Marta Rukoz

TQoS: Transactional and QoS-Aware Selection Algorithm for Automatic Web Service Composition

Web Services are the most famous implementation of service-oriented architectures that has brought some challenging research issues. One of these is the composition, i.e., the capability to recursively construct a composite Web service as a workflow of other existing Web services, which are developed by different organizations and offer diverse functionalities (e.g., ticket purchase, payment), transactional properties (e.g., compensatable or not), and Quality of Service (QoS) values (e.g., execution price, success rate). The selection of a Web service, for each activity of the workflow, meeting the users requirements, is still an important challenge. Indeed, the selection of one Web service among a set of them that fulfill some functionalities is a critical task, generally depending on a combined evaluation of QoS. However, the conventional QoS-aware composition approaches do not consider the transactional constraints during the composition process. This paper addresses the issue of selecting and composing Web services not only according to their functional requirements but also to their transactional properties and QoS characteristics. We propose a selection algorithm that satisfies users preferences, expressed as weights over QoS criteria and as risk levels defining semantically the transactional requirements. Proofs and experimental results are presented.

QoS-Driven Selection of Web Services for Transactional Composition

Web services composition has been gaining interest over the last years as it leverages the capabilities to offer complex operations resulting from the aggregation of Web services offered by different organizations. As composite Web services are often long-running, loosely coupled and cross-organizational applications, advanced transactional support is required to ensure reliable execution. In addition, in the presence of multiple Web services with equivalent functionality, users will discriminate the alternatives based on their quality of service (QoS). This paper address the issue of selecting and composing Web services not only according to their functional requirements but also to their transactional properties and QoS characteristics defined using a quality model. In this model, Web services are selected in a way that satisfies user preferences. These preferences are expressed as weights over QoS criterion and as risk level defining semantically the transactional requirements.

Embedding spatial information into image content description for scene retrieval

This article presents @D-TSR, an image content representation describing the spatial layout with triangular relationships of visual entities, which can be symbolic objects or low-level visual features. A semi-local implementation of @D-TSR is also proposed, making the description robust to viewpoint changes. We evaluate @D-TSR for image retrieval under the query-by-example paradigm, on contents represented with interest points in a bag-of-features model: it improves state-of-the-art techniques, in terms of retrieval quality as well as of execution time, and is scalable. Finally, its effectiveness is evaluated on a topical scenario dedicated to scene retrieval in datasets of city landmarks.

CPN-TWS: a coloured petri-net approach for transactional-QoS driven Web Service composition

In this paper, we address the issue of selecting and composing Web Services (WSs) considering functional and Quality of Service (QoS) requirements combined with transactional properties. We formalise the WS functional, QoS and transactional properties as well as the WS Composition problem. We have extended Coloured Petri Net (CPN) formalism to incorporate transactional WSs properties in the composition process. We present and prove our CPN Transactional WS (CPN-TWS) selection algorithm that satisfies the user query functional conditions expressed as input and output attributes, QoS requirements represented by weights over criteria, and transactional properties expressed as a risk level. The result of our algorithm is a CPN corresponding to a Transactional Composite WS whose WS components locally optimise the QoS. We present experimental results to study the performance and the efficiency of our selection algorithm compared with an exhaustive one.

Quadtree representations for storage and manipulation of clusters of images

This article gives an overview of different approaches proposed for the storage and manipulation of clusters of images. Clustering images consists of grouping together images having a defined relationship. In this article, images are represented by quadtrees implemented in a hierarchical or linear way. The discussion, presented at the end of the article, allows selecting a quadtree-based representation well adapted to a specific area of application or to the characteristics of the manipulated images.

Modeling dynamic recovery strategy for composite web services execution

During the execution of Composite Web Services (CWS), a component Web Service (WS) can fail and can be repaired with strategies such WS retry, substitution, compensation, roll-back, replication, or checkpointing. Each strategy behaves differently on different scenarios, impacting the CWS QoS. We propose a non intrusive dynamic fault tolerant model that analyses several levels of information: environment state, execution state, and QoS criteria, to dynamically decide the best recovery strategy when a failure occurs. We present an experimental study to evaluate the model and determine the impact on QoS parameters of different recovery strategies; and evaluate the intrusiveness of our strategy during the normal execution of CWSs.

Web Service Selection for Transactional Composition

In this work we address the issue of selecting and composing Web Services (WSs) considering functional requirements and transactional properties. We formalize the WS composition problem using the user query, the transactional properties of WSs and the composite WSs definition. We extends Colored Petri Net (CPN) formalism to incorporate transactional WSs properties. We present a CPN-WS selection algorithm that satisfies the user query functional conditions expressed as input and output attributes, and transactional properties expressed as a risk level. The result of our algorithm is a Colored Petri-Net allowing to execute a transactional composite WS.

Storage and Management of Similar Images

Numerical images are becoming more and more important and an increasing emphasis on multimedia applications has resulted in large volumes of images. However, images need a large memory space to be stored, so their efficient storage and retrieval generate challenges to the database community. This paper proposes a new algorithm for an efficient storage of sets of images. It is based on a version approach used in databases. It shows how to store and operate on similar images; two images are defined as similar if the quad-trees encoding them have only few different nodes. A data structure called Generic Quad-Tree (GQT) is proposed. It optimizes the memory space required to store similar images and allows an efficient navigation among them. An Image Tree stores the ancestors and descendants of an image, like a version hierarchy. Using the Image Tree, the Generic Quad-Tree allows an image to share common parts with its ancestors and descendants. The GQT approach and some algorithms for reading, modifying or removing images from the Generic Quad-Tree are described. Examples using black and white images and gray scale images are presented.

FaCETa: Backward and Forward Recovery for Execution of Transactional Composite WS

In distributed software contexts, Web Services (WSs) that provide transactional properties are useful to guarantee reliable Transactional Composite WSs (TCWSs) execution and to ensure the whole system consistent state even in presence of failures. Failures during the execution of a TCWS can be repaired by forward or backward recovery processes, according to the component WSs transactional properties. In this paper, we present the architecture and an implementation of a framework, called FaCETa, for efficient, fault tolerant, and correct distributed execution of TCWSs. FaCETa relies on WSs replacement, on a compensation protocol, and on unrolling processes of Colored Petri-Nets to support failures. We implemented FaCETa in a Message Passing Interface (MPI) cluster of PCs in order to analyze and compare the behavior of the recovery techniques and the intrusiveness of the framework.

FACETA*: Checkpointing for Transactional Composite Web Service Execution based on Petri-Nets

Abstract Failures during the execution of Transactional Composite Web Services (TCWSs) can be repaired by forward or back–ward recovery processes, according to the component WSs transactional properties. In previous works, we presented TCWS fault tolerant execution approaches relying on WSs replacement, on a compensation protocol, and on unrolling processes of Colored Petri-Nets (CPNs) to support forward and backward recovery. We represent a TCWS and its corresponding backward recovery process by CPNs. Even though these recovery processes ensure system consistency, backward recovery means that users do not get the desired answer to their queries and forward recovery could imply long waiting time for users to finally get the desired response. In this paper, we present an alternative fault tolerant approach in which, in case of failures, the unrolling process of the CPN controlling the execution of a TCWS is check–pointed and the execution flow goes on as much as it is possible. In this way, users can have partial responses as soon as they are received and can re-submit the checkpointed CPN to re-start its execution from an advanced point of execution (checkpoint). We present the checkpointing algorithm integrated to our previous work.