Re: [SystemSafety] Modelling and coding guidelines: "Unambiguous Graphical Representation"

From: Peter Bernard Ladkin < >
Date: Wed, 16 Mar 2016 08:23:26 +0100


On 2016-03-16 00:58 , Les Chambers wrote:
> The issue of making a safety case with the argument of proven in use
> resonates with me

Good. You've unfortunately missed the open-comment period on the final draft of the TS, though. For info, the process has been ongoing since Spring 2013.

> At the point of making
> the safety case we had roughly 200 control computers operating in the field
> for a year with no failures or safety incidents. The accumulated operating
> hours allowed us to use certain equations that looked fantastic on paper and
> were probably the most compelling element of the case. In my engineering
> black-and-white heart though, I know this was complete BS.

The equations aren't the point. It is the conditions on the data that are the point.

How were the usage logs?

If you couldn't have done all of that (and more), then the TS would say that your SW is not (yet) "proven in use".

> But your disclosure that:
> 'The IEC is about to publish a technical specification on the criteria to be
> fulfilled for a component to be considered "proven in use".'
> ... is of grave concern to me,

> 1. The concept of "same" does not apply to a software product. No two
> software products are the "same", ever.

It certainly - and obviously - does apply.

If two instantiations are not identical then you can't lump them together for the purpose of joint evaluation. People have been clear on that for four decades.

> 2. The concept of "identical environment" does not apply to software. As
> with item 1 there is no such thing. In the software context, there is no
> human verifiable criteria you could apply to defining one environment as
> identical to another.

Sure there is. The SW receives inputs. You can know what those parameters are. You better know what those parameters are. If you don't know what they are, you have other problems than trying to assure the SW for your system component as "proven in use". For example, you couldn't perform an adequate hazard analysis.

> What is the IEC thinking! If what they're doing involves software they are
> formalising and promulgating utter BS.

First, let me refer you to the current edition of IEC 61508, Parts 2 and 3. The conditions on "proven in use" for SW are to my mind incoherent. So I would agree with you that, as things stand, IEC 61508 is currently promoting an inappropriate approach to this.

It will continue to do so until the TS is published. The point of the TS is to try to fix this anomaly by giving more appropriate guidance for SW.

However, I'm not sure it's appropriate to describe IEC as "thinking". It might be worth a couple of words as to how it functions. It is a facilitating organisation which provides and supports a process of turning a project into an international standard. Anyone can propose a standardisation project. If it meets certain preliminary tests, then it will be proposed by a national standards organisation (that of the originator) to the IEC, a call goes out from the IEC to national standards organisations worldwide, and then from those national standards organisations to engineers generally, for volunteers to participate on the project. Those people are often designated by their organisations who have an interest in a resulting standard. Sometimes, as in my case, we just do it because we think (or hope!) it is a valuable thing to do.

In this case, someone thought it would be a good idea to have coherent guidance on evaluating safety-critical SW as "proven in use", and proposed it to the IEC. I can actually name names. Indeed, four people on this list are in the IEC working group.

This is not the first time the subject of evaluation of existing SW has been discussed here. It would have been appropriate for comments to be directed to the IEC at an earlier point. All IEC projects have to address each submitted comment. The comments process is pretty bureaucratic, but it is designed in part to weed out non-serious commentary and to direct commentary specifically towards changing an existing document: you have to specify exactly which paragraphs or sentences in the document are unsatisfactory, why they are unsatisfactory, and explicitly suggest a substitute, including omission if appropriate.

This is common to the ISO, IEC, CENELEC. There are people here who think the standards process is broken; indeed I think it is broken in certain respects, but I think the commentary aspect works moderately well, even though there is plenty of room for improvement. A recent revision of a European standard for safety-critical digital systems on railways was abandoned, because the draft garnered 5,000 comments or so, and they couldn't be worked through by the committee in the time period available for completion of the project. The project had to be formally abandoned. That is how the fact of no consensus translates into a result.

Two main failings of the standardisation process from my point of view are that:

> With this kind of Stone Age thinking they should go back to standardising
> nuts and bolts.

People have accused me of all sorts of mental misdemeanors throughout the years, but "Stone Age thinking" is genuinely new. For the record, my leopard skins are all faux but my knotty club is real enough.

PBL Prof. Peter Bernard Ladkin, Faculty of Technology, University of Bielefeld, 33594 Bielefeld, Germany Je suis Charlie
Tel+msg +49 (0)521 880 7319 www.rvs.uni-bielefeld.de



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