1,265 research outputs found
Towards Streaming Evaluation of Queries with Correlation in Complex Event Processing
Complex event processing (CEP) has gained a lot of attention for evaluating complex patterns over high-throughput data streams. Recently, new algorithms for the evaluation of CEP patterns have emerged with strong guarantees of efficiency, i.e. constant update-time per tuple and constant-delay enumeration. Unfortunately, these techniques are restricted for patterns with local filters, limiting the possibility of using joins for correlating the data of events that are far apart.
In this paper, we embark on the search for efficient evaluation algorithms of CEP patterns with joins. We start by formalizing the so-called partition-by operator, a standard operator in data stream management systems to correlate contiguous events on streams. Although this operator is a restricted version of a join query, we show that partition-by (without iteration) is equally expressive as hierarchical queries, the biggest class of full conjunctive queries that can be evaluated with constant update-time and constant-delay enumeration over streams. To evaluate queries with partition-by we introduce an automata model, called chain complex event automata (chain-CEA), an extension of complex event automata that can compare data values by using equalities and disequalities. We show that this model admits determinization and is expressive enough to capture queries with partition-by. More importantly, we provide an algorithm with constant update time and constant delay enumeration for evaluating any query definable by chain-CEA, showing that all CEP queries with partition-by can be evaluated with these strong guarantees of efficiency
On the Expressiveness of Languages for Complex Event Recognition
Complex Event Recognition (CER for short) has recently gained attention as a mechanism for detecting patterns in streams of continuously arriving event data. Numerous CER systems and languages have been proposed in the literature, commonly based on combining operations from regular expressions (sequencing, iteration, and disjunction) and relational algebra (e.g., joins and filters). While these languages are naturally first-order, meaning that variables can only bind single elements, they also provide capabilities for filtering sets of events that occur inside iterative patterns; for example requiring sequences of numbers to be increasing. Unfortunately, these type of filters usually present ad-hoc syntax and under-defined semantics, precisely because variables cannot bind sets of events. As a result, CER languages that provide filtering of sequences commonly lack rigorous semantics and their expressive power is not understood.
In this paper we embark on two tasks: First, to define a denotational semantics for CER that naturally allows to bind and filter sets of events; and second, to compare the expressive power of this semantics with that of CER languages that only allow for binding single events. Concretely, we introduce Set-Oriented Complex Event Logic (SO-CEL for short), a variation of the CER language introduced in [Grez et al., 2019] in which all variables bind to sets of matched events. We then compare SO-CEL with CEL, the CER language of [Grez et al., 2019] where variables bind single events. We show that they are equivalent in expressive power when restricted to unary predicates but, surprisingly, incomparable in general. Nevertheless, we show that if we restrict to sets of binary predicates, then SO-CEL is strictly more expressive than CEL. To get a better understanding of the expressive power, computational capabilities, and limitations of SO-CEL, we also investigate the relationship between SO-CEL and Complex Event Automata (CEA), a natural computational model for CER languages. We define a property on CEA called the *-property and show that, under unary predicates, SO-CEL captures precisely the subclass of CEA that satisfy this property. Finally, we identify the operations that SO-CEL is lacking to characterize CEA and introduce a natural extension of the language that captures the complete class of CEA under unary predicates
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Solidification Morphology Analysis of SLM of Cu Powder
The solidification morphology analysis of fine Cu powder melted by a raster
scanned energy beam from a focused Nd:YAG laser is presented here. The powder was
processed inside of sealed chamber where it was subjected to a high vacuum cycle. The
laser fusion process consisted raster scanning a narrow rectangular pattern with a high
density of scanning lines, the chamber was purged with inert gas during the process. Up
to a 3.3 mm/s laser travel speed and maximum laser power level of 240 W were used to
melt a 2 mm thick bed of loose powder. The resulting solidified ingots were separated
into categories based on their shape integrity. Metallographic analysis by means of
optical microscopy and scanning electron microscopy was performed on the cross section
and longitudinal section of the ingots with homogeneous surface and complete shape
integrity. Characterization revealed an elongated columnar grain structure with a grain
orientation along the direction of the laser travel direction, some degree of porosity was
observed too in some of the specimens. It was observed that grains diameter ranged from
10 to 100 µm and contained a two phase eutectic microstructure of copper and it oxides.
Oxygen content was accounted from a 5.5 up to 8.1 atomic percent, a small percentage of
chlorine was present, too. A 2 to 8 percent variation in the Vickers microhardness values
were found between the different specimens when measured along the longitudinal
section. These HV values corresponded to approximate 20-25% cold rolled oxygen free
copper (80-90 HV). The ingots thus produced suggest that a multilayer structure from Cu
powder could be build by the SLM process having sufficiently adequate compositional,
microstructure and mechanical properties for functional applications.Mechanical Engineerin
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Mechanical Behavior of SLS Components in Relation to the Build Orientation During the Sintering Process as Measured by ESPI
Selective Laser Sintering (SLS) allows producing real parts from CAD data from materials with
different characteristics compared to the final model, presenting dissimilar mechanical properties
between the prototype and the product. The purpose of this work is to correlate the mechanical
behavior of beam-type specimens produced by SLS with build orientation angle used as a process
parameter, also attempting to demonstrate how this parameter affects the accuracy of the
Empirical Similitude Method (ESM). ESM presents itself as a valuable tool when creating scale
models with SLS, specifically in the framework of evolutionary product design. More
specifically, the Young modulus variation of test specimens of well-known dimensions and
material (DuraformTM PA2
), will be characterized by loading them within the elastic range. The
resulting elastic deformations will be measured using the technique of Electronic Speckle Pattern
Interferometry (ESPI) for small deformations in an out-of-plane configuration, contrasting these
results with the elastic theory of deformations. As a main result, it was found that there exists a
linear correlation between the build angle and the elastic modulus of the parts. Secondly, it was
demonstrated empirically that the ESM predicts better the mechanical response of the part than
TSM. Moreover, a 30% error reduction can be achieved within the ESM when using the build
orientation angle as a process parameter.Mechanical Engineerin
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Direct Metal Laser Fabrication of Cu Slabs from Powder Precursor: Surface Depth of Melt and Furnace Temperature Issues
A DMLF processing unit based on a raster-scanned 80 W CO2 laser beam has been
developed to process single layers of metallic powder precursor. The process chamber
provides atmosphere control (high vacuum and inert gas refill) besides temperature
elevation up to 700 o
C. In this work, copper powder precursor is confined inside a
refractory steel mask surrounded by an aluminum oxide jacket that acts as insulator. The
powder layers can have thicknesses of 0,5 and 1 mm. An infrared pyrometer measures
in real time the temperature at one location in the surface of the powder bed. Scan
speed, scan step, and furnace temperature have been varied to find combinations of such
parameters that render surface melting and maximum densification. Partially melted
samples were produced and their mass density was measured. Micro-hardness and
surface roughness were also measured along the resolidified surface, the first rendering
an average of 80,6 HV compared to the 90-105 HV of oxygen free copper, while the
second resulting in an 8 μm Ra value. Maximum melt of depth achieved is ~0,15 mm
followed by a sintered layer.Mechanical Engineerin
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