Synchronous atomic broadcast for redundant broadcast channels

We propose a synchronous atomic broadcast protocol for distributed real-time systems based on redundant broadcast channels. The protocol can tolerate a finite number f of concurrent processor crash failures, channel adapter performance failures and channel omission failures. Its message cost is optimal: when no failures occur only f+1 messages are sent per broadcast. The cost implications of providing tolerance to other failure classes are also investigated.     

Probabilistic clock synchronization

A probabilistic method is proposed for reading remote clocks in distributed systems subject to unbounded random communication delays. The method can achieve clock synchronization precisions superior to those attainable by previously published clock synchronization algorithms. Its use is illustrated by presenting a time service which maintains externally (and hence, internally) synchronized clocks in the presence of process, communication and clock failures. Flaviu Cristian is a computer scientist at the IBM Almaden Research Center in San Jose, California. He received his PhD from the University of Grenoble, France, in 1979. After carrying out research in operating systems and programming methodology in France, and working on the specification, design, and verification of fault-tolerant programs in England, he joined IBM in 1982. Since then he has worked in the area of fault-tolerant distributed protocols and systems. He has participated in the design and implementation of a highly available system prototype at the Almaden Research Center and has reviewed and consulted for several fault-tolerant distributed system designs, both in Europe and in the American divisions of IBM. He is now a technical leader in the design of a new U.S. Air Traffic Control System which must satisfy very stringent availability requirements.     

Prognostic significance of 25‐hydroxivitamin D entirely explained by a higher comorbidity burden: Experience from a South‐Eastern European Dialysis Cohort

Vitamin D deficiency is still a common problem particularly in the elderly and in individuals with various degrees of renal impairment. The present study aimed to evaluate the association between plasma concentrations of 25(OH)D and death in a large cohort of prevalent patients on hemodialysis (HD) from south‐east Romania, a typical Balkan region. This is an observational prospective study that included a total of 570 patients on maintenance HD. Study patients were classified into three groups by baseline 25(OH)D levels: (1) sufficient 25(OH)D—i.e., >30 ng/mL; (2) insufficient 25(OH)D—i.e., between 10 and 29 ng/mL; and (3) deficient 25(OH)D—i.e., <10 ng/mL. During the follow‐up period of 14 months, 68 patients (11.9%) died, the Kaplan–Meier analysis showing significant differences in all‐cause mortality for chronic kidney disease patients in different 25(OH)D groups (P = 0.002). Unadjusted Cox regression analysis also showed significant differences in survival. The multivariate Cox regression model showed no significant differences in survival according to vitamin D levels. Hazard ratio for death in the “<10 ng/mL” group was 1.619 (P = 0.190) and in the “10–30 ng/mL” group was 0.837 (P = 0.609). In our dialysis population with a high comorbidity burden, low 25(OH)D concentration was not associated with mortality in the adjusted Cox model, suggesting that vitamin D deficiency could represent only a non‐specific marker for a poor health status, with less impact on mortality.     

Reaching agreement on processor-group membrship in synchronous distributed systems

Reaching agreement on the identity of correctly functioning processors of a distributed system in the presence of random communication delays, failures and processor joins is a fundamental problem in fault-tolerant distributed systems. Assuming a synchronous communication network that is not subject to partition occurrences, we specify the processor-group membership problem and we propose three simple protocols for solving it. The protocols provide all correct processors with consistent views of the processor-group membership and guarantee bounded processor failure detection and join delays. Flaviu Cristian is a computer scientist at the IBM Almaden Research Center in San Jose, California. He received his PhD from the University of Grenoble, France, in 1979. After carrying out research in operating systems and programming methodology in France and working on the specification, design, and verification of fault-tolerant software in England, he joined IBM in 1982. Since then he has worked in the area of fault-tolerant distributed systems and protocols. He has participated in the design and implementation of a highly available distributed system prototype at the Almaden Research Center, has reviewed and consulted for several fault-tolerant distributed system designs, both in Europe and the American divisions of IBM, and is now a technical leader in the design of a new Air Traffic Control System for the US which must satisfy very stringent availability requirements.     

