Software solutions to automate the procurement of web services are gaining importance when technology evolves, the number of providers increases and the needs of the clients become more complex. There are several proposals in this field, but they all have important drawbacks, namely: many of them are not able to check offers and demands for internal consistency; selecting the best offer usually relies on evaluating linear objective functions, which is quite a naive solution; the language to express offers is usually less expressive than the language to express demands; and, last but not least, providers cannot impose constraints on their clients. In this article, we present a solution to overcome these problems that relies on constraint programming; furthermore, we present a run-time framework, some experimental results, and a comparison with other proposals.

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Web services bring programmers a new way to develop advanced applications able to integrate any group of services on the Internet into a single solution. Web services procurement (WSP) is focussed on the acquisition of web services, including some complex tasks such as the specification of demands, the search for available offers, and the best choice selection. Although the technology to support them already exists, there are only a few approaches wherein qualityof-service in demands and offers is taken into account, in addition to functionality. In this paper, we present some implementation issues on a quality-aware approach to WSP, whose solution is mainly based on using mathematical constraints to define quality-of-service in demands and offers.

Abstract. Most Semantic Web Services discovery approaches are not well suited when using complex relational, arithmetic and logical expressions, because they are usually based on Description Logics. Moreover, these kind of expressions usually appear when discovery is performed including Quality-of-Service conditions. In this work, we present an hybrid discovery process for Semantic Web Services that takes care of QoS conditions. Our approach splits discovery into stages, using different engines in each one, depending on its search nature. This architecture is extensible and loosely coupled, allowing the addition of discovery engines at will. In order to perform QoS-aware discovery, we propose a stage that uses Constraint Programming, that allows to use complex QoS conditions within discovery queries. Furthermore, it is possible to obtain the optimal offer that fulfills a given demand using this approach.

Abstract. In the context of web service procurement (WSP), temporal-awareness refers to managing service demands and offers which are subject to validity periods, i.e. their evaluation depends not only on quality of service (QoS) values but also on time. For example, the QoS of some web services can be considered critical in working hours (9:00 to 17:00 from Monday to Friday) and irrelevant at any other moment. Until now, the expressiveness of such temporal-aware specifications has been quite limited. As far as we know, most proposals have considered validity periods to be composed of a single temporal interval. Other proposals, which could allow more expressive time-dependent specifications, have not performed a detailed study about all the underlying complexities of such approach, in spite of the fact that dealing with complex expressions on temporality is not a trivial task at all. As a matter of fact, it requires a special design of the so-called procurement tasks (consistency and conformance checking, and optimal selection). In this paper, we present a constraint-based approach to temporal-aware WSP. Using constraints allows a great deal of expressiveness, so that not only demands and offers can be assigned validity periods but also their conditions can be assigned (possibly multiple) validity temporal subintervals. Apart from revising the semantics of procurement tasks, which we previously presented in the first edition of the ICSOC conferences, we also introduce the notion of the covering set of a demand, a topic which is closely related to temporality.

Service Level Agreement (SLA) regulates the provisioning of a service by defining a set of guarantees. Each guarantee sets a Service Level Objective (SLO) on some service metrics, and optionally a compensation that is applied when the SLO is unfulfilled or overfulfilled. Currently, there are software tools and research proposals that use the information about compensations to automate and optimise certain parts of the service management. However, they assume that compensations are well defined, which is too optimistic in some circumstances and can lead to undesirable situations. In this article we discuss about the notion of validity of guarantees with a compensation, which we refer to as compensable guarantees (CG). We describe an abstract model of CGs and we provide a technique that leverages constraint satisfaction problem solvers to automatically validate them. We also present a materialisation of the model of CGs in iAgree, a language to specify SLAs and a tooling support that implements our whole approach. An assessment over 319 CGs taken from 24 real-world SLAs suggests that the expressiveness and effectiveness of our proposal can pave the way for using CGs in a safer and more reliable way.

Abstract. As government agencies and business become more dependent on web services, software solutions to automate their procurement gain importance. Current approaches for automating the procurement of web services suffer from an important drawback: neither uncertainty measures nor non-linear, and complex relations among parameters can be used by providers to specify quality-ofservice in offers. In this paper, we look deeply into the roots of this drawback and present a proposal which overcomes it. The key point to achieve this improvement has been using the constraint programming as a formal basis, since it endows the model with a very powerful expressiveness. A XML-based implementation is presented along with some experimental results and comparisons with other approaches.

Abstract. Service Level Agreements (SLA) describe the rights and obligations of parties involved (typically the service consumer and the service provider); amongst other information they could include the definition of compensations: penalties and/or rewards depending on the level of service provided. We coin the concept of Compensable SLAs to such that include compensation information inside. In such a context, in spite of important steps towards the automation of the management of SLAs have been given, the expression of compensations remains as an important challenge to be addressed. In this paper we aim to provide a characterization model to create Compensable SLAs; specifically, the main contributions include: (i) the conceptualization of the Compensation Function to express consistently penalties and rewards. (ii) a model for Compensable SLAs as a set of guarantees that associate Service Level Objectives with Compensation Functions. We provide some properties and aspects that have been used to analyse two real-world SLAs.