Fresh-Variable Automata: Application to Service Composition

Automated reasoning about systems with infinite domains requires an extension of regular automata to infinite alphabets. Existing formalisms of such automata cope with the infiniteness of the alphabet by adding to the automaton a set of registers or pebbles, or by attributing the alphabet by labels from an auxiliary finite alphabet that is read by an intermediate transducer. These formalisms involve a complicated mechanism on top of the transition function of automata over finite alphabets and are therefore difficult to understand and to work with. We introduce and study variable finite automata over infinite alphabets (VFA). VFA form a natural and simple extension of regular (and ω-regular) automata, in which the alphabet consists of letters as well as variables that range over the infinite alphabet domain. Thus, VFAs have the same structure as regular automata, only that some of the transitions are labeled by variables. We compare VFA with existing formalisms, and study their closure properties and classical decision problems. We consider the settings of both finite and infinite words. In addition, we identify and study the deterministic fragment of VFA. We show that while this fragment is sufficiently strong to express many interesting properties, it is closed under union, intersection, and complementation, and its nonemptiness and containment problems are decidable. Finally, we describe a determinization process for a determinizable subset of VFA.

The automatic composition of Web services requires the existing services to be encoded in a semantic form, such as ontology. If Web services are represented with ontology, we can automatic compose Web services by means of their semantics. Ontology modeling is divided into two steps: input/output parameters modeling and Web services modeling. First, we collect Web services' input/output parameters, represent them with classes in ontology, and transform Web services to properties in ontology, define relations between classes and properties. Then we judge Web services' relations based on relations between properties transformed from these Web services and estimate whether these Web services can be composed.

Efficient Composition of Semantic Web Services with End-to-End QoS Optimization

Automated composition of Web services has received much interest in the last decade, as it supports B2B applications. It aims at selecting and inter-connecting services provided by different partners in response to client requests. Planning techniques are used widely in the literature to describe Web services composition problem. However, since Web services proliferate day after day, classical planners are no longer well suited to compose Web services in a reasonable time. This weakness is due to the explosion of the search space caused by the large number of services and the broad range of data exchanged among services. In this paper, we propose a decentralised multi-agent approach to solve the Web services composition problem at runtime. Our model consists of a set of Web service agents where each agent has a set of services organised in a graph. Responding to a request, agents propose partial plans which are partial paths in the graph, then they coordinate their partial plans to provide the best global plan for the submitted request. The analysis of the complexity and results of the implementation show the ability of our approach to scale up when compared to the of state-of-the-art techniques for automated web service composition.

The web service composition problem with respect to behavioural descriptions deals with the automatic synthesis of a coordinator web service that controls a set of web services to reach a goal state. Despite its importance, however, solving the problem for a general case (when the coordinator has only partial observations) remains doubly exponential in the number of variables in web service descriptions. Toward this challenge, we propose two novel (signature preserving and subsuming) approximation-based approaches using abstraction and refinement. Given a set of web service behavioural descriptions and a reachability goal, we automatically construct abstract web services which have less variables using over-approximation. If our method identifies a coordinator web service, the coordinator is guaranteed to control the given web services to reach the goal state no matter how they behave. Otherwise, our method refines the current abstraction by adding some variables that have strong dependency on the goal variables.

The goal of the Web service composition (WSC) problem is to find an optimal composition of web services to satisfy a given request using their syntactic and/or semantic features. In this paper, in particular, we study the Quality of Services (QoS)-driven WSC problem to optimize service quality criteria, e.g., response time and/or throughput. We propose a novel solution based on Learning-based Depth First Search (LDFS). Given a set of web service descriptions including QoS information and a requirement web service, we reduce the QoS-driven WSC problem into a planning problem on a state-transition system. We then find the optimal solution for the problem using a dynamic programming based on LDFS which recently has shown a promising result.

