Paik, H. Maamar, Z. Jovanovic, J.
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July McCalla, G. In the last years both industry and academia have shown a great interest in ensuring consistent cooperation for business-critical services, with contractually agreed levels of Quality of Service. Service Level Agreement specifications as well as techniques for their evaluation are nowadays irremissible assets. This paper presents Puppet Pick UP Performance Evaluation Test-bed , an approach and a tool for the automatic generation of test-beds to empirically evaluate the QoS features of a Web Service under development.
Specifically, the generation exploits the information about the coordinating scenario be it choreography or orchestration , the service description WSDL and the specification of the agreements WS-Agreement. Although WSs constitute a quite new drift in software application development, research in this technology has already evolved through a few stages. Initially the focus was in mechanisms which could allow for the loose interconnection among services independently developed and implemented on different machines.
Such vision can only be achieved through the disciplined usage of standard notations and protocols, and in fact the WS domain is characterized by a strong boost toward standardization. Thus key achievements at this stage have been the establishment of common service descriptions, the definition of open service directories for storing and retrieving such descriptions, and the enactment of dynamic discovering and binding mechanisms. Nevertheless, the need soon arose of allowing for more complex scenarios, beyond simple point-to-point interactions .
The standardization of adequate mechanisms for services composition and interaction constituted thus the next stage, which is still very active. Two directions currently lead the scene within two different, but related, contexts : the first aims at defining the composition of services referred to as orchestration , the second at describing how related services should cooperate to perform a given L.
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Bertolino, G. De Angelis, and A. Polini task choreography. Both interpretations of service integration provide a means to describe interacting scenarios. On one side, orchestration approaches foresee the availability of an execution engine that, by executing the code defining the orchestration, reproduces the specified interactions; as a fact, this need limits the applicability of orchestration to those cases in which a governing organization is in charge for defining the business process.
In contrast, the choreography approach foresees the availability of a specification of the interactions to which the various services must conform, but it does not introduce per se any mechanism for forcing such interactions. Eventually, the openness of the environment characterizing the SOA paradigm naturally led to the pursuit of mechanisms for specifying the provided levels of Quality of Service QoS and establishing an agreement on them, in line with the widely accepted idea that service delivery cannot just focus on functional aspects, and ignore QoS-related properties.
Indeed, not only for Service Oriented systems, but for many other kinds of enterprise applications  , communication networks and embedded systems , solutions that do not put adequate consideration of non functional aspects  are no longer acceptable. Correspondingly, in recent years much research has been devoted to methodologies for QoS evaluation, including predictive and empirical techniques .
Predictive approaches are crucial during the design and the development of a software system, to shape the quality of the final product : they perform analytical QoS evaluation, based on suitable models, such as Petri Nets or Queueing Networks. But increasingly modern applications are deployed over complex platforms i.
In such cases, empirical approaches, i. However, such approaches require the development of expensive and time consuming prototypes , on which representative benchmarks of the system in operation can be run. In this last direction, however, when computer-processable specifications exist, and code-factories can be used to automatically generate a running prototype from a given specification, there is large room for the adoption of empirical approaches.
In particular, and this is the position we take in this work, given the high availability of standardized computer processable information, WSs and related technologies [2,10,18,22,14] yield very promising opportunities for the application of empirical approaches to QoS evaluation. According to this intuition, in this paper we introduce an approach, called Puppet Pick UP Performance Evaluation Test-bed , which realizes the automatic derivation of test-beds for evaluating the desired QoS characteristics for a service under development, before it is deployed.
In particular, we are interested in assessing that a specific service implementation can afford the required level of QoS e.
To this purpose, Puppet relies on the availability of the QoS specification of both the service under evaluation and the interacting services. In the next section we provide a basic background on the emerging languages for the definition of SLAs. Then, in Sec.
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Successively, in Sec. An exploratory example is presented in Sec. In Sec. Traditionally, agreements were expressed informally, not in machine-readable form. In software engineering quite basic notions of agreements were established by means of Interface Description Languages .
SLAs aim at ensuring a consistent cooperation for business-critical services.
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The approach introduced in this paper has been conceived to be as independent as possible of a specific SLA language. Indeed, concerning the goal of the work, any SLA languages predicating on the concepts we are considering are equivalent. However, when it comes to developing a specific implementation of our conceptual environment, we obviously need to consider a specific technology. Hence, in the remainder of the paper, we will focus on a proof-of-concept development carried on using the WS-Agreement language . To make the paper self-contained, we report below the background notions behind its current proposal.
The main assets of the language concern the specification of domain-independent elements of a simple contracting process. Such generic definitions can be augmented with domain-specific concepts. As shown in Fig. The Context element is used to describe the involved parties and other aspects of an agreement not representing obligations of parties, such as its expiration date. An agreement can be defined for one or more contexts. The defined consensus or obligations of a party core in a WS-Agreement specification are expressed by means of Terms. Special elements e. The obligation terms are organized in two logical parts.
The first specifies the involved services by means of the Service Description Terms. Such part primarily describes the functional aspects of a service that will be delivered under an agreement. WS—Agreement Structure term for the service description is defined by means of its name, and the name of the service which it refers to. In some case, a domain-specific description of the service may be conditional to specific runtime constraints. A special kind of Service Description Terms is the Service Reference, which defines a pointer to a description of a service, rather than describing it explicitly into the agreement.
The second part of the terms definition specifies measurable guarantees associated with the other terms in the agreement and that can be fulfilled or violated. A Guarantee Term definition consists of the obliged party i. In general, the information contained into the fields of a Guarantee Term are expressed by means of domain-specific languages. The basic assumption of our approach is that such a description indeed exists.
This is not an unrealistic assumption, as the global definition of applications resulting from the dynamic integration of unrelated services is seen as one of the most relevant factors at the basis of the take-off of the Service Oriented paradigm. Our view1 is that the integration of services offers major guarantees, and will be fostered, by the existence of predefined choreographies and the definition of orchestration.
Given a definition of the integration of different services, based on choreography or orchestration, our objective is to provide a tool to support the QoS evaluation of services to be integrated, but still under development. The scenario we envisage is depicted in Fig.
The process continues with the annotation of the composition with QoS attributes for each service involved in the integration. In Fig.
For this step we distinguish between the case of a choreography and that of an orchestration. In the former case, the organization that released the specification of the composition is in charge of augmenting the specification with QoS attributes.
go here Developers of services will take such a specification as a reference for their implementation, expecting that their required services do the same. As a consequence each developer of a service is interested in evaluating that it actually can provide the required service according to the specified QoS.