Two Formal Approaches for Web Services: Process Algebras & Action Languages

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1 SAPIENZA UNIVERSITÀ DI ROMA DOTTORATO DI RICERCA IN INGEGNERIA INFORMATICA XX CICLO 2008 Two Formal Approaches for Web Services: Process Algebras & Action Languages Antonella Chirichiello

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3 SAPIENZA UNIVERSITÀ DI ROMA DOTTORATO DI RICERCA IN INGEGNERIA INFORMATICA XX CICLO Antonella Chirichiello Two Formal Approaches for Web Services: Process Algebras & Action Languages Thesis Committee Prof. Marco Schaerf Prof. Luigia Carlucci Aiello Prof. Roberto Baldoni (Advisor) Reviewers Prof. Enrico Giunchiglia Prof. Yves Lespérance

4 AUTHOR S ADDRESS: Antonella Chirichiello Dipartimento di Informatica e Sistemistica Antonio Ruberti Sapienza Università di Roma Via Ariosto 25, I Roma, Italy. chirichiello@dis.uniroma1.it WWW: chirichiello/

5 To my beloved Andrea

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7 Abstract Web services emerged recently as a promising way to develop applications through the Internet. Nowadays, an increasing number of companies and organizations only implement their core business and outsource other application services over Internet. A central challenge is the development of modelling techniques and tools for enabling the (semi)-automatic composition and analysis of these services, taking into account their semantic and behavioral properties. Research and standards are exploring a variety of approaches, which can be broken along several dimensions. A key dimension concerns whether the focus is on message passing (as found in WSDL and BPEL4WS) or actions performed (as found in OWL-S). Another key dimension concerns how behaviors should be modelled, including both the behavior of individual Web services and also the desired or actual behaviors of compositions of Web services. Possibilities here include flowchart-based approaches (e.g., workflow, BPEL4WS), automata-based models, or goal-driven approaches (e.g., OWL-S). This dissertation addresses the problem of engineering composite Web services, i.e., designing services that combine existing services to satisfy new requirements. Specifically, the research objective is to describe, simulate, (automatically) compose, verify, and develop Web services. Motivated by the challenge discussed above, we have experimented with two formalisms, namely process algebras and action languages, and investigated how they differ w.r.t. the research issues. On the one hand, we argue that the essential facets of Web Services, and especially, those useful to understand their interaction, can be described using process algebra languages. In fact, Web service description and execution languages, such as BPEL4WS, are essentially process description languages. They are based on primitives for behavior description and message exchange, which can also be found in process algebras. Our investigation has proved that the Web service community can benefit from the sophisticated languages developed in the process algebra area. Process algebras can be used to provide solutions to a number of challenges raised by the Web service paradigm, since they are very well studied and have formal foundation for investigating various aspects of interests of process-based systems in general. Furthermore, they are equipped with tools that are effective at verifying that compositions of services conform their requirements and respect desired properties. Process iii

8 algebras can be used also to develop certified Web services following a well-defined method. To this end, we suggest a general framework based on a mapping between process algebras and Web services written in BPEL4WS, and illustrate both the modelling of services by process algebras and the use of reasoning tools. On the other hand, we present a declarative approach to Web services composition based on reasoning about actions formalisms, and in particular on action languages. Action languages are attractive because they are concise and because their semantics is based on a theory of causality: the meaning of a domain description in such languages can be represented as a transition diagram, a directed graph with vertices corresponding to states of the world and edges denoting the changes caused by the occurrence/nonoccurence of actions. Various action languages have been proposed in literature, offering different capabilities and expressiveness. In this dissertation we focus on the action language C+. Our approach is based on a vision of services similar to OWL-S process model. Atomic services are described by specifying their preconditions and effects in an action theory. We show how the problem of Web services composition can be formulated in the action theory as a planning task. Such a characterization allows us to use the tool CCALC that implements (a fragment of) C+, for finding the solution to the problem and automatically synthesizing a composite service for the client request. The idea of using reasoning about action formalisms, mainly Situation Calculus, in the context of Web services is not new. Very few proposals exploit the expressiveness capabilities of action languages for workflows specification. The contribution of our investigation to the research in this direction is the presentation of a declarative approach which is very flexible. We specify the effects of the activities of Web services without any specific knowledge about how they are implemented and how they should be coordinated. Furthermore, the entailment relation that characterizes the action language enables us to reason about the correctness of the specification of a composite service in terms of achieving a client request. Although promising, this approach is based on a simplified assumption of complete knowledge, which in the context of the Web is unrealistic. As a first attempt to investigate complex scenarios involving incomplete information, we describe an extension of the action language framework for dealing with actions that explicitly affect knowledge. Specifically, we propose a generalization of the action language C+ by introducing epistemic modalities in the underlying logic to distinguish what is known about the world from what is unknown. Then, we suggest how the expressive capabilities of the new language could be exploited for describing the additional aspects of services related to information requirements and knowledge gain.

