems? This book is for you

In-depth volume covers development of systems under the IEC 61508, ISO 26262, EN 50128, and IEC 62304 standards

In June, I told you of an upcoming book by my colleague Chris Hobbs, who works as a software safety specialist here at QNX Software Systems. Well, I’m happy to say that the book is now available. It’s called Embedded Software Development for Safety-Critical Systems and it explores design practices for building medical devices, railway control systems, industrial control systems, and, of course, automotive ADAS devices.

The book:

• covers the development of safety-critical systems under ISO 26262, IEC 61508, EN 50128, and IEC 62304
• helps developers learn how to justify their work to external auditors
• discusses the advantages and disadvantages of architectural and design practices recommended in the standards, including replication and diversification, anomaly detection, and so-called “safety bag” systems
• examines the use of open-source components in safety-critical systems

Interested? I invite to you to visit the CRC Press website, where you can view the full Table of Contents and, of course, order the book.

A version of this post originally appeared on the QNX Auto Blog.

Developing software for safety-critical systems? Have I got a book for you

Chris Hobbs is the only person I know who holds a math degree with a specialization in mathematical philosophy. In fact, before I met him, I didn’t know such a thing even existed. But guess what? That’s one of the things I really like about Chris. The more I hang out with him, the more I learn.

Come to think of it, helping people learn has become something of a specialty for Chris. He is, for example, a flying instructor and the author of Flying Beyond: The Canadian Commercial Pilot Textbook. And, as a software safety specialist at QNX Software Systems, he regularly provides advice to customers building systems that must comply with functional safety standards like IEC 61508, EN 5012x, and ISO 26262.

Chris has already written a number of papers on software safety, some of which I have had the great privilege to edit. You can find several of them on the QNX website. But recently, Chris upped the ante and wrote an entire book on the subject, titled Embedded Software Development for Safety-Critical Systems. The book:

• covers the development of safety-critical systems under ISO 26262, IEC 61508, EN 50128, and IEC 62304
• helps readers understand and apply remarkably esoteric development practices and be prepared to justify their work to external auditors
• discusses the advantages and disadvantages of architectural and design practices recommended in the standards, including replication and diversification, anomaly detection, and so-called “safety bag” systems
• examines the use of open-source components in safety-critical systems

I haven’t yet had a chance to review the book, but at 358 pages, it promises to be a substantial read.

Interested? Well, you can’t get the book just yet. But you can pre-order it today and get one of the first copies off the press. It’s scheduled for release September 1.

A version of this post appeared in the QNX Auto Blog.

Breaking up is hard to do

Separation can be painful. But often, the failure to separate can result in even more pain over the long haul.

No, I’m not talking love, marriage, or other affairs of the human heart. I am talking software design. In particular, the design of complex software systems that must perform safety-critical functions. The software, for example, in a medical device, automotive ADAS unit, or train-control system.

In systems like these, separation is critical: software components must be cleanly isolated from one another. Otherwise, you risk the chance that the behavior of one component will inadvertently interfere with the behavior of another. For this reason, component isolation is a key thrust of functional safety standards like IEC 61508 and ISO 26262.

Several forms of interference, all undesirable.

Interference can take many forms. For instance, a component could improperly use file descriptors or flash memory needed by other components. Or it could enter a tight loop under a failure condition and starve a more-critical component of CPU time. Or it could write to the private memory of another component.

You could, of course, run every component on separate hardware. But that becomes an expensive proposition. Moreover, the market trend is toward hardware consolidation, which, for reasons of economy, merges previously discrete systems onto a single platform.

It’s important, then, to embrace software-based separation techniques. These include OS mechanisms to prevent resource deprivation, time starvation, data corruption, and so on. For instance, the adaptive time partitioning provided by the QNX Neutrino OS can ensure that a software component always gets a minimum percentage of CPU time, whenever it needs it. That way, other components can't prevent it from running, either unintentionally or maliciously.

Software separation is as much art as science. In fact, my colleague Yi Zheng goes further than that. She argues that there is as yet no precise methodology for separating system functions. There are no textbooks, no pat answers.

So is separation only a matter of asking the right questions? That would be an oversimplification, of course. Skill also comes into play, as does experience, not to mention a good dose of thoroughness. But really, you should read Yi’s article, “The Art of Separation”, in Electronic Design and judge for yourself.

Can a safety-critical system be over-engineered?

Too much of a good thing?

It's a rhetorical question, of course. But hear me out.

As you can imagine, many safe systems must be designed to handle scenarios outside their intended scope. For instance, in many jurisdictions, passenger elevators must be capable of handling 11 times more weight than their recommended maximum — you just never know what people will haul into an elevator car. So, if the stated limit for a passenger elevator is 2000 pounds, the actual limit is closer to 22,000 pounds. (Do me a favor and avoid the temptation to test this for yourself.)

Nonetheless, over-engineering can sometimes be too much of a good thing. This is especially true when an over-engineered component imposes an unanticipated stress on the larger system. In fact, focusing on a specific safety issue without considering overall system dependability can sometimes yield little or no benefit — or even introduce new problems. The engineer must always keep the big picture in mind.

