Anonymous Asynchronous Systems: The Case of Failure Detectors

Due the multiplicity of loci of control, a main issue distributed systems have to cope with lies in the uncertainty on the system state created by the adversaries that are asynchrony, failures, dynamicity, mobility, etc. Considering message-passing systems, this paper considers the uncertainty created by the net effect of three of these adversaries, namely, asynchrony, failures, and anonymity. This means that, in addition to be asynchronous and crash-prone, the processes have no identity. Trivially, agreement problems (e.g., consensus) that cannot be solved in presence of asynchrony and failures cannot be solved either when adding anonymity. The paper consequently proposes anonymous failure detectors to circumvent these impossibilities. It has several contributions. First it presents three classes of failure detectors (denoted AP, A∩ and A∑) and show that they are the anonymous counterparts of the classes of perfect failure detectors, eventual leader failure detectors and quorum failure detectors, respectively. The class A∑ is new and showing it is the anonymous counterpart of the class ∑ is not trivial. Then, the paper presents and proves correct a genuinely anonymous consensus algorithm based on the pair of anonymous failure detector classes (A∩, A∑) (“genuinely” means that, not only processes have no identity, but no process is aware of the total number of processes). This new algorithm is not a “straightforward extension” of an algorithm designed for non-anonymous systems. To benefit from A∑, it uses a novel message exchange pattern where each phase of every round is made up of sub-rounds in which appropriate control information is exchanged. Finally, the paper discusses the notions of failure detector class hierarchy and weakest failure detector class for a given problem in the context of anonymous systems

Identification of a new strain of Actinomadura isolated from Saharan soil and partial characterization of its antifungal compounds

One promising strain Actinomadura sp. AC170, isolated from Algerian Saharan soil, with strong antifungal activity against pathogenic and toxinogenic fungi, was selected for further studies. The 16S rRNA results showed a distinct phylogenetic lineage from the other species within the Actinomadura genus. The production of antibiotic substances was investigated using GYEA solid medium. The butanolic extract contained four bioactive spots detected on thin layer chromatography plates. Among these antibiotics, a complex called 170A, which showed the more interesting antifungal activity, was selected and purified by reverse-phase HPLC. This complex is composed of four compounds. Ultraviolet-visible, infrared, mass and H nuclear magnetic resonance spectroscopy studies showed that these molecules contain an aromatic ring substituted by aliphatic chains. These compounds differ from the known antibiotics produced by Actinomadura species.Key words: Actinomadura, taxonomy, antifungal compounds, Saharan soil

Qualité microbiologique du lait cru ovin collecte dans la steppe centrale de l’Algérie

L’objectif de l’étude consiste à évaluer la qualité microbiologique du lait cru ovin collecté localement, en milieu steppique Algérien dans la région de Djelfa situé à 300 km au sud d’Alger. Au total 51 échantillons de lait ont été prélevés pour des fins analytiques. La contamination moyenne est de 2,3.107 cfu/mL pour la flore totale contre 1,1.105 pour les coliformes totaux et 1,5.104 pour les coliformes fécaux. Pour la flore fongique, les laits sont plus contaminés en levures qu’en moisissures (2,4.105 vs 3,4.103 germes/mL respectivement). Les Streptocoques fécaux sont détectés chez 43,14% des échantillons par contre E.coli n’est décelée que chez 17,65% des laits. Par ailleurs, 28% des prélèvements sont positifs vis-à-vis de Clostridium contre 9,8% pour Staphylococcus aureus. En revanche, tous les laits sont exempts de Salmonella. Pour ce qui est de la recherche des anticorps brucelliques, 13,73% des laits analysés sont positifs au Ring-test. La mauvaise qualité du lait reflète bien les conditions déplorables de production et la non-observance des bonnes pratiques hygiéniques lors de la traiteMots-clés : brebis, lait cru, qualité hygiénique, microorganismes pathogènes, Algérie.Microbiological quality of raw ewe’s milk in Algerian middle steppeThe objective of this study was to assess the microbiological quality of raw milk sheep collected in Algerian steppe. A total of 51 samples of bulk milk were examined. The results obtained show the unsatisfactory quality of milk analyzed. The average contamination is 2,3.107 cfu/ml for the total flora against 1,1.105 for total coliforms and 1,5.104 for fecal coliforms. For the fungal flora, milk is more contamined with yeast than fungi (2,4.105 vs 3,4.103 germes/ml respectively). Fecal streptococci were detected in 43,14% of the samples were E.coli is detected in 17,65% of the milk. In addition, 28% of samples are positive for Clostridium against 9,8% for Staphylococcus aureus. All milk are free from Salmonella. In terms of Brucella antibody, 13,73% of milk are ring test positive. The poor quality of milk reflects the deplorable conditions of production and non-observance of good hygiene practices during milking.Keywords : Ewe, Raw milk, Hygienic quality, Pathogens, Algeria