9 Acknowledgments I would like to thank all of the people that have consistently helped me during the years of my Ph.D. First of all, I am grateful to Luigia Carlucci Aiello who encouraged me throughout all phases of my Ph.D. studies, and to my supervisor Marco Schaerf, who gave me the opportunity to work in his group as a research assistant. I thank my supervisor also for the travel opportunities and for the possibility he gave me to participate within the Project ASTRO, funded by the Italian Ministry for Research under FIRB framework (funds for basic research), which partially supported my work. My gratitude is also due to Marco Cadoli, Paolo Liberatore, and Amedeo Cesta, for their patience concerning technical questions, and for the fruitful discussions that provided me with many valuable ideas and insights. A special thank goes to Vladimir Liftschitz for taking the time to read and comment on early drafts of some parts of this dissertation, and for suggesting to me several improvements. I am also thankful to the committee members and the external reviewers, for their advises about the contents and presentation of this dissertation. Many thanks to Andrea Vitaletti, Lucas Bordeaux, and in particular to Gwen Salaün, for their scientific and human support. They have been precious mentors and colleagues since the very beginning of my studies. I have also benifited from many discussions with teachers and friends of the Diparimento di Informatica e Sistemistica Antonio Ruberti. I must mention Vincenzo Bonifaci, Vittorio Amos Ziparo, Adnan Mian Noor, Giuseppe Paolo Settembre, Luca Allulli, Marco Fratarcangeli, Federico Pecora, Paolo Romano, Fabio Patrizi and Andrea Ribichini. Additionally, I would like to thank Maria Grazia Giacon and Giuseppina Melita, secretaries of the Diparimento di Informatica e Sistemistica Antonio Ruberti, for their constant encouragement in these last years of hard work and study. Last, but not least, I want to express my special gratitude to my father and mother, Giuseppe and Giuseppa, to my brother Mirko, and to my dearest friends Alessandra, Caterina and her lovely daughter Sara, Lucilla, Cristina, Michelangelo, Filomena, Sabrina, Emanuele, Agostino, for cheering up my life. Without their support, nothing of this work could have been accomplished. Finally, I want to thank my beloved Andrea. He has been patient, understanding, and above all, always encouraging me to do my best. v

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11 Contents Abstract Acknowledgments Contents List of Figures List of Tables iii v ix xi xiii 1 Introduction Automated Web Service Composition Research Objectives and Contributions Outline Web Services Service Oriented Computing Web Service Approach to SOC Web Service Description WSDL Web Service Composition BPEL4WS Semantic Web Services OWL-S OWL-S versus WSDL/BPEL Example: A Stock Management System Remarks vii

12 3 Formal Approaches for Web Services Labelled Transition Systems Petri Nets Process Algebras AI Planning Hierarchical Task Networks Planning as Model Checking PDDL and Estimated Regression Planning Situation Calculus Event Calculus Action Languages Multi-Agent Systems Remarks Representing and Reasoning about Web Services with Process Algebras Introduction to Process Algebras Common Process Algebras Constructs CCS LOTOS Description of Web Services Composition of Web Services Choreography Orchestration Automated Reasoning on Web Services Negotiation Among Web Services What Does Negotiation Involve? Negotiation Using LOTOS/CADP Development of Web Services From Process Algebras to BPEL On Correctness and Completeness of the Encoding Example: A Stock Management System cont d Specification and Validation Translation into BPEL Comparison with Related Work Remarks Representing and Reasoning About Web Services with Action Languages The Action Language C Multivalued Propositional Formulas Syntax of C Nonmonotonic Causal Theories

13 5.1.4 Semantics of C Transition Systems, Paths, Plans and Goals Reasoning about Actions The Causal Calculator (CCALC) Describing and Reasoning About Web Services with C Description of Web Services Composition of Web Services Example: The On-line Municipal Agency Dealing with Incomplete Information Towards an Epistemic Extension of the Action Language C Using KC+ for Web Services Comparison with Related Work Remarks Discussion and Conclusions Critical Evaluation of the Two Proposed Approaches Summary Future Perspectives A Translation of the LOTOS Choice Operator into BPEL 179 A.1 Preliminaries A.2 Basic Interactions A.3 Conclusion B Abbreviations for C+ Causal Laws 193 Bibliography 216