The article describes a method for developing and modelling composite web-services. Web-service composition is used to derive new functionality from the interaction of existing web-services. Composite web-services are built in several stages: specifications (determining the type of service); development of the structure of the service based on the algebra of services; service composition modelling; selecting a service variant and generating a service. The main elements of the proposed approach are the algebra of services and web-services interaction models (basic and composite). The above approach formally presents the consideration of the main aspects in solving problems related to the construction of effective composite web-services and the selection of mathematical models, namely: description of web-services, determining the structure of web-services, taking into account the dynamics of information changes, taking into account the main uncertainties in building the structure web-services. Based on the approach, a method for constructing composite web-services has been developed. The method is based on an algebraic description of a web-service based on a specialized algebra of services. The method consists of the following steps: description of the task of developing a web-service; development of the structure of a web service based on the algebra of services; formal description of the structure and clarification of service operations; building models of functioning and interaction of service components; building simulation models of the composite service; defining the final structure of the composite service; web-service implementation. The main tool for describing composite services is the algebra of services, and the model building tool is colouredPetri nets implemented using СPN Tools. As an example of the application of the developed method, the construction of a pharmaceutical service is described, in which the modellingof the interaction of web-services based on Petri nets is used. The process of building a composite model of a web-service is considered. The process begins with building a pharmaceutical web-service model structure based on the service algebra. Based on the service algebra, operators were developed to implement composite services. The structure of the model is developed, which is presented in the form of a formal description. The process of building pharmaceutical service models in the СPN Tools environment is described in detail.

The increasing popularity of Web services for application integration has strengthened the need for automated Web service composition and joint execution. A lot of attention has been paid recently towards techniques for creating a composite Web service from individual Web services based on user requirements, and driven by a variety of criteria. However, the major lacuna so far in Web service composition is the lack of a holistic requirements-driven approach for modeling the Web service execution lifecycle. In this paper we present such an approach based on our earlier work on context-driven Web service modeling. In particular, we separate requirements into two parts - functional and extrafunctional requirements (FRs and EFRs, respectively). We express FRs as commitments made by individual Web ser-vices towards the composite Web service, and EFRs as rules that constrain the behavior of the individual Web services while they execute against their FRs. We also present a conceptual model that can be used for tracking requirements during Web service execution.

Aiming at the hot and difficult issues in Web services and service composition, this paper analyzes and studies the selection of Web services and service composition from the functional and non functional attributes of Web services, and mainly makes the following work and innovation. First of all, the functional attributes of Web services are studied, Web services on various service provision platforms are collected, and Web services are classified. Secondly, aiming at the shortcomings of existing Web service QoS attribute evaluation models, an improved method is proposed, which introduces a comprehensive evaluation mechanism of variable weight vector. Based on the constant weight comprehensive evaluation method, the state variable weight vector is established to dynamically adjust the attribute weights of various service QoS indicators, so as to improve the accuracy and objectivity of the Web service QoS attribute evaluation. Finally, in view of the defects and deficiencies of traditional algorithms in the selection of Web service composition, the particle swarm optimization algorithm with linearly decreasing inertia weight and learning factor is introduced, which better balances the self cognition and social learning ability of particles, and improves the search speed and global search ability of particles. And a large number of experiments are carried out to compare it with the traditional algorithm, which verifies the effectiveness and superiority of the improved particle swarm optimization algorithm.

Web service composition makes it possible for Web service reused, assembled, providing value-added service. The key problem of Web service composition is to find an effective modeling language and to describe it with composition definition language. BPEL4WS is a newly process definition language which is relatively complex and difficult to use. And BPEL4WS only supports static binding of existing services, and put binding of process and services as a known condition. But most of time dynamic service composition is needed to cater to the high dynamic environment. In this paper, firstly a process modeling language e_SPDL based on XML is proposed by simplifying and extending BPEL4WS. e_SPDL is used in the prototype system (e_Scope4WS) which supporting Web service composition. The basic structure and control structure of e_SPDL are presented in detail, and a composite service instance described in e_SPDL is given. Then the concept of service container is introduced in e_SPDL which provides the basis for dynamic service execution strategy. And the structure of service container is given. Finally the dynamic execution strategy of the process (i.e. composite service) based on e_SPDL and service container is given.

