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Service-oriented architecture enables an environment where businesses can expose services for use by their collaborators and their peer organizations. In this dynamic environment, organizations require the use of service level agreements (SLAs) to assure the quality of service (QoS) standards of services provided by their collaborators. In an ad-hoc workflow scenario, a business may need to perform real-time composition of existing services in response to consumer requests. In this work, we suggest that, in parallel to traditional web service composition, the business must also compose the existing SLAs in order to ensure the service levels that must be guaranteed to new consumers. Ultimately, this approach to SLA composition must align with the overarching principles of the provider and the priorities of the consumer. In this paper, we introduce a model and representations of service level agreement attributes appropriate for managing a service provider's expectations when adding new partners. Our evaluations suggest that the SLA composition can efficiently run concurrently with traditional service composition.

A service level agreement, SLA, is a technical contract between two types of businesses, producers and consumers. A SLA captures the agreed-upon terms between organizations with respect to quality of service (QoS) and other related concerns. In simple cases, one consumer forms a SLA with a producer. In more complex cases, a consumer may form a SLA that defines a set of producer businesses. Considering a service-oriented computing environment, capabilities are shared via the implementation of web services exposed by a producer organization. The ultimate goal of service-oriented computing is for consumers to access these shared capabilities on-demand. As such, in cases where businesses have longstanding relationships, such as workflow and supply chain environments, peer companies that share services must be able to assure a level of service to their underlying customers [4] and [8]. New specifications, such as the Web Service Level Agreement (WSLA) and Web Service Agreement (WS-Agreement) [2] enable SLAs to be associated with an individual web service or even groups of web services. These specifications define an eXtensible Markup Language (XML)-based data model that can be used along with the Web Service Description Language (WSDL) documents that traditionally describe the web services. These specifications provide a significant opportunity. Organizations can specify QoS-related concerns in concert with the functionality concerns already captured in the WSDL files. As a result, when a new organization searches for a pertinent web service, the SLA-enhanced WSDL file can be used to determine the appropriateness of the service to meet the required business need. Furthermore organizations can use the SLA-enhanced WSDL file to negotiate the QoS terms. Although these SLA technologies and specifications present new opportunities for service-oriented business processes, there are a number of significant barriers. When a consumer organization must create a new business capability that requires the workflow composition of multiple web services, then that organization will also need to understand the composite impact of the underlying SLAs. Consequently, in addition to composing web services that are functionally compatible, the organization will need to ensure that the web services are compatible with regard to their service levels. Also, the product of all the SLAs for a composition of web services must be within the required threshold of feasibility as defined by the end users. As the service-oriented computing paradigm increases in popularity, the consumer will have the option of many similar services that may meet a particular requirement. As such, the composition of web services that is most efficient for a particular business purpose will rest on the organization's ability to understand and optimize the corresponding composition of SLAs. To deal with the aforementioned issues, we introduce the phrase, workflow composition of SLAs. Our approach suggests the multi-dimensional evaluation of existing agreed-upon QoS standards in order to predict the standards possible for the introduction of new agreements. While the notions of multi-dimensional analysis, optimization, dynamic programming are not new [9], [10], [12], [17] and [35] in this work, we identify the specific SLA-based attributes that allows for the introduction of new partners. Furthermore we develop a set of principles and the associated process that utilize the SLA information to estimate service levels. This approach favors services with clean request/response (RPC-type) communication, generally known as WSDL-based web services. Further investigation would be required to assess this approach as it relates to REST-based services [37]. In this work, we investigate several research issues relevant to the integration of web services-based workflow: 1. What SLA measures and principles are appropriate to support QoS-based assessment of existing service level guarantees? 2. Given a group of SLAs and knowledge about current consumer service level needs, can an on-demand request be analyzed against existing SLAs to guarantee a certain service level for a new consumer? The paper proceeds in the following section with a discussion of related work. In Section 3, we discuss how the SLA-based QoS assessment values are derived from higher-level organizational principles. The formal details of the attributes are defined in Section 4, and how the attributes are physically captured in markup languages in shown in Section 5. Finally, in Section 6 we evaluate the performance SLA composition as it runs parallel with traditional service composition routines.

When provider organizations expose their services for consumption by their peers, it is important for them to understand what they are guaranteeing. Moreover, consumers that receive such commitments must abide by their own commitments such that the providers can meet their guarantees to all consumers. In this work, we introduce a method of assessing an organization based on both pending and existing SLAs. Since the estimation of QoS measures for web services has been investigated in great detail, in this paper, we make a varied contribution. Our work builds on the existing studies by considering the QoS guarantees, as captured in SLAs, such that provider and consumer concerns can be modeled independently. This work also considers that SLA assessment occurs in a separate process than standard QoS estimation at service composition time. Here, we introduce high-level criteria that can be created from the aggregation of a comprehensive list of lower-level QoS-based attributes. This variation to related work facilitates automated cross-enterprise SLA negotiation. In this paper, we define a two-step process for composing SLAs and evaluating their efficiency. Consistent with results in related work that estimate QoS on fully operational web services, we have found through simulated experimentation that the management of third-party SLA information can be composed efficiently in parallel with the actual operational service composition. In future work, we plan to implement our approach within a real operational setting as opposed to the simulation setting that is represented in this paper. In the real operational setting, data will be produce using a variety of distributions. As such, WS-Agreement files will be appended to WSDL files and evaluated in a network environment for performance and feasibility. In addition, we plan to extend the SLA composition paradigm to protocols that mandate the discovery process within web service registries. In this way, it may be possible for adaptive software (or intelligent agents) to negotiate SLAs, in real time, while they perform on-demand, service discovery.