Specification of Service Level Agreements: Problems, Principles and Practices

Software intensive systems (SIS) increasingly influence the ability of enterprises to be competitive in continuously changing market situations. The integration of these systems into organizations, and in particular the subsequent exploitation, maintenance and service activities, have become of utmost importance. Unfortunately the area of exploitation and operation, also known as service management, is still rather immature. Service management covers services such as performance and availability support, end-user and help desk support, education, and maintenance. One of the main concepts of service management is the Service Level Agreement (SLA). The goal of an SLA is to bridge the gap between service provider and users or customers. However, there exist many problems and unsolved questions regarding the specification and the quantification of SLAs. This paper addresses the specification of SLAs on the basis of three well-founded service management principles, respectively: 'continuity in service management, the pit/shell principle of a service, and the principle of specifying the quality of both a service process and a service object. Finally, the paper addresses the validation of these principles in practice.     

基于语义网技术的服务等级协议协商机制

针对服务等级协议(SLA)的自动协商中用于协商的SLA元素及协商过程缺乏语义描述的问题,提出了一种基于语义网技术的SLA协商机制。首先提出一种协商本体Osn,该本体专门描述直接用于协商过程的SLA元素;设计了SLA协商元素的映射函数及协商评估函数并使用Osn描述这些函数,基于描述逻辑对Osn的主要概念及概念之间的关系给出了形式化描述,为Osn提供了可满足的语义模型。进而提出了一种针对SLA协商的讨价还价模型,通过对有关命题及定理的证明说明采用该模型可以形成帕累托最优的协商出价;采用该模型基于OWL-S与统一建模语言(UML)的映射关系设计了针对SLA协商的服务本体。实例研究的结果表明Osn描述的知识可以形成满足协商参与者利益最大化需求的出价序列。从而说明:Osn可以对服务本体提供针对任意SLA协商的参数类型支持;面向SLA协商的讨价还价模型可以形成双方可接受的SLA。     

Toward the automation of a QoS-driven SLA establishment in the Cloud

Composite software as a service (SaaS)-based SOA offers opportunities for enterprises to offer value-added services. The cornerstone for such a business is service level agreements between Cloud customers and Cloud providers. In spite of the hype surrounding composite SaaS, standardized methods that enable a reliable management of service level agreements starting from the SLA derivation from the customer requirements to the SLA establishment between the two stockholders are still missing. To overcome such a drawback, we propose a method for SLA establishment guided by QoS for composite SaaS. Our method provides: (1) a requirement specification language for the Cloud customer to define the composition schemas of the requested services along with its QoS constraints; (2) a Cloud provider offer specification language and method to help in identifying the services and resources that satisfy the customer requirements; and (3) an SLA document definition language and method to specify a deployable composite SaaS on the Cloud. Our approach for SLA establishment embraces model-driven architecture principles to automate the SLA document generation from the customer requirements document. The automation is handled through model transformations along with enrichment algorithms to ensure the generation of complete SLA documents.     

Dynamic SLAs management in service oriented environments

The increasing adoption of service oriented architectures across different administrative domains, forces service providers to use effective mechanisms and strategies of resource management in order for them to be able to guarantee the quality levels their customers demands during service provisioning. Service level agreements (SLA) are the most common mechanism used to establish agreements on the quality of a service (QoS) between a service provider and a service consumer. The WS-Agreement specification, developed by the Open Grid Forum, is a Web Service protocol to establish agreements on the QoS level to be guaranteed in the provision of a service. The committed agreement cannot be modified during service provision and is effective until all activities pertaining to it are finished or until one of the signing party decides to terminate it. In B2B scenarios where several service providers are involved in the composition of a service, and each of them plays both the parts of provider and customer, several one-to-one SLAs need to be signed. In such a rigid context the global QoS of the final service can be strongly affected by any violation on each single SLA. In order to prevent such violations, SLAs need to adapt to any possible needs that might come up during service provision. In this work we focus on the WS-Agreement specification and propose to enhance the flexibility of its approach. We integrate new functionality to the protocol that enable the parties of a WS-Agreement to re-negotiate and modify its terms during the service provision, and show how a typical scenario of service composition can benefit from our proposal.     

