Editorial: special section on dependable real-time systems

Abstract

VER time, the discipline of computing systems has evolved and expanded in a multifaceted manner to cover many distinct and (sometimes) disparate streams. These encompass themes in system architecture, performance modeling, real-time, and fault tolerance, among several others. Many of these areas (fortunately or unfortunately?) have evolved separately and become established as discrete fields, although the overall design of computers and applications still, and inherently, involves their synergistic association. To open a topic of debate: This synergism is often lacking or, if present, not particularly acknowledged as coming from the pertinent area and, at times, reinvented. Among the offshoots of the broad discipline of computing systems, fault tolerance and real-time, in particular, are increasingly being viewed as areas with potential for synergy. The concept of fault tolerance is expanding in scope to address the broader area of dependability: reliability, performability, availability, safety, the consideration of temporal properties in defining “correct” services towards the goal of reliably delivering desired services at specified times. Similarly, the real-time community is increasingly incorporating notions of dependability within its definitions of provision of timely services. Realistically, the entire range of current system designs is reaching a complexity level that necessitates joint consideration of fault tolerance and real-time features, regardless of which gets primary emphasis. Examples abound: communication networks, transaction processing, and flight, industrial, medical, or automotive control all clearly require consideration of both dependability and timeliness. It is of interest to note that the two communities tackle largely overlapping sets of problems which can be broadly stated as: “assured delivery of correctly executed services” and constraints, i.e., “resource management, guarantees, and efficiency.” Interestingly, the fields of fault tolerance and real-time address these problems with different perspectives and aim at optimizing different sets of objectives within the same problems. For example, the fault tolerance emphasis involves reliability of the mechanisms providing for delivery of correctly executed services; the real-time community puts the emphasis on guaranteeing delivery of services in a timely manner. Similarly, resource management issues are addressed as redundancy management issues, versus issues of efficient use of uniprocessor/multiprocessors for task allocation from the realtime systems perspective. It is actually quite a long list of issues that get addressed differently by the fault-tolerant and real-time viewpoints. Moreover, the two fields rely on very similar fundamentals and techniques from engineering, mathematics, and theory. Indeed, at times this fosters a very healthy development of ideas and solutions from varied perspectives; often enough, it involves reinventing the wheel. Both the disciplines of fault tolerance and real-time have matured, with their individual seminal contributions, development of terminology, and conceptual basis. Perhaps this maturity now fosters a better chance in understanding complementary issues which have, over time, been developed with different perspectives, emphasis, and presentation. However, as they do tackle the common problem of “assured delivery of correctly executed services,” it is the intent of this special issue to address and encourage the integration of these two (perceived?) diverse areas toward a cohesive formulation of dependable real-time service and systems. In many ways, the notion and popularity of the term “Quality of Service (QoS)” already represents the juxtaposition of many fault-tolerant and real-time services. This special issue on dependable real-time systems endeavors to develop the area of systems with composite attributes of real-time and dependability within a common framework. The thrust is on understanding system models, identifying issues and concepts used across both communities for design and analysis, and elucidating the context and relevance of concepts synergistically across the areas of dependability and real time. The flavor of this special issue is both theoretical and experimental in scope to expand the underlying principles of the areas of both dependability and real-time. We paraphrase some topics from the call for papers which we believe depict some of the areas where there is significant work needed to link the fields of dependability and real-time. These include (in part):