Carlos Müller

Improving Temporal-Awareness of WS-Agreement

WS-Agreement (WS-Ag) is a proposed recommendation of the Open Grid Forum that provides a schema to describe SLAs and a protocol to create them based on a mechanism of templates. However, although it identifies the necessity of specifying temporal-aware agreement terms (e.g. the response time is 30 ms from 8:00h to 17:00h and 15 ms from 17:00h to 8:00h), to the best of our knowledge, there are no existing proposals that deal with that necessity. We propose an extension that gives WS-Ag support to temporality. This allows describing expressive validity periods such as those composed by several periodic or non-periodic intervals and it applies not only to the agreement terms themselves but also to other parts of WS-Ag such as creation constraints and preferences about the service properties. In addition, in this paper we propose a preference XML schemato describe preferences over any set of service properties using any kind of utility function. In further research we will study a concrete specification for those utility functions.

Automated Analysis of Conflicts in WS-Agreement

WS-Agreement is one of the most widely used SLA specifications. An advantage of WS-Agreement over other agreement metamodels is that it allows one to define conditional and optional term sets inside an agreement document, which are commonly found features in real-world agreements. Unfortunately, they increase the complexity of the automated detection and explanation of conflicts between SLA terms, leading to new kinds of conflicts that are not supported by current techniques. Furthermore, creating a general-purpose conflict analyser in WS-Agreement is a hard task since it should understand the semantics of an unbounded number of languages that can be used in the eight extension points that WS-Agreement includes for the sake of flexibility. In this article, we address these issues by providing a conflict classification for SLAs that includes new conflicts derived from the use of conditional and optional term sets; and a novel language-agnostic technique based on constraint satisfaction problems to automatically detect and explain these conflicts. In pursuing these results, we defined some WS-Agreement concepts as well as a fully-fledged WS-Agreement-compliant language. The developed technique and its reference implementation have been thoroughly validated.

SALMonADA: a platform for monitoring and explaining violations of WS-agreement-compliant documents

Quality assurance techniques have been developed to supervise the service quality (QoS) agreed between service-based systems (SBSs) consumers and providers. Such QoS is usually included in service level agreements (SLAs) and thus, SLA monitoring platforms have been developed supporting violation detection. However, just a few of them provide explanation of the violations caused by observed QoS at monitoring time, but not in an user-friendly format. Therefore, we propose a general monitoring and analysis conceptual reference model and we instantiated it with SALMonADA, a SBS that notifies the clients with violations and their causes in their own easy-to-understand specification terms. In addition, our platform performs an early analysis notification that avoids delays in the client notification time when a violation takes place. Moreover, we have implemented a web application as a SALMonADA client, to prove how it monitors, analyses and reports to their clients the service level fulfillment of real services subject to a SLA specified with WS-Agreement.

An Initial Approach to Explaining SLA Inconsistencies

An SLA signed by all interested parties must be created carefully, avoiding contradictions between terms, because their terms could carry penalties in case of failure. However, this consistency checking may become a challenging task depending on the complexity of the agreement. As a consequence, an automated way of checking the consistency of an SLA document and returning the set of inconsistent terms of the agreement would be very appealing from a practical point of view. For instance, it enables the development of software tools that make the creation of correct SLAs and the consistency checking of imported SLAs easier for users. In this paper, we present the problem of explaining WS-Agreement inconsistencies as a constraint satisfaction problem (CSP), and then we use a CSP solver together with an explanation engine to check the consistency and return the inconsistent terms. Furthermore, a proof-of-concept using Choco solver in conjunction with the Palm explanation engine has been developed.

Explaining the Non-compliance between Templates and Agreement Offers in WS-Agreement

A common approach to the process of reaching agreements is the publication of templates that guide parties to create agreement offers that are then sent for approval to the template publisher. In such scenario, a common issue the template publisher must address is to check whether the agreement offer received is compliant or not with the template. Furthermore, in the latter case, an automated explanation of the reasons of such non-compliance is very appealing. Unfortunately, although there are proposals that deal with checking the compliance, the problem of providing an automated explanation to the non-compliance has not yet been studied in this context. In this paper, we take a subset of the WS-Agreement recommendation as a starting point and we provide a rigorous definition of the explanation for the non-compliance between templates and agreement offers. Furthermore, we propose the use of constraint satisfaction problem (CSP) solvers to implement it and provide a proof-of-concept implementation. The advantage of using CSPs is that it allows expressive service level objectives inside SLAs.

Towards a Formal Specification of SLAs with Compensations

In Cooperative Information Systems, service level agreements (SLA) can be used to describe the rights and obligations of parties involved in the transactions (typically the service consumer and the service provider); amongst other information, SLA could define guarantees associated with the idea of service level objectives (SLOs) that normally represent key performance indicators of either the consumer or the provider. In case the guarantee is under-fulfilled or over-fulfilled SLAs could also define some compensations (i.e. penalties or rewards). In such a context, during the last years there have been important steps towards the automation of the management of SLAs, however the formalization of compensations in SLAs still remains as an important challenge.

