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.