Regulators of IEC 62304 have put a lot of energy into normalizing how to handle SOUPs (Software Of Unknown Provenance) for software of classes B and C (software that is in a position to potentially harm people in a non-benign way). The definition says: “Software that is already developed and generally available and that has not been developed for the purpose of being incorporated into the MEDICAL DEVICE (also known as “off the-shelf software”) or software previously developed for which adequate records of the development PROCESSES are not available”. To sum up: everything that hasn’t been built according to the norm.
What I’ve seen in the trenches indicates that this distrust in SOUPs is a bit misplaced: in my projects, carefully chosen libraries contain several dozen times less bugs than home-made code before verification. Why?
- Released libraries are finished software.
- They are used by many more developers than code being called in only one context and thus have a higher probability that bugs have already been found and fixed.
- The rise of open source software along with excellent processes (automated builds, TDD, gitflow with systematic reviews of pull request…) and psychological motivators (the name of developers permanently and publicly attached to every commit incentivizes perfection in code) has dramatically increased the quality of free libraries compared to ten years ago, when 62304 was first released.
But I understand the theoretical need of regulators: if there was no SOUP policy, it would be too easy to pretend that a major part of the code is a SOUP and not apply the regulation at all. I can’t imagine a norm that doesn’t think that what’s coming from outside of its jurisdiction could be better.
Norms are norms and auditors are paid to verify compliance to a norm, not to argue about how well or bad the norm was written. I’ve heard that SOUPs are one of the top favorite areas for auditors to look for defects in your implementation of IEC 62304 (the other one being risk analysis): be warned.
So how do we handle this mandatory and not-so-useful activity? Here are a few hints to maximize productivity.
- You need a list of dependencies and their versions. In some programming environments (nuget, bower, npm…), there is a clear list of these dependencies and their versions (package.config, package.json, bower.config…): try to generate the SOUP list from these files.
Dependencies in a bower.json file - It’s a good idea to take advantage of this list to perform a thorough inventory of licenses and do what’s required to be clear. For example, many open source libraries require your software documentation (typically online help) to quote them. And maybe you’ll find one or two that’s not free for commercial use and that needs to be replaced – the sooner the better.
- 62304 requires specifications for SOUPs, including performance criteria. This is a tricky business: some of your SOUPs are a lot more complex than your medical device (the base library of your favorite language, the OS): you can’t possibility retro-spec them entirely. My preferred approach is to document the requirements of the main behaviors of the library that you actually use – a projection of its features on your special use case.
- You should always try to wrap the external dependencies of your code in, well, wrapper classes. This prevents this external namespace to creep all over your code. It helps to easily change the library with another functionally similar implementation someday. In the context of SOUPs, the public interface of the wrapper makes very clear which part of the SOUP you use, and which part you don’t. This can serve as a boundary to limit your SOUP specification effort.
The SOUP wrapper acts as a Facade and helps limit the specification and unit testing effort to features that are really used. - 62304 requires you to test these requirements. That’s something developers spontaneously do when choosing a library: make sure the library works, test a few edge cases. But you need to do it again every time you upgrade the library. For the latter reason, I strongly suggest unit tests that you can link to the specification (so that they end up in the traceability matrix) and use to test the mandatory performance requirements (for example by using the MaxTime attribute in NUnit). These unit tests will help you make sure the next version of the library works with very little extra effort.
- When they are available, you could run the unit tests of the library itself and use their results as a proof of quality. You will still have to deal with writing your own requirements and linking them to the tests. In practice my teams often have had problems with libraries having a few failed tests related to features we didn’t use, which triggered cumbersome justification; in this case we just skipped the library unit tests.
- You are required to perform a risk analysis of your SOUPs and add mitigation strategies as required. This is theoretically a good idea, but I’ve often found it very difficult to put in practice with general-purpose libraries, because their impact cannot be bound to a single feature. In some cases – databases, ORMs, mappers – almost any features could potentially be compromised. As always with risk analysis, there is a temptation to assess every possible failure mode, which would lead to an overwhelming analysis that never gets finished. My advice here would be to trust your gut feeling and choose a selected handful of risks where the brainpower consumed performing risk analysis will be most valuable. There are less failure modes in SOUPs than in your code; use your time on the risks that really threaten patients. Don’t get stuck in an impossible thorough analysis of everything that could possibly go wrong in things that are more complex than what you produce.
- You are also required to perform a list of known bugs and assess the risk your software incurs because of them. It’s a demanding endeavor: in practice my projects tend to use dozens of libraries, some of them have hundreds of bugs, and others don’t publish bugs at all; when they do, it is often difficult to tell which versions of the library have the bug without testing them. I suggest you don’t waste your time with this before the end of the project because you are likely to upgrade your libraries until then and because more known bugs are likely to be closed with newer versions. The ROI of this activity seems very low. I would be glad if this requirement was stripped out of the next version, or adapted to be more cost-effective.
- Operating Systems are a special kind of SOUP. Of course you don’t want to retro-specify and test what it took your vendor decades of work with thousands of developers. But there is an alternative approach. These days, a lot of emphasis has been put on cybersecurity for medical devices – and this is good, patient data is sacred and hackers are on the brink of cyberwar to get it. You must harden your OS – and maybe brand it along the way. My recommendation would be for you to specify, document and test the hardened OS and not the base OS. This way, the OS spec is really useful and has a realistic scope.
- SOUPs of SOUPs. Developers often ask how they should handle SOUPs of SOUPs – the dependencies of the libraries themselves. Of course you can’t handle all dependencies recursively, you would be overwhelmed. Treat your direct dependencies; their own dependencies are an implementation detail. The tests that verify the requirements you wrote for what you use in the SOUP will exercise the lines of code of the SOUP dependencies that you actually use. Their possible failure would be ways to trigger the failure mode of the level 1 SOUP that you already considered in your risk analysis; you don’t need to analyze them separately.
Whatever the hardships in producing the required documentation, resist the temptation to code for yourself what others have done. Reinventing the wheel is a waste of time. Remember, your goal in agile development is customer feedback and delight, not library writing. The thrill of writing cutting-edge technical code is what I suspect entices many developers into rolling their own version of existing stuff, and not good project governance; this an area where a responsible mindset – adult self-supervision – is of particular importance. Developing with an agile mindset implies going as fast as you can by removing waste; missed opportunities of good reuse are horrendous waste. Your immature code will always more buggy and more poorly designed than a library written by people dedicated to it, maybe working on it full-time for several years, and that has lived in production for several releases in many different contexts. In this regard I think that the writers of 62304 have done a very dangerous job in discouraging people to use reliable libraries and creating an incentive to write brittle home-made code instead, which would have a very negative effect on overall medical device reliability and safety. A few month ago I stumbled upon a concrete example of this : a developer I know decided to write his own XML generation routine to avoid the lengthy, boring and absurd (according to him) process of documenting an off-the-shelf library. Don’t ever do this. SOUPs are good. Always use SOUPs when they make sense. Accept the pointless burden (automating as much as you can) and write the required doc.
Let me take advantage of this tribune to deeply thank all the open source contributors in the world.
Agile Med Dev 