Temporal-Awareness in SLAs: Why Should We Be Concerned?

Traditionally, Service Level Agreements have been decomposed in two sets of properties: functionals (what) and non-functionals (how). However, in our opinion, there has been a third key element that has had a minor attention from academy: temporal awareness (when). We believe temporality is a main concern that should be addressed in realistic scenarios. In doing so, this position paper discuss our experience in extending the specification WS-Agreement with a temporal Domain Specific Language; importantly, main aim of the paper is to provoke a debate about the importance of temporality in SLAs.

On the automated analysis of WS–Agreement documents: Applications to the processes of creating and monitoring agreements

A medida que los SLAs empiezan a ser necesarios en servicios y aplicaciones usadas por el gran publico y en ambientes XaaS donde existen penalizaciones por su incumplimiento, aumenta la demanda y la complejidad de los sistemas que dan soporte al ciclo de vida de los SLAs, en adelante, soluciones dirigidas por SLAs. Tras revisar el estado del arte, hemos concluido que existe un amplio espacio de mejora en las tecnologias para desarrollar soluciones dirigidas por SLAs con dos dimensiones claramente diferenciables: (1) los lenguajes para especificar SLAs, y (2) las tecnicas para extraer informacion util sobre los SLAs, que en adelante llamaremos tecnicas de analisis. En cuanto a los lenguajes de especificacion, las mejoras pueden venir de conseguir lenguajes i) independientes del dominio, ii) conformes a recomendaciones estandar, iii) con la expresividad necesaria para describir de manera sencilla los objetivos de nivel de servicio (SLOs), las restricciones y los periodos de validez de sus diferentes elementos, iv) que dispongan de criterios para verificar propiedades basicas tales como la consistencia, la conformidad y la optimalidad y, llegado el caso, explicar por que no es posible verificarlas de un modo que permita al usuario un facil diagnostico y reparacion. En cuanto a las tecnicas de analisis propuestas hasta la fecha, encontramos que presentan algunas de las carencias habituales de las tecnologias emergentes, por lo que la mejora puede venir de: desarrollar implementaciones de referencia lo suficientemente completas, con un diseno orientado a su reutilizacion, abiertas a ser extendidas de manera efectiva y, por ultimo, disponer de un interfaz de usuario facil de usar. El objetivo global de esta tesis es mejorar el soporte actualmente existente para desarrollar soluciones dirigidas por SLAs tomando como referencia las mejoras anteriormente identificadas, centrando su estudio en la recomendacion WS�Agreement, propuesta por la Open Grid Forum para especificar y crear acuerdos de nivel servicio. Sus principales contribuciones son un lenguaje de especificacion WS�Agreement compliant que hemos denominado iAgree, y un entorno para desarrollar soluciones dirigidas por SLAs que hemos denominado IDEAS que tiene como componente principal un analizador de documentos iAgree que hemos denominado ADA. Como contribuciones menores se han desarrollado soluciones para mejorar el soporte disponible actualmente para la creacion y monitorizacion de acuerdos de nivel de servicios. La clave de nuestra propuesta para mejorar el lenguaje de especificacion de acuerdos pasa por definir lo que hemos dado a llamar un configuracion WS�Agreemen (WSAC) que dispone de los sublenguajes necesarios para describir las distintas partes de un documento de acuerdo. En cuanto a nuestra propuesta de mejora para las tecnicas de analisis, la clave ha sido organizar dichas tecnicas en un catalogo de operaciones basicas de analisis que pueden ser combinadas para soportar soluciones dirigidas por SLAs mas complejas. La aplicabilidad de nuestros resultados se restringe a aquellos SLAs que pueden interpretarse como problemas de satisfaccion de restricciones, lo cual, en base a nuestra experiencia, es suficiente para abordar muchas situaciones de gran interes.

An Elasticity-Aware Governance Platform for Cloud Service Delivery

In cloud service provisioning scenarios with a changing demand from consumers, it is appealing for cloud providers to leverage only a limited amount of the virtualized resources required to provide the service. However, it is not easy to determine how much resources are required to satisfy consumers expectations in terms of Quality of Service (QoS). Some existing frameworks provide mechanisms to adapt the required cloud resources in the service delivery, also called an elastic service, but only for consumers with the same QoS expectations. The problem arises when the service provider must deal with several consumers, each demanding a different QoS for the service. In such an scenario, cloud resources provisioning must deal with trade-offs between different QoS, while fulfilling these QoS, within the same service deployment. In this paper we propose an elasticity-aware governance platform for cloud service delivery that reacts to the dynamic service load introduced by consumers demand. Such a reaction consists of provisioning the required amount of cloud resources to satisfy the different QoS that is offered to the consumers by means of several service level agreements. The proposed platform aims to keep under control the QoS experienced by multiple service consumers while maintaining a controlled cost.

Towards Compensable SLAs

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.