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15 List of Figures 2.1 A Service Oriented Architecture The Web Service Architecture A Composite Web Service The Web Service Stack A WSDL Service Specification Web Services Choreography Web Services Orchestration Orchestration vs. Choreography Behavioral Interfaces: Local Store and Supplier Overview of the Service-to-be and its Mates Overview of the Process Algebras Approach An Example of Choreography An Example of Orchestration Example of Executions of CCS Processes: Process1 and Process Example of Executions of CCS Processes: Process3 and Process Datatypes Dependence Graph Process Involved in the Negotiation Steps Transition Diagram for C+ Action Description AD Transition Diagram for KC+ Action Description AD k xi

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17 List of Tables 4.1 Practical Assessment of the Approach Correspondence Between Process Algebras (LOTOS) and BPEL Constructs Translation of C+ to Causal Theories Common Epistemic Axioms A.1 Encoding the LOTOS Choice into BPEL A.2 Encoding the LOTOS Choice into BPEL (cont d) A.3 Encoding the LOTOS Choice into BPEL (cont d) B.1 Abbreviations for Causal Laws in C B.2 Abbreviations for Causal Laws in C+ cont nd xiii

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19 Chapter 1 Introduction Web services are network-based software components deployed and then accessed through the Internet using standard interface description languages and uniform communication protocols. Each service solves a precise task, and may communicate with other services by exchanging messages. Nowadays, an increasing number of companies and organizations implement their core business and outsource other application services over the Internet. Thus, the ability to efficiently and effectively select and integrate interorganizational and heterogeneous services on the Web at runtime, is an important step towards the development of Web service applications. 1.1 Automated Web Service Composition Given a repository of services and a client request, the Web service composition problem involves finding multiple services that can be put together in correct order of execution to satisfy the client request. The multiple services should be composed preferably in an automated fashion. The word automated in this context refers to the fact that the methodology should be able to construct the composite Web service from already existing ones with minimal developer intervention. An important characteristic of Web service composition is that it can be iterated, thus allowing the definition of increasingly complex applications by aggregating components: the composite service can be used as a component service by other (composite) services. Automated Web service composition is emerging as a paradigm for enabling application integration within and across organizational boundaries. Several initiatives have been conducted with the intention to provide platforms (e.g., IBM s Web- Sphere Toolkit [IBM, WebSphere], Sun s Open Net Environment [Sun Microsystem, ONE] and Microsoft s.net [Microsoft, Net]) and languages (e.g., SOAP [W3C, 2007a], WSDL [W3C, 2001a; W3C, 2007b; W3C, 2007c; W3C, 2007d], BPEL4WS 1

20 2 1. Introduction [Curbera et al., 2002], OWL-S [DMAL, 2006]) that support and allow an easy integration of of such heterogeneous systems. At the same time, this trend has triggered a considerable number of research efforts on the composition of Web services. Research cuts across many areas of computer science - Data Integration, Artificial Intelligence (Knowledge Representation, Planning and Reasoning About Actions), Theoretical Computer Science (Program Synthesis and Verification), and Software Engineering (Software Components) - and comes with numerous proposals that address several aspects of Web service composition including Web service description, simulation, verification and development. Formal approaches play a fundamental role in this context. They allow us to deal with Web services and then to tackle several issues in an abstract way. For example, they can help us to define unambiguous semantics for the languages and protocols that underpin existing Web service infrastructures, and provide a basis for checking the conformance and compliance of bundled services. They can also empower dynamic discovery and binding with compatibility checks against behavioral properties. Despite all the efforts, the automated composition of Web services is still a complex task and it is not clear which method serves it best. Broadly speaking, we can say that all the proposed techniques fall into one of these two categories: methods based on a predefined workflow model and methods based on Artificial Intelligence (AI) planning. Methods Based on a Predefined Workflow Model. This approach is adopted widely in industry. It considers the composite service similar to a workflow that describes the interaction protocol among component services and specifies the control and data flow among services. In this case the automation means that the method can locate the correct services if the abstract workflow model is given. The industry approach looks at Web services mainly from the runtime perspective of functions, data and control flows. It uses XML-based standards like SOAP, UDDI [OASIS, 2002], and WSDL to formalize the specification of Web services, and BPEL4WS (BPEL for short, recently evolved into WS-BPEL [OASIS, 2007]) that provides the basis for manually specifying composite Web services using a procedural language that coordinates the activities of other services. The Web Service Definition Language (WSDL) describes a Web service via its set of visible operations. They can be thought of as message endpoints, or set of messages that the service can send and receive. An operation is either reactive, in which a message is received by the service, or proactive, in which a message is sent out from the service. A reactive operation can be declared one-way, i.e., it does not return a response, or request-response, and the return type is also declared. A proactive operation can be declared either as notification, i.e., the message is sent out without waiting for a response, or as solicit-response, which blocks the execution of the service till the arrival of the response, whose type is also declared. Receive and response