基于Parlay/OSA的服务水平协议分析与改进

服务水平协议是保证服务提供的关键技术,在服务提供领域日显其重要性。文章首先概述了服务水平协议的组成与功能,分析了Paraly/OSA架构下引入的商业模型,讨论组成商业模型的四个商业角色之间的服务关系。在此基础上,进一步分析了目前的Parlay规范对服务水平协议的支持,对组成服务水平协议的离线协议与在线协议的分析可知Parlay规范对服务水平协议的支持存在着极大的不足。最后,提出定义链表型的数据结构增强服务能力水平区分,完善服务水平协议,并分析了其优点、可行性以及带来的挑战。     

A resource oriented integration architecture for the Internet of Things: A business process perspective

The vision of the Internet of Things (IoT) foresees a future Internet incorporating smart physical objects that offer hosted functionality as IoT services. These services when integrated with the traditional enterprise level services form the creation of ambient intelligence for a wide range of applications. To facilitate seamless access and service life cycle management of large, distributed and heterogeneous IoT resources, service oriented computing and resource oriented approaches have been widely used as promising technologies. However, a reference architecture integrating IoT services into either of these two technologies is still an open research challenge. In this article, we adopt the resource oriented approach to provide an end-to-end integration architecture of front-end IoT devices with the back-end business process applications. The proposed architecture promises a programmer friendly access to IoT services, an event management mechanism to propagate context information of IoT devices, a service replacement facility upon service failure, and a decentralized execution of the IoT aware business processes.     

An SLA-based cloud computing that facilitates resource allocation in the distributed data centers of a cloud provider

The number of cloud service users has increased worldwide, and cloud service providers have been deploying and operating data centers to serve the globally distributed cloud users. The resource capacity of a data center is limited, so distributing the load to global data centers will be effective in providing stable services. Another issue in cloud computing is the need for providers to guarantee the service level agreements (SLAs) established with consumers. Whereas various load balancing algorithms have been developed, it is necessary to avoid SLA violations (e.g., service response time) when a cloud provider allocates the load to data centers geographically distributed across the world. Considering load balancing and guaranteed SLA, therefore, this paper proposes an SLA-based cloud computing framework to facilitate resource allocation that takes into account the workload and geographical location of distributed data centers. The contributions of this paper include: (1) the design of a cloud computing framework that includes an automated SLA negotiation mechanism and a workload- and location-aware resource allocation scheme (WLARA), and (2) the implementation of an agent-based cloud testbed of the proposed framework. Using the testbed, experiments were conducted to compare the proposed schemes with related approaches. Empirical results show that the proposed WLARA performs better than other related approaches (e.g., round robin, greedy, and manual allocation) in terms of SLA violations and the provider’s profits. We also show that using the automated SLA negotiation mechanism supports providers in earning higher profits.     

The WSLA Framework: Specifying and Monitoring Service Level Agreements for Web Services

We describe a novel framework for specifying and monitoring Service Level Agreements (SLA) for Web Services. SLA monitoring and enforcement become increasingly important in a Web Service environment where enterprise applications and services rely on services that may be subscribed dynamically and on-demand. For economic and practical reasons, we want an automated provisioning process for both the service itself as well as the SLA managment system that measures and monitors the QoS parameters, checks the agreed-upon service levels, and reports violations to the authorized parties involved in the SLA management process. Our approach to these issues is presented in this paper. The Web Service Level Agreement (WSLA) framework is targeted at defining and monitoring SLAs for Web Services. Although WSLA has been designed for a Web Services environment, it is applicable as well to any inter-domain management scenario, such as business process and service management, or the management of networks, systems and applications in general. The WSLA framework consists of a flexible and extensible language based on XML Schema and a runtime architecture comprising several SLA monitoring services, which may be outsourced to third parties to ensure a maximum of objectivity. WSLA enables service customers and providers to unambiguously define a wide variety of SLAs, specify the SLA parameters and the way they are measured, and relate them to managed resource instrumentations. Upon receipt of an SLA specification, the WSLA monitoring services are automatically configured to enforce the SLA. An implementation of the WSLA framework, termed SLA Compliance Monitor, is publicly available as part of the IBM Web Services Toolkit.     

Service Violation Monitoring Model for Detecting and Tracing Bandwidth Abuse

Bandwidth abuse is a critical Internet service violation. However, its origins are difficult to detect and trace given similarities between abusive and normal traffic. So far, there is no capable and scalable mechanism to deal with bandwidth abuse. This paper proposes a distributed edge-to-edge model for monitoring service level agreement (SLA) violations and tracing abusive traffic to its origins. The mechanism of policing misbehaving user traffic at a single random early detection (RED) gateway is used in the distributed monitoring of SLA violations, including violations carried out through several gateways. Each RED gateway reports misbehaving users who have been sent notifications of traffic policing to an SLA monitoring unit. Misbehaving users are considered suspicious users and their consumed bandwidth shares are aggregated at every gateway to be compared with SLA-specified ratios. Bandwidth is abused when SLA-specified ratios are exceeded. By reporting bandwidth abuse, illegitimate users can be isolated from legitimate ones and source hosts of abusive traffic may be traced. Approximate simulation results show that the proposed model can detect any SLA violation and identify abusive users. In addition, the proposed model can trace user violations back to their source machines in real time.     