'Nano' Morphology and Element Signatures of Early Life on Earth: A New Tool for Assessing Biogenicity

The relatively young technology of NanoSIMS is unlocking an exciting new level of information from organic matter in ancient sediments. We are using this technique to characterize Proterozoic organic material that is clearly biogenic as a guide for interpreting controversial organic structures in either terrestrial or extraterrestrial samples. NanoSIMS is secondary ion mass spectrometry for trace element and isotope analysis at sub-micron resolution. In 2005, Robert et al. [1] combined NanoSIMS element maps with optical microscopic imagery in an effort to develop a new method for assessing biogenicity of Precambrian structures. The ability of NanoSIMS to map simultaneously the distribution of organic elements with a 50 nm spatial resolution provides new biologic markers that could help define the timing of life s development on Earth. The current study corroborates the work of Robert et al. and builds on their study by using NanoSIMS to map C, N (as CN), S, Si and O of both excellently preserved microfossils and less well preserved, non-descript organics in Proterozoic chert from the ca. 0.8 Ga Bitter Springs Formation of Australia

Diversification in the Archean Biosphere: Insight from NanoSIMS of Microstructures in the Farrel Quartzite of Australia

The nature of early life on Earth is difficult to assess because potential Early Archean biosignatures are commonly poorly preserved. Interpretations of such materials have been contested, and abiotic or epigenetic derivations have been proposed (summarized in [1]). Yet, an understanding of Archean life is of astrobiological importance, as knowledge of early evolutionary processes on Earth could provide insight to development of life on other planets. A recently-discovered assemblage of organic microstructures in approx.3 Ga charts of the Farrel Quartzite (FQ) of Australia [2-4] includes unusual spindle-like forms and a variety of spheroids. If biogenicity and syngeneity of these forms could be substantiated, the FQ assemblage would provide a new view of Archean life. Our work uses NanoSIMS to further assess the biogenicity and syngeneity of FQ microstructures. In prior NanoSIMS studies [5-6], we gained an understanding of nano-scale elemental distributions in undisputed microfossils from the Neoproterozoic Bitter Springs Formation of Australia. Those results provide a new tool with which to evaluate poorly preserved materials that we might find in Archean sediments and possibly in extraterrestrial materials. We have applied this tool to the FQ forms

"Nano" Scale Biosignatures and the Search for Extraterrestrial Life

A critical step in the search for remnants of potential life forms on other planets lies in our ability to recognize indigenous fragments of ancient microbes preserved in some of Earth's oldest rocks. To this end, we are building a database of nano-scale chemical and morphological characteristics of some of Earth's oldest organic microfossils. We are primarily using the new technology of Nano-Secondary ion mass spectrometry (NanoSIMS) which provides in-situ, nano-scale elemental analysis of trace quantities of organic residues. The initial step was to characterize element composition of well-preserved, organic microfossils from the late Proterozoic (0.8 Ga) Bitter Springs Formation of Australia. Results from that work provide morphologic detail and nitrogen/carbon ratios that appear to reflect the well-established biological origin of these 0.8 Ga fossils

