242 research outputs found
Fast Lean Erasure-Coded Atomic Memory Object
In this work, we propose FLECKS, an algorithm which implements atomic memory objects in a multi-writer multi-reader (MWMR) setting in asynchronous networks and server failures. FLECKS substantially reduces storage and communication costs over its replication-based counterparts by employing erasure-codes. FLECKS outperforms the previously proposed algorithms in terms of the metrics that to deliver good performance such as storage cost per object, communication cost a high fault-tolerance of clients and servers, guaranteed liveness of operation, and a given number of communication rounds per operation, etc. We provide proofs for liveness and atomicity properties of FLECKS and derive worst-case latency bounds for the operations. We implemented and deployed FLECKS in cloud-based clusters and demonstrate that FLECKS has substantially lower storage and bandwidth costs, and significantly lower latency of operations than the replication-based mechanisms
Highly Scalable Algorithms for Robust String Barcoding
String barcoding is a recently introduced technique for genomic-based
identification of microorganisms. In this paper we describe the engineering of
highly scalable algorithms for robust string barcoding. Our methods enable
distinguisher selection based on whole genomic sequences of hundreds of
microorganisms of up to bacterial size on a well-equipped workstation, and can
be easily parallelized to further extend the applicability range to thousands
of bacterial size genomes. Experimental results on both randomly generated and
NCBI genomic data show that whole-genome based selection results in a number of
distinguishers nearly matching the information theoretic lower bounds for the
problem
ARES: Adaptive, Reconfigurable, Erasure coded, atomic Storage
Atomicity or strong consistency is one of the fundamental, most intuitive,
and hardest to provide primitives in distributed shared memory emulations. To
ensure survivability, scalability, and availability of a storage service in the
presence of failures, traditional approaches for atomic memory emulation, in
message passing environments, replicate the objects across multiple servers.
Compared to replication based algorithms, erasure code-based atomic memory
algorithms has much lower storage and communication costs, but usually, they
are harder to design. The difficulty of designing atomic memory algorithms
further grows, when the set of servers may be changed to ensure survivability
of the service over software and hardware upgrades, while avoiding service
interruptions. Atomic memory algorithms for performing server reconfiguration,
in the replicated systems, are very few, complex, and are still part of an
active area of research; reconfigurations of erasure-code based algorithms are
non-existent.
In this work, we present ARES, an algorithmic framework that allows
reconfiguration of the underlying servers, and is particularly suitable for
erasure-code based algorithms emulating atomic objects. ARES introduces new
configurations while keeping the service available. To use with ARES we also
propose a new, and to our knowledge, the first two-round erasure code based
algorithm TREAS, for emulating multi-writer, multi-reader (MWMR) atomic objects
in asynchronous, message-passing environments, with near-optimal communication
and storage costs. Our algorithms can tolerate crash failures of any client and
some fraction of servers, and yet, guarantee safety and liveness property.
Moreover, by bringing together the advantages of ARES and TREAS, we propose an
optimized algorithm where new configurations can be installed without the
objects values passing through the reconfiguration clients
RADON: Repairable Atomic Data Object in Networks
Erasure codes offer an efficient way to decrease storage and communication costs while implementing atomic memory service in asynchronous distributed storage systems. In this paper, we provide erasure-code-based algorithms having the additional ability to perform background repair of crashed nodes. A repair operation of a node in the crashed state is triggered externally, and is carried out by the concerned node via message exchanges with other active nodes in the system. Upon completion of repair, the node re-enters active state, and resumes participation in ongoing and future read, write, and repair operations. To guarantee liveness and atomicity simultaneously, existing works assume either the presence of nodes with stable storage, or presence of nodes that never crash during the execution. We demand neither of these; instead we consider a natural, yet practical network stability condition N1 that only restricts the number of nodes in the crashed/repair state during broadcast of any message.
We present an erasure-code based algorithm RADON_{C} that is always live, and guarantees atomicity as long as condition N1 holds. In situations when the number of concurrent writes is limited, RADON_{C} has significantly improved storage and communication cost over a replication-based algorithm RADON_{R}, which also works under N1. We further show how a slightly stronger network stability condition N2 can be used to construct algorithms that never violate atomicity. The guarantee of atomicity comes at the expense of having an additional phase during the read and write operations
Nature of light rain during presence and absence of bright band
This paper reports the evolution of rain drop size distribution (DSD) during bright band (BB) and no-BB (NBB) conditions of low intensity rainfall events as observed by a vertically pointing Micro Rain Radar (MRR) over Pune (18.58°N, 73.92°E), India. The BB is identified by enhanced radar reflectivity factor Z (dBZ) at the 0°C isotherm. The gradient of hydrometeor fall velocity is found to be a good indicator in identifying the melting layer when enhanced radar reflectivity at melting layer is not prominent. The storm structures as observed by the MRR are compared with CloudSat observations that provide evidence of ice hydrometeor at ~ −60°C with clear indication of BB at 0°C. Storm heights at warmer than 0°C are evident during NBB conditions from CloudSat. This suggests that warm rain processes are responsible for producing rain during NBB conditions. During BB conditions, bimodal DSDs below the melting layer are observed at lower altitudes. The DSDs of shallow warm precipitating systems of NBB conditions are monomodal at all the altitudes. Significantly, normalized DSDs are found to be bimodal for BB conditions, and monomodal for NBB conditions which confirm different dominant microphysical processes. It is found that the observed bimodal DSDs during BB conditions are mainly due to the collision, coalescence and break-up processes. During NBB conditions, number and size of large raindrops grow while reaching the ground without much breakup. The radar reflectivity and rainfall intensity R (mmh − 1) relationship of the form Z = aR b are found out for BB and NBB conditions. Existing different microphysical processes lead to large coefficient in the Z–R relationship with small exponent during BB conditions while during NBB conditions the coefficients are small with large exponents
AlCl3,6H2O/KI/H2O/CH3CN : a new alternate system for dehydration of oximes and amides in hydrated media
Dehydration of oximes and amides to nitriles was
carried out using the AlCl3â6H2O/KI/H2O/CH3CN system. It
produced isoquinoline derivatives 8a-c (Bischler Naperialski
reaction) when reacted with amides 7a-c in hydrated
media. Also, the keto oximes produced anilides (Beckmann
rearrangement) with the system under the same reaction
conditions
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