Case in point: the SS Eastland. In 1915 this passenger ship rolled over, killing more than 840 passengers and crew. The Eastland Memorial Society explains what happened:

"...the Eastland's top-heaviness was largely due to the amount and weight of the lifeboats required on her... after the sinking of the Titanic in 1912, a general panic led to the irrational demand for more lifesaving lifeboat capacity for passengers of ships.

Lawmakers unfamiliar with naval engineering did not realize that lifeboats cannot always save all lives, if they can save any at all. In conformance to new safety provisions of the 1915 Seaman’s Act, the lifeboats had been added to a ship already known to list easily... lifeboats made the Eastland less not more safe..."

There you have it. A well-intentioned safety feature that achieved the very opposite of its intended purpose.

Fast forward to the 21st century. Recently, my colleague Chris Hobbs wrote a whitepaper on how a narrow design approach can subtly work its way into engineering decisions. Here's the scenario he uses for discussion:

"The system is a very simple, hypothetical in-cab controller (for an equally hypothetical) ATO system running a driverless Light Rapid Transit (LRT) system...

Our hypothetical controller has already proven itself in Rome and several other locations. Now a new customer is considering it for an LRT ATO in the La Paz-El Alto metropolitan area in Bolivia. La Paz-El Alto has almost 2.5 million inhabitants living at an elevation that rises above 4,100 meters (13,600 ft.—higher than Mount Erebus). This is a significant change in context, because the threat of soft and hard memory errors caused by cosmic rays increases with elevation. The customer asks for proof that our system can still meet its safety requirements when the risk of soft memory errors caused by radiation is included in our dependability estimates..."

So where should the engineer go from here? How can he or she ensure that the right concerns are being addressed? That is what Chris endeavours to answer. (Spoiler alert: The paper determines that, in this hypothetical case, software detection of soft memory errors isn't a particularly useful solution.)

Highly recommended.

10 truths about building safe embedded software systems

I wish I could remember his exact words. But it has been a long time — 20 years — and my memory has probably added words that he never wrote and removed words that he did write. That said, this is how I remember it:

"We all strive to write bug-free code. But in the real world, bugs can and do occur. Rather than pretend this isn't so, we should adopt a mission-critical mindset and create software architectures that can contain errors and recover from them intelligently."

The "he" in question is my late (and great) colleague Dan Hildebrand. I'm sure that Dan's original sentences were more nuanced and to the point. But the important thing is that he grokked the importance of "culture" when it comes to designing software for safety-critical systems. A culture in which the right attitudes and the right questions, not just the right techniques, are embraced and encouraged.

Which brings me to a paper written by my colleagues Chris Hobbs and Yi Zheng. It's titled "Ten truths about building safe embedded software systems" and, sure enough, the first truth is about culture. I quote:

"A safety culture is not only a culture in which engineers are permitted to raise questions related to safety, but a culture in which they are encouraged to think of each decision in that light..."

I was particularly delighted to read truth #5, which echoes Dan's advice with notable fidelity:

"Failures will occur: build a system that will recover or move to its design safe state..."

I also remember Dan writing about the importance of software architectures that allow you to diagnose and repair issues in a field-deployed system. Which brings us to truth #10:

"Our responsibility for a safe system does not end when the product is released. It continues until the last device and the last system are retired."

Dan argued for the importance of these truths in 1993. If anything, they are even more important today, when so much more depends on software. If you care about safe software design, you owe it to yourself to read the paper.

QNX provides OS for new IEC 61508 certified robotics middleware

This just in: Systems Engineering Consultants (SEC), a leading realtime technology company in Japan, has developed new robotics middleware that runs on the QNX Neutrino RTOS Safe Kernel. Like the Safe Kernel, SEC’s middleware is certified to the IEC 61508 standard at Safety Integrity Level 3, or SIL3. (If you’re new to IEC 61508, this certification provides independent validation that a product offers a very high level of reliability when used in safety-critical systems.)

SEC designed the middleware, dubbed RTMSafety, to help manufacturers create safety-related systems for robots, including factory automation robots and personal-care robots used in medical and elderly care settings. RTMSafety allows robotics elements, such as actuators and sensors, to be treated as modular, reusable components.

According to Shintaro Sakurai, an executive director in the engineering division of SEC, “SEC has been working with industry groups and robotics societies to promote component standardization, which we believe will eliminate cost issues that have prevented commercialization of robotics. After much R&D effort, we are getting ready to move into the business phase to offer our customers an IEC 61508 certified middleware platform to run on the QNX Neutrino RTOS Safe Kernel.”

Suggested reading
To learn more about RTMSafety, read the press release.

To learn more about the design of safety-critical systems, read these whitepapers:
Using an IEC 61508-Certified RTOS Kernel for Safety-Critical Systems
Building Functional Safety into Complex Software Systems, Part I
Building Functional Safety into Complex Software Systems, Part II

And to learn more about QNX Software Systems' certified operating systems, visit the QNX Neutrino product page.