On Synchronous and Asynchronous Interaction in Distributed Systems

When considering distributed systems, it is a central issue how to deal with interactions between components. In this paper, we investigate the paradigms of synchronous and asynchronous interaction in the context of distributed systems. We investigate to what extent or under which conditions synchronous interaction is a valid concept for specification and implementation of such systems. We choose Petri nets as our system model and consider different notions of distribution by associating locations to elements of nets. First, we investigate the concept of simultaneity which is inherent in the semantics of Petri nets when transitions have multiple input places. We assume that tokens may only be taken instantaneously by transitions on the same location. We exhibit a hierarchy of `asynchronous' Petri net classes by different assumptions on possible distributions. Alternatively, we assume that the synchronisations specified in a Petri net are crucial system properties. Hence transitions and their preplaces may no longer placed on separate locations. We then answer the question which systems may be implemented in a distributed way without restricting concurrency, assuming that locations are inherently sequential. It turns out that in both settings we find semi-structural properties of Petri nets describing exactly the problematic situations for interactions in distributed systems.Comment: 26 pages. An extended abstract of this paper appeared in Proceedings 33rd International Symposium on Mathematical Foundations of Computer Science (MFCS 2008), Torun, Poland, August 2008 (E. Ochmanski & J. Tyszkiewicz, eds.), LNCS 5162, Springer, 2008, pp. 16-3

Augmenting High-Level Petri Nets to Support GALS Distributed Embedded Systems Specification

Part 9: Embedded Systems and Petri NetsInternational audienceHigh-level Petri net classes are suited to specify concurrent processes with emphasis both in control and data processing, making them appropriate to specify distributed embedded systems (DES). Embedded systems components are usually synchronous, which means that DES can be seen as Globally-Asynchronous Locally-Synchronous (GALS) systems. This paper proposes to include in high-level Petri nets a set of concepts already introduced for low-level Petri nets allowing the specification of GALS systems, namely time domains, test arcs and priorities. Additionally, this paper proposes external messages and three types of (high-level) asynchronous communication channels, to specify the interaction between distributed components based on message exchange. With these extensions, GALS-DES can be specified using high-level Petri nets. The resulting models include the specification of each component with well-defined boundaries and interface, and also the explicit specification of the asynchronous interaction between components. These models will be used not only to specify the system behavior, but also to be the input for model-checking tools (supporting its verification) and automatic code generation tools (supporting its implementation in software and hardware platforms), giving a contribution to the model-based development approach and hardware-software co-design of DES based on high-level Petri nets

Symmetric and Asymmetric Asynchronous Interaction

We investigate classes of systems based on different interaction patterns with the aim of achieving distributability. As our system model we use Petri nets. In Petri nets, an inherent concept of simultaneity is built in, since when a transition has more than one preplace, it can be crucial that tokens are removed instantaneously. When modelling a system which is intended to be implemented in a distributed way by a Petri net, this built-in concept of synchronous interaction may be problematic. To investigate this we consider asynchronous implementations of nets, in which removing tokens from places can no longer be considered as instantaneous. We model this by inserting silent (unobservable) transitions between transitions and some of their preplaces. We investigate three such implementations, differing in the selection of preplaces of a transition from which the removal of a token is considered time consuming, and the possibility of collecting the tokens in a given order. We investigate the effect of these different transformations of instantaneous interaction into asynchronous interaction patterns by comparing the behaviours of nets before and after insertion of the silent transitions. We exhibit for which classes of Petri nets we obtain equivalent behaviour with respect to failures equivalence. It turns out that the resulting hierarchy of Petri net classes can be described by semi-structural properties. For two of the classes we obtain precise characterisations; for the remaining class we obtain lower and upper bounds. We briefly comment on possible applications of our results to Message Sequence Charts.Comment: 27 pages. An extended abstract of this paper was presented at the first Interaction and Concurrency Experience (ICE'08) on Synchronous and Asynchronous Interactions in Concurrent Distributed Systems, and will appear in Electronic Notes in Theoretical Computer Science, Elsevie

Toward Sequentializing Overparallelized Protocol Code

In our ongoing work, we use constraint automata to compile protocol specifications expressed as Reo connectors into efficient executable code, e.g., in C. We have by now studied this automata based compilation approach rather well, and have devised effective solutions to some of its problems. Because our approach is based on constraint automata, the approach, its problems, and our solutions are in fact useful and relevant well beyond the specific case of compiling Reo. In this short paper, we identify and analyze two such rather unexpected problems.Comment: In Proceedings ICE 2014, arXiv:1410.701

Peripatetic electronic teachers in higher education

This paper explores the idea of information and communications technology providing a medium enabling higher education teachers to act as freelance agents. The notion of a ‘Peripatetic Electronic Teacher’ (PET) is introduced to encapsulate this idea. PETs would exist as multiple telepresences (pedagogical, professional, managerial and commercial) in PET‐worlds; global networked environments which support advanced multimedia features. The central defining rationale of a pedagogical presence is described in detail and some implications for the adoption of the PET‐world paradigm are discussed. The ideas described in this paper were developed by the author during a recently completed Short‐Term British Telecom Research Fellowship, based at the BT Adastral Park

Exploring the virtual classroom: What students need to know (and teachers should consider)

Technological improvements in many countries have meant that institutions offering distance education programmes now have more options available to them to communicate and interact with their students, and increasingly, attention is being turned to the potential of Web2 technologies to facilitate synchronous interaction. This study explores the affordances and limitations of an online virtual classroom, Adobe Connect Pro, when used in the learning programmes of two groups of undergraduate and postgraduate education students. Results indicate that while both groups gained value from using the classroom, they also found it a completely new environment, and one to which many had trouble transferring the interaction and communication skills developed in other contexts. The reasons for this related to three specific areas of knowledge – technical, procedural and operational, that were identified as being critical to student performance in this environment. The study suggests that educators and course designers need to embed strategies into their online offerings to enable students to develop these, if they are to gain substantial benefit from the availability of virtual classrooms. Additionally, the study identified that when making design decisions about online learning environments, it is very much a matter of horses for courses when selecting tools for specific purposes. While the virtual classroom proved useful for developing social connection and a sense of community, it may not be so beneficial for supporting deeper learning