State-of-the-art of long-term preservation in product lifecycle management

Providing access to digital information for the indefinite future is the intention of long-term digital preservation systems. One application domain that certainly needs to implement such long-term digital preservation processes is the design and engineering industry. In this industry, products are designed, manufactured, and operated with the help of sophisticated software tools provided by product lifecycle management (PLM) systems. During all PLM phases, including geographically distributed cross-domain and cross-company collaboration, a huge amount of heterogeneous digital product data and metadata is created. Legal and economic requirements demand that this product data has to be archived and preserved for a long-time period. Unfortunately, the software that is able to interpret the data will become obsolete earlier than the data since the software and hardware lifecycle is relatively short-lived compared to a product lifecycle. Companies in the engineering industry begin to realize that their data is in danger of becoming unusable while the products are in operation for several decades. To address this issue, different academic and industrial initiatives have been initiated that try to solve this problem. This article provides an overview of these projects including their motivations, identified problems, and proposed solutions. The studied projects are also verified against a classification of important aspects regarding scope and functionality of digital preservation in the engineering industry. Finally, future research topics are identified.     

On the complexity of deciding avoidability of sets of partial words

Blanchet-Sadri et al. have shown that Avoidability, or the problem of deciding the avoidability of a finite set of partial words over an alphabet of size k≥2, is NP-hard [F. Blanchet-Sadri, R. Jungers, J. Palumbo, Testing avoidability on sets of partial words is hard, Theoret. Comput. Sci. 410 (2009) 968-972]. Building on their work, we analyze in this paper the complexity of natural variations on the problem. While some of them are NP-hard, others are shown to be efficiently decidable. Using some combinatorial properties of de Bruijn graphs, we establish a correspondence between lengths of cycles in such graphs and periods of avoiding words, resulting in a tight bound for periods of avoiding words. We also prove that Avoidability can be solved in polynomial space, and reduces in polynomial time to the problem of deciding the avoidability of a finite set of partial words of equal length over the binary alphabet. We give a polynomial bound on the period of an infinite avoiding word, in the case of sets of full words, in terms of two parameters: the length and the number of words in the set. We give a polynomial space algorithm to decide if a finite set of partial words is avoided by a non-ultimately periodic infinite word. The same algorithm also decides if the number of words of length n avoiding a given finite set of partial words grows polynomially or exponentially with n.     

On the number of term orders

By an admissible order on a finite subsetQ of ⁿ we mean the restriction toQ of a linear order on ⁿ compatible with the group structure of ⁿ and such that ⁿ is contained in the positive cone of the order. We first derive upper and lower bounds on the number of admissible orders on a given setQ in terms of the dimensionn and the cardinality ofQ. Better estimates are possible if the setQ enjoys symmetry properties and in the case whereQ is a discrete hyperbox of the form In the latter case, we also give asymptotic results as d _k for fixedn. We finally present algorithms which compute the set of admissible orders that extend a given binary relation onQ and their number. The algorithms are useful in connection with the construction of universal Gröbner bases.AMS Classification: primary 06F20 secondary 06-04, 11N25     

Best Practice in Managing HTTP-Based Client Sessions

Most organizations now have substantial investments in their online Internet presences. For major financial institutions and retailers, the Internet provides both a cost effective means of presenting their offerings to customers, and a method of delivering a personalised 24/7 presence. In almost all cases, the preferred method of delivering these services is over common HTTP. Due to limitations within the protocol, there is no in-built facility to identify or track a particular customer (or session) uniquely within an application. Thus the connection between the customer’s Web browser and the organisation's Web service is commonly referred to as being “stateless”. Because of this, organizations have been forced to adopt custom methods of managing client sessions if they wish to maintain state.An important aspect of correctly managing state information through session IDs relates directly to authentication processes. While it is possible to insist that a client using an organization's Web application provide authentication information for each “restricted” page or data submission, it would soon become tedious and untenable. Thus session IDs are not only used to follow clients throughout the Web application, they are also used to identify each unique, authenticated user — thereby indirectly regulating access to site content or information.     

A Contactless Inductive Velocity Reconstruction Method for Metallic and Semiconducting Melts

The knowledge of the flow in metallic and semiconducting melts is important for a number of industrial problems. Optical methods cannot be applied due to the opaqueness of those fluids. Ultrasonic methods have problems with wetting and thermal stability when applied in hot metallic melts. Evidently, contactless methods for velocity determination would be highly desirable. In some applications, even a coarse knowledge of the flow topology and the direction of the main eddies would be of high value. The study examines the possibility of velocity reconstruction in electrically conducting fluids from external measurements of different induced magnetic fields. It is shown that, for a reliable velocity reconstruction, the effect of the electric potential at the fluid boundary on the induced magnetic field should be taken into account.