基于接纳控制和ILP资源调度模型的BDAaaS系统架构

为了使云计算平台为大数据分析提供有效支持,提出一种大数据分析即服务(BDAaaS)的系统架构;首先,当用户向系统提交大数据分析应用(BDAA)时,通过接纳控制器评估任务的执行时间和成本并作出接纳决策;然后,通过服务等级协议(SLA)管理器根据任务的服务质量(QoS)需求制定SLA;最后,利用提出的整数线性规划(ILP)资源调度模型,以最小化执行成本为目标,在满足SLA下合理调度资源来执行任务;仿真结果表明,提出的方案能够有效降低任务执行时间,具有有效性和可行性。     

Constructing scalable Internet of Things services based on their event‐driven models

Even though many IoT (Internet of Things) applications have been implemented based on distributed events, how to construct scalable IoT services is still unclear. In this paper, we first discuss representing physical entities as IoT resources in the cyber world and integrate them into IoT services and then use an event session mechanism to express the coordination logic in an IoT service system. Given their event‐driven models, the scalable IoT services are constructed through decoupling one service at behavior from others, where running atomic service instances with high concurrency is our first focus, and distributed execution of an IoT business process is our second focus. Our last focus is to make each distributed business process execution unit hold its properties in the whole process, which requires modeling the environment as a prerequisite to compute its properties, and some assume‐guarantee rules for composing the services from the environment's perspective. We then propose a platform to support the construction, where according to the behavior decoupling features, one IoT business process is decomposed into pieces of event composition logic and business computation logic, together with separating the data dependency of computation functions in each process fragment, such that it can be concurrently executed and distributed. A practical application is implemented to concept‐prove our work. Copyright © 2015 John Wiley & Sons, Ltd.     

Semantics and pragmatics of Real-Time Maude

At present, designers of real-time systems face a dilemma between expressiveness and automatic verification: if they can specify some aspects of their system in some automaton-based formalism, then automatic verification is possible; but more complex system components may be hard or impossible to express in such decidable formalisms. These more complex components may still be simulated; but there is then little support for their formal analysis. The main goal of Real-Time Maude is to provide a way out of this dilemma, while complementing both decision procedures and simulation tools. Real-Time Maude emphasizes ease and generality of specification, including support for distributed real-time object-based systems. Because of its generality, falling outside of decidable system classes, the formal analyses supported—including symbolic simulation, breadth-first search for failures of safety properties, and model checking of time-bounded temporal logic properties—are in general incomplete (although they are complete for discrete time). These analysis techniques have been shown useful in finding subtle bugs of complex systems, clearly outside the scope of current decision procedures. This paper describes both the semantics of Real-Time Maude specifications, and of the formal analyses supported by the tool. It also explains the tool's pragmatics, both in the use of its features, and in its application to concrete examples.     

On the ontological expressiveness of information systems analysis and design grammars

Abstract. Information systems analysis and design (ISAD) methodologies provide facilities for describing existing or conceived real-world systems. These facilities are ontologically expressive if they are capable of describing all real-world phenomena completely and clearly. In this paper we formally examine the notion of the ontological expressiveness of a grammar and discuss some of its implications for the design and use of ISAD methodologies. We identify some generic ways in which ontological expressiveness may be undermined in a grammar and some potential consequences of these violations. We also examine ontological expressiveness within the context of some other desirable features that might be considered in the design of ISAD methodologies.     

Automatic test case generation for WS-Agreements using combinatorial testing

In the scope of the applications developed under the service-based paradigm, Service Level Agreements (SLAs) are a standard mechanism used to flexibly specify the Quality of Service (QoS) that must be delivered. These agreements contain the conditions negotiated between the service provider and consumers as well as the potential penalties derived from the violation of such conditions. In this context, it is important to assure that the service based application (SBA) behaves as expected in order to avoid potential consequences like penalties or dissatisfaction between the stakeholders that have negotiated and signed the SLA. In this article we address the testing of SLAs specified using the WS-Agreement standard by means of applying testing techniques such as the Classification Tree Method and Combinatorial Testing to generate test cases. From the content of the individual terms of the SLA, we identify situations that need to be tested. We also obtain a set of constraints based on the SLA specification and the behavior of the SBA in order to guarantee the testability of the test cases. Furthermore, we define three different coverage strategies with the aim at grading the intensity of the tests. Finally, we have developed a tool named SLACT (SLA Combinatorial Testing) in order to automate the process and we have applied the whole approach to an eHealth case study.     

