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Resource Contention in Real-time Systems
The divide—and—conquer method is extensively used for system design. For real-time systems the separated components execute concurrently using some common computational infrastructure and this can lead to contention for system resources, such as processors, memory, communication channels, and so on. Unless the resource contention is accommodated, then a system built from the composition of components may not function as expected and the “proven” behaviour of the components can be invalid. To overcome this uncertainty a divide—conquer—and—system-composition method is required. This thesis takes a different approach to many of the existing notations which focus on descriptions of behaviour. The Composite Transition System notation and algebra presented here enables the resource usage of the components to be specified and combined to form a composite system of concurrently executing components. By relating the composite system to the realisable behaviour of the system resources provided by the common infrastructure it becomes possible to determine any violation of the constraints imposed by the system resources. If the composite system model is then constrained by the resource behaviours then it is possible through an extraction operation to determine the modified behaviour of the components that will yield a system free of resource contention. Component specification, concurrent composition, the application of system level constraints and extraction are applied in this thesis to a system encountered in a commercial application. The purpose of this example is to demonstrate contention modelling and the mathematics of the notation, rather than to prove any specific properties of the application. Deployment of the notation to more complex applications will require the development of software tools to compute concurrent composition and extraction, and this is the motivation for the mathematical treatment in this thesis
Discovery and validation for composite services on the semantic web
urrent technology for locating and validating composite services are not sufficient due to the following reasons. • Current frameworks do not have the capacity to create complete service descriptions since they do not model all the functional aspects together (i.e. the purpose of a service, state transitions, data transformations). Those that deal with behavioural descriptions are unable to model the ordering constraints between concurrent interactions completely since they do not consider the time taken by interactions. Furthermore, there is no mechanism to assess the correctness of a functional description. • Existing semantic-based matching techniques cannot locate services that conform to global constraints. Semantic-based techniques use ontological relationships to perform mappings between the terms in service descriptions and user requests. Therefore, unlike techniques that perform either direct string matching or schema matching, semantic-based approaches can match descriptions created with different terminologies and achieve a higher recall. Global constraints relate to restrictions on values of two or more attributes of multiple constituent services. • Current techniques that generate and validate global communication models of composite services yield inaccurate results (i.e. detect phantom deadlocks or ignore actual deadlocks) since they either (i) do not support all types of interactions (i.e. only send and receive, not service and invoke) or (ii) do not consider the time taken by interactions. This thesis presents novel ideas to deal with the stated limitations. First, we propose two formalisms (WS-ALUE and WS-π-calculus) for creating functional and behavioural descriptions respectively. WS-ALUE extends the Description Logic language ALUE with some new predicates and models all the functional aspects together. WS-π-calculus extends π-calculus with Interval Time Logic (ITL) axioms. ITL axioms accurately model temporal relationships between concurrent interactions. A technique comparing a WS-π-calculus description of a service against its WS-ALUE description is introduced to detect any errors that are not equally reflected in both descriptions. We propose novel semantic-based matching techniques to locate composite services that conform to global constraints. These constraints are of two types: strictly dependent or independent. A constraint is of the former type if the values that should be assigned to all the remaining restricted attributes can be uniquely determined once a value is assigned to one. Any global constraint that is not strictly dependent is independent. A complete and correct technique that locates services that conform to strictly dependent constraints in polynomial time, is defined using a three-dimensional data cube. The proposed approach that deals with independent constraints is correct, but not complete, and is a heuristic approach. It incorporates user defined objective functions, greedy algorithms and domain rules to locate conforming services. We propose a new approach to generate global communication models (of composite services) that are free of deadlocks and synchronisation conflicts. This approach is an extension of a transitive temporal reasoning mechanism