Conditions for Set Agreement with an Application to Synchronous Systems

The $k$-set agreement problem is a generalization of the consensus problem: considering a system made up of $n$ processes where each process proposes a value, each non-faulty process has to decide a value such that a decided value is a proposed value, and no more than $k$ different values are decided. While this problem cannot be solved in an asynchronous system prone to $t$ process crashes when $t \geq k$, it can always be solved in a synchronous system; $\lfloor \frac{t}{k} \rfloor +1$ is then a lower bound on the number of rounds (consecutive communication steps) for the non-faulty processes to decide. The {\it condition-based} approach has been introduced in the consensus context. Its aim was to both circumvent the consensus impossibility in asynchronous systems, and allow for more efficient consensus algorithms in synchronous systems. This paper addresses the condition-based approach in the context of the $k$-set agreement problem. It has two main contributions. The first is the definition of a framework that allows defining conditions suited to the $\ell$-set agreement problem. More precisely, a condition is defined as a set of input vectors such that each of its input vectors can be seen as ``encoding'' $\ell$ values, namely, the values that can be decided from that vector. A condition is characterized by the parameters $t$, $\ell$, and a parameter denoted $d$ such that the greater $d+\ell$, the least constraining the condition (i.e., it includes more and more input vectors when $d+\ell$ increases, and there is a condition that includes all the input vectors when $d+\ell>t$). The conditions characterized by the triple of parameters $t$, $d$ and $\ell$ define the class of conditions denoted ${\cal S}_t^{d,\ell}$, $0\leq d\leq t$, $1\leq \ell \leq n-1$. The properties of the sets ${\cal S}_t^{d,\ell}$are investigated, and it is shown that they have a lattice structure. The second contribution is a generic synchronous $k$-set agreement algorithm based on a condition $C\in {\cal S}_t^{d,\ell}$, i.e., a condition suitedto the $\ell$-set agreement problem, for $\ell \leq k$. This algorithm requires at most $\left\lfloor \frac{d-1+\ell}{k} \right\rfloor +1$ rounds when the input vector belongs to $C$, and $\left\lfloor \frac{t}{k} \right\rfloor +1$ rounds otherwise. (Interestingly, this algorithm includes as particular cases the classical synchronous $k$-set agreement algorithm that requires $\left\lfloor \frac{t}{k} \right\rfloor+1$ rounds (case $d=t$ and $\ell=1$), and the synchronous consensus condition-based algorithm that terminates in $d+1$ rounds when the input vector belongs to the condition, and in $t+1$ rounds otherwise (case $k=\ell=1$).

Names Trump Malice: Tiny Mobile Agents Can Tolerate Byzantine Failures

We introduce a new theoretical model of ad hoc mobile computing in which agents have severely restricted memory, highly unpredictable movement and no initial knowledge of the system. Each agent’s memory can store O(1) bits, plus a unique identifier, and O(1) other agents’ identifiers. Inputs are distributed across n agents, and all agents must converge to the correct output value. We give a universal construction that proves the model is surprisingly strong: It can solve any decision problem in NSPACE(n log n). Moreover, the construction is robust with respect to Byzantine failures of a constant number of agents

Overview of the results of the organics PET Study of the cometary samples returned from comet Wild 2 by the Stardust mission

This presenation will provide an overview of the efforts and results produced by the Organics Preliminary Examination Team during their studies of the samples returned from comet Wild 2 by the Stardust spacecraft

Supernova Remnants and Star Formation in the Large Magellanic Cloud

It has often been suggested that supernova remnants (SNRs) can trigger star formation. To investigate the relationship between SNRs and star formation, we have examined the known sample of 45 SNRs in the Large Magellanic Cloud to search for associated young stellar objects (YSOs) and molecular clouds. We find seven SNRs associated with both YSOs and molecular clouds, three SNRs associated with YSOs but not molecular clouds, and eight SNRs near molecular clouds but not associated with YSOs. Among the 10 SNRs associated with YSOs, the association between the YSOs and SNRs can be either rejected or cannot be convincingly established for eight cases. Only two SNRs have YSOs closely aligned along their rims; however, the time elapsed since the SNR began to interact with the YSOs' natal clouds is much shorter than the contraction timescales of the YSOs, and thus we do not see any evidence of SNR-triggered star formation in the LMC. The 15 SNRs that are near molecular clouds may trigger star formation in the future when the SNR shocks have slowed down to <45 km/s. We discuss how SNRs can alter the physical properties and abundances of YSOs.Comment: 24 pages, 5 figures, 1 table, Accepted for publication in the August 2010 edition of the Astronomical Journa