Designing biocompatible Ti-based metallic glasses for implant applications

Ti-based metallic glasses show high potential for implant applications; they overcome in several crucial respects their well-established biocompatible crystalline counterparts, e.g. improved corrosion properties, higher fracture strength and wear resistance, increased elastic strain range and lower Young's modulus. However, some of the elements required for glass formation (e.g. Cu, Ni) are harmful for the human body. We critically reviewed the biological safety and glass forming tendency in Ti of 27 elements. This can be used as a basis for the future designing of novel amorphous Ti-based implant alloys entirely free of harmful additions. In this paper, two first alloys were developed: Ti₇₅Zr₁₀Si₁₅ and Ti₆₀Nb₁₅Zr₁₀Si₁₅. The overheating temperature of the melt before casting can be used as the controlling parameter to produce fully amorphous materials or bcc-Ti-phase reinforced metallic glass nano-composites. The beneficial effect of Nb addition on the glass-formation and amorphous phase stability was assessed by X-ray diffraction, transmission electron microscopy and differential scanning calorimetry. Crystallization and mechanical behavior of ribbons are influenced by the amount and distribution of the nano-scaled bcc phase existing in the as-cast state. Their electrochemical stability in Ringer's solution at 310 K was found to be significantly better than that of commercial Ti-based biomaterials; no indication for pitting corrosion was recorded.     

Simulation-based Testing of Communication Protocols for Dependable Embedded Systems

We present a novel approach to testing fault-tolerant and real-time protocol implementations. Cesium, our testing environment, executes the protocols in a centralized simulator of the distributed system. It simulates the occurrence of inputs and the failure scenarios the protocols are designed to tolerate, while automatically verifying that the required safety and timeliness properties hold at all times during test experiments. Within this framework, the human tester can define failure operations that simulate every failure class studied in the literature. We apply our approach to two fault-tolerant protocols typical in embedded systems. The results show that Cesium can pinpoint implementation errors that would be very difficult to identify in a real system, and can also compute accurate performance predictions that would be problematic to measure in the real embedded platform without ad hoc hardware instrumentation.     

Ex-vivo model of colon cancer in normothermic conditions: Applications in nanomedicine

ABSTRACT

Colorectal cancer is one of the most accessible experimental models for the study of neoplasia, being widely used in experimental nanomedicine. Although in-vitro and in-vivo models are available and commonly used in this field, the experimental results cannot accurately predict the clinical efficiency of the tested nanobioconjugate. Ex-vivo models are particularly important in research, providing the most similar conditions to natural ones. Currently, there are few such models, existing ones being done under hypothermia, this resulting in modification of cellular metabolism. Taking this in account, we considered important to test an ex-vivo experimental colon cancer model, experiment conducted under normothermia, whose feasibility has been demonstrated previously by our team. In the present study, we underscored the usefulness that such an experimental model may have in nanomedicine, by demonstrating a positive gold nanoparticles accumulation in tumor cells by passive targeting, while maintaining cell viability for a sufficient period of time.     

Integrating External and Internal Clock Synchronization

We address the problem of how to integrate fault-tolerant external and internal clock synchronization. In this paper we propose a new external/internal clock synchronization algorithm which provides both external and internal clock synchronization for as long as a majority of the reference time servers (servers with access to reference time) stay correct. When half or more of the reference time servers are faulty, the algorithm degrades to a fault-tolerant internal clock synchronization algorithm. We prove that at least 2 F+1 reference time servers are necessary for achieving external clock synchronization when up to F reference time servers can suffer arbitrary failures, thus the proposed algorithm provides maximum fault-tolerance. In this paper we also derive lower bounds for the best maximum external deviation achievable in standard mode and the best drift rate achievable in degraded mode. Our algorithm is optimal with respect to these two bounds: (1) the maximum external deviation is optimal in standard mode, and (2) the drift rate of the clocks is optimal in standard and degraded mode.