Diversity for off-the-Shelf Components

"Commercial-off-the-shelf" (COTS) or, generally, "offthe-shelf" (OTS) software items are increasingly used in building systems, instead of only relying on bespoke software items1. This trend is driven by a wish to reduce costs, and by some hope that greater re-use of software may lead to higher quality (via more feedback from use). Thus, for instance, the U.S. Dept of Defence policy is now to encourage the use of COTS items. This trend extends to critical systems with high dependability requirements, like a computer-based railway signalling systems by Alcatel (Austria) [1]. A serious problem with OTS items (software, but also complex digital hardware) is that they often lack the guarantee of good development practice, and the extensive documentation of it, which are traditionally the basis for accepting/certifying software for critical applications. Even for commercial applications with modest dependability requirements, using OTS items requires some trust that they will not become a "weak link", making the final product intolerably unreliable. If the OTS items have already seen much operational use, this experience could be used to forecast their dependability in a new context; but this experience is seldom documented with sufficient accuracy and detail to allow confident predictions. Much of the on-going discussion about OTS items addresses this issue. A notorious case of a U.S. warship being disabled by a crash of Windows NT in 1997 [2] is often quoted to illustrate the problems with dependence on COTS items. The debate is open about how COTS items could be certified to have sufficient reliability but what is clear is that a solution will not be widely available any time soon.