Generating a contract checker for an SLA language

SLAng is a language for expressing Service LevelAgreements (SLAs) under development as part of the Europeanproject TAPAS. It is defined using a meta-model, an instance ofthe Meta-Object Facility (MOF) model, in which the relationshipbetween the syntax of the language and its domain of applicationis explicitly represented, and the violation semantics ofthe language defined using Object Constraint Language (OCL)constraints. The concrete syntax of the language is the XMLMeta-data Interchange (XMI) mapping of the syntactic part ofthe meta-model. In this paper we describe how the Java MetadataInterface (JMI) mapping can be applied to the meta-modelof the language to generate interfaces and classes to create andquery SLAs and relevant service monitoring data in memory;and how an OCL interpreter can be applied to check violationconstraints over this data, resulting in the implementation of acontract checker that is highly likely to respect the semantics ofthe language

Precise service level agreements

SLAng is an XML language for defining service level agreements, the part of a contract between the client and provider of an Internet service that describes the quality attributes that the service is required to possess. We define the semantics of SLAng precisely by modelling the syntax of the language in UML, then embedding the language model in an environmental model that describes the structure and behaviour of services. The presence of SLAng elements imposes behavioural constraints on service elements, and the precise definition of these constraints using OCL constitutes the semantic description of the language. We use the semantics to define a notion of SLA compatibility, and an extension to UML that enables the modelling of service situations as a precursor to analysis, implementation and provisioning activities

Language support for service-level agreements for application-service provision

My thesis is that practical language support can be provided for Service-Level Agreements (SLAs) for Application-Service Provision (ASP), which is better than that provided by pre-existing languages in that: it provides greater assistance in expressing conditions that mitigate the risks inherent in ASP; and disputes related to agreements expressed in this manner may be more easily resolved in so as to respect the original intent of the parties. I support this thesis by establishing requirements for SLAs for ASP based on an account of a typical ASP infrastructure and business model. These identify the particular risks inherent in ASP, permit comparisons between ASP SLA languages, and guide the development of an abstract, extensible, domain-specific language, SLAng. SLAng is defined using a meta-modelling approach that allows a high degree of precision in the specification of its semantics, traceability from SLA to language specification, and the testing of the language and SLAs to ensure they capture the original intent of the parties. SLAng supports the expression of mutually-monitorable SLAs, for which the determination of compliance depends only on events visible to both client and provider of the service. I demonstrate that such SLAs are the most monitorable possible in a typical ASP scenario, given current monitoring technology, and describe an approximately-monitorable constraint on the accuracy of evidence used to administer such SLAs. SLAng is shown to be of practical use in a case study, evaluated against the original requirements, and compared with pre-existing languages. The evaluation of SLAng is enhanced using metrics developed to assist in assessing the contribution of a domain-specific language specification to encoding the meaning of statements in that language

Efficient online monitoring of web-service SLAs

If an organization depends on the service quality provided by another organization it often enters into a bilateral service level agreement (SLA), which mitigates outsourcing risks by associating penalty payments with poor service quality. Once these agreements are entered into, it becomes necessary to monitor their conditions, which will commonly relate to timeliness, reliability and request throughput, at runtime. We show how these conditions can be translated into timed automata. Acceptance of a timed word by a timed automaton can be decided in quadratic time and because the timed automata can operate while messages are exchanged at runtime there is effectively only a linear run-time overhead. We present an implementation to derive on-line monitors for web services automatically from SLAs using an Eclipse plugin. We evaluate the efficiency and scalability of this approach using a large-scale case study in a service-oriented computational grid

Service-level agreements for electronic services

The potential of communication networks and middleware to enable the composition of services across organizational boundaries remains incompletely realized. In this paper, we argue that this is in part due to outsourcing risks and describe the possible contribution of Service-Level Agreements (SLAs) to mitigating these risks. For SLAs to be effective, it should be difficult to disregard their original provisions in the event of a dispute between the parties. Properties of understandability, precision, and monitorability ensure that the original intent of an SLA can be recovered and compared to trustworthy accounts of service behavior to resolve disputes fairly and without ambiguity. We describe the design and evaluation of a domain-specific language for SLAs that tend to exhibit these properties and discuss the impact of monitorability requirements on service-provision practices

The Cannon Hall Muscat grape

The Canon Hall Muscat table grape grown for export in Western Australia is consistent with the early descriptions and the present appearance of the original  Cannon Hall Muscat vine, and is clearly the same variety. It has the diploid number of chromosomes and the use of its name for tetraploid sports of Muscat of Alexandria is not justified.Die Rebensorte Cannon Hall MuscatDie Tafeltraube Canon Hall Muscat, die in Westaustralien zum Export angebaut wird, gleicht der ursprünglichen Cannon-Hall-Muscat-Rebe, da die erstgenannte eindeutig den alten Beschreibungen und dem heutigen Aussehen der letzteren entspricht. Canon Hall Muscat ist diploid, und deshalb sollte dieser Name nicht für tetraploide Mutanten von Muscat of Alexandria gebraucht werden