严格服务等级约定的区分Web服务技术研究

随着Internet的普及和Web应用的发展，满足客户不同业务需求成为人们关注的问题．该文基于业务类的访问资源需求和服务等级给出了一种资源划分策略，用来确定业务类在访问率波动较小条件下达到服务级别所需的资源；提出一种资源效用控制机制，用来适应访问的突发性和保证业务类服务等级要求．通过按周期进行资源划分，同时利用资源控制机制来适应周期内的访问突发，得到了严格服务等级约定的区分Web服务(简称SLADS)．文章通过建立原型与其它方法进行了实验比较，结果表明SLADS方法能够有效支持业务类的严格服务等级和提高资源利用率．     

A fast and scalable approach for IoT service selection based on a physical service model

Information Systems (ISs) have become one of the crucial tools for various organizations in managing and coordinating business processes. Now we are entering the era of the Internet of Things (IoT). IoT is a paradigm in which real-world physical things can be connected to the Internet and provide services through the computing devices attached. The IoT infrastructure is starting to be integrated with ISs thereby diminishing the boundaries between the physical world and the business IT systems. With the development of IoT technologies, the number of connected things and their available physical services are increasing rapidly. Thus, selecting an appropriate service that satisfies a user’s requirements from such services becomes a time-consuming challenge. To address this issue, we propose a Physical Service Model (PSM) as a common conceptual model to describe heterogeneous IoT physical services. PSM contains three core concepts (device, resource, and service) and specifies their relationships. Based on the proposed PSM, we define three types of Quality of Service (QoS) attributes and rate candidate services according to user requirements. To dynamically rate QoS values and select an appropriate physical service, we propose a Physical Service Selection (PSS) method that takes a user preference and an absolute dominance relationship among physical services into account. Finally, experiments are conducted to evaluate the performance of the proposed method.     

An adaptive resource management scheme in cloud computing

There are various significant issues in resource allocation, such as maximum computing performance and green computing, which have attracted researchers’ attention recently. Therefore, how to accomplish tasks with the lowest cost has become an important issue, especially considering the rate at which the resources on the Earth are being used. The goal of this research is to design a sub-optimal resource allocation system in a cloud computing environment. A prediction mechanism is realized by using support vector regressions (SVRs) to estimate the number of resource utilization according to the SLA of each process, and the resources are redistributed based on the current status of all virtual machines installed in physical machines. Notably, a resource dispatch mechanism using genetic algorithms (GAs) is proposed in this study to determine the reallocation of resources. The experimental results show that the proposed scheme achieves an effective configuration via reaching an agreement between the utilization of resources within physical machines monitored by a physical machine monitor and service level agreements (SLA) between virtual machines operators and a cloud services provider. In addition, our proposed mechanism can fully utilize hardware resources and maintain desirable performance in the cloud environment.     

Parsing with restricted quantification: an initial demonstration1

The primary goal of this paper is to illustrate how smaller deductive search spaces can be obtained by extending a logical language with restricted quantification and tailoring an inference system to this extension. The illustration examines the search spaces for a bottom-up parse of a sentence with a series of four strongly equivalent grammars. The grammars are stated in logical languages of increasing expressiveness, each restatement resulting in a more concise grammar and a smaller search space. A secondary goal is to point out an area where further research could yield results useful to the design of efficient parsers, particularly for grammatical formalisms that rely heavily on feature systems.     

Recent Advances in Real-Time Maude

This paper gives an overview of recent advances in Real-Time Maude. Real-Time Maude extends the Maude rewriting logic tool to support formal specification and analysis of object-based real-time systems. It emphasizes ease and generality of specification and supports a spectrum of analysis methods, including symbolic simulation, unbounded and time-bounded reachability analysis, and LTL model checking. Real-Time Maude can be used to specify and analyze many systems that, due to their unbounded features, such as unbounded data structures or dynamic object and message creation, cannot be modeled by current timed/hybrid automaton-based tools. We illustrate this expressiveness and generality by summarizing two case studies: (i) an advanced scheduling algorithm with unbounded queues; and (ii) a state-of-the-art wireless sensor network algorithm. Finally, we give some (often easily checkable) conditions that ensure that Real-Time Maude's analysis methods are complete, also for dense time, for object-based real-time systems. In practice, our result implies that Real-Time Maude's time-bounded search and model checking of LTL time-bounded formulas are complete decision procedures for a large and useful class of non-Zeno real-time systems that fall outside the scope of systems that can be modeled in decidable fragments of hybrid automata, including the sensor network case study discussed in this paper.     

SLA在DiffServ计费系统中的应用研究

DiffServ的引入使得ISP可以根据不同的数据类型和用户服务要求来提供不同QoS水平的服务。但是,如果ISP不能对所提供的服务进行有效的计费管理,那么DiffServ的这个优点仍然不能得到充分利用。因此,有必要研究一种灵活的适应DiffServ要求的计费管理方法。该文所提出的计费系统通过SLA监控信息将市场激励机制引入计费管理,可以很好地指导用户网络行为、改善网络状态,这将有助于DiffServ环境下对不同QoS级别的服务计费管理。