The run-time binding of web services has been recently put forward in order to support rapid and dynamic web service compositions. With the growing number of alternative web services that provide the same functionality but differ in quality parameters, the service composition becomes a decision problem on which component services should be selected such that user's end-to-end QoS requirements (e.g. availability, response time) and preferences (e.g. price) are satisfied. Although very efficient, local selection strategy fails short in handling global QoS requirements. Solutions based on global optimization, on the other hand, can handle global constraints, but their poor performance renders them inappropriate for applications with dynamic and real-time requirements. In this paper we address this problem and propose a solution that combines global optimization with local selection techniques to benefit from the advantages of both worlds. The proposed solution consists of two steps: first, we use mixed integer programming (MIP) to find the optimal decomposition of global QoS constraints into local constraints. Second, we use distributed local selection to find the best web services that satisfy these local constraints. The results of experimental evaluation indicate that our approach significantly outperforms existing solutions in terms of computation time while achieving close-to-optimal results.

A web service is a software system that supports interoperable machine to machine interaction over the network. It can be any kind of service provided on the web that can exchange data between applications. In simple terms, it is any program that is callable by another program across the web using standard protocols. Web services are special because they are independent of the platform, programming language, and model design. In current web applications, more businesses are gradually publishing their business as services over the web. This growing number of web services available within an organization and on the Web raises a new and challenging search problem: locating desired web services. Searching for web services with conventional web search engines is insufficient in this context. In this paper, a latent semantic web service discovery and composition framework, addressing both discovery and composition of web services, is introduced. The framework provides a graphical visualization interface that can facilitate users in finding the desired web service beyond the general keyword search and provide composition of complex services if the service does not exist. A novel approach based on an information retrieval technique known as latent semantic analysis is applied to a large web service descriptions data set collection. By using co-occurrence patterns across the entire data collection, and then using those patterns to infer semantic relationships between documents, relevant results in a ranked order can be retrieved. Moreover, a back to front algorithm is applied to the composition search interface in order to compose complex web services in full or partial terms from pre-existing web services when the requested service does not exist. An experimental study conducted on a collection of 2525 publicity available web service shows improved performance to the technique applied in obtaining much better search results.

A High-level Petri Net Based Approach for Modeling and Composition of Web Services

Model checking is an automatic verification technique, which provides an answer to the question whether a program, given as a state-transition system, satisfies its specification, given in terms of a temporal-logic formula. Model checking is very well studied as far as boolean finite-state programs are concerned [CGP99]. To take into account several sources of infinity such as an unknown number of processes or infinite data structures, the classical setting has been extended in several orthogonal directions. In the context of concurrent programs, one may ask whether a specification is satisfied independently of the number of participating processes. This question is referred to as parameterized verification (see [Esp14] for an overview). Second, a system may have to cope with variables ranging over an infinite domain such as the natural numbers or finite strings. Depending on the operations that are allowed on this domain, system executions can then be described as words over an infinite alphabet, possibly equipped with one or several binary relations such as equality or a total order. Those words are usually referred to as data words [BDM+11], [BMSS09]. Many models and results from both areas, parameterized verification and data words, smoothly extend the classical finite-state approach and, in particular, provide decidable instances of the modelchecking problem. Our concern in this talk will be distributed algorithms, where an unknown number of (identical) processes cooperate to achieve a common goal. However, assuming perfectly identical processes, even simple tasks such as electing a leader cannot always be accomplished. One may, therefore, assume that every process is equipped with a unique process identifier from an unbounded domain, and that identifiers can be compared with one another wrt. a total order. Thus, when modeling distributed algorithms, one has to cope with both sources of infinity mentioned above: the number of processes and infinite data. This may be one reason why there have been only a few approaches to the formal verification of distributed algorithms [KVW12]. In this talk, we survey recent developments in the areas of parameterized verification and data words, and we demonstrate how they can be exploited towards a framework for the formal verification of distributed algorithms [ABG15].

Automated reasoning about systems with infinite domains requires an extension of automata, and in particular, finite automata, to infinite alphabets. One such model is Variable Finite Automata (VFA). VFAs are finite automata whose alphabet is interpreted as variables that range over an infinite domain. On top of their simple and intuitive structure, VFAs have many appealing properties. One such property is a deterministic fragment (DVFA), which is closed under the Boolean operations, and whose containment and emptiness problems are decidable. These properties are rare amongst the many different models for automata over infinite alphabets. In this paper, we continue to explore the advantages of DVFAs, and show that they have a canonical form, which proves them to be a particularly robust model that is easy to reason about and use in practice. Building on these results, we construct an efficient learning algorithm for DVFAs, based on the \(\textsc {L}^*\) algorithm for regular languages.