21 1.1. Automated Web Service Composition 3 types are defined by using XML Schema [W3C, 2001b]. WSDL provides a functioncentric description of Web services covering inputs and outputs. A service is viewed both as a server (via its reactive operations) and as a client (via its proactive operations). Business Process Execution Language for Web Services (BPEL4WS - BPEL for short) is the de facto standard language for describing workflows in Web service area. A BPEL process, also called business process, describes the behavior of an orchestrator by means of workflow constructs and communication primitives. BPEL introduces a stateful model of Web services interacting by exchanging sequences of messages between business partners. The major parts of a BPEL process definition consist of (i) partners of the business process (Web services that the process interacts with), (ii) a set of variables that keep the state of the process, and (iii) an activity defining the logic behind the interactions between the process and its partners. Activities that can be performed are categorized into basic, structured, and scope-related activities. Basic activities perform simple operations like receive, reply, invoke and others. Structured activities impose an execution order on a collection of activities and can be nested. Then, scope-related activities enable defining logical units of work and delineating the reversible behavior of each unit. Generally, workflow descriptions encoded in BPEL are written by domain experts. Thus, they won t change until the experts that wrote them decide to modify them. Industry likes workflows because they offer predictable performance: once a company develops a workflow, the actors are expected to follow it to the letter. Moreover, the workflow implementers can analyze and modify it if data from past experiences shows inefficiencies. These workflows also have some degree of fault tolerance thanks to their fault-handling mechanisms. However, they are largely rigid. The only flexibility in dynamically adapting the workflow schema is in the specification of binding details at runtime, and in executing specific branches in it. Methods Based on AI Planning. This approach has been investigated mainly in academic communities where the focus is on reasoning about Web services by explicitly declaring their preconditions and effects with terms defined in ontologies. If the user can specify the preconditions and effects required by the composite service, a plan or process is generated automatically by logical theorem provers or AI planners without knowledge of a predefined workflow. During the planning, the business logic can provide constraints in the planning setting. In this case, the automation means that the method can generate the abstract workflow model automatically. The Semantic Web [Berners-Lee, Hendler, & Lassila, 2001] provides a common framework that allows data to be shared and reused across application, enterprises, and community boundaries. It is a collaborative effort led by the World Wide Web Consortium (W3C) [W3C, World Wide Web Consortium] with participation from a large number of researchers and industrial partners. The goal is to provide machinereadable descriptions of Web services, in order to enable automated discovery, nego-

22 4 1. Introduction tiation with, composition, enactment, and monitoring of Web services. The Semantic Web project has resulted in initiatives like OWL-S, which is an OWL-based ontology [W3C, 2004a] language designed to model and describe the meanings and intended usage of Web services more precisely than is currently supported by industrial standards. OWL-S describes Web services in terms of their process model. The basic building block of the OWL-S process model is the notion of process. This includes both atomic and composite processes. Processes are specified to have inputs, outputs, preconditions, and (conditional) effects (IOPEs). Inputs specify the information that the service requires for its execution, whereas outputs are generated as a result of the execution. Preconditions specify conditions that must be satisfied for a service to execute correctly, whereas effects describe the conditions in the world that must be satisfied after the execution of the service. Composite processes can be created by combining atomic processes using a set of flow control constructs such as Sequence, Split, Split+Join Unordered, Choice, If-Then-Else, Iterate, and Repeat-Until. OWL-S is the only Web service language that can be directly connected with AI planning. In general, a planning problem is described as a tuple S, S 0, G, A, Γ, where S is the set of all possible states of the world S 0 S denotes the set of initial states of the world G S denotes set of goal states of the world the planner attempts to reach A is the set of actions the planner can perform in attempting to change one state to another state in the world Γ S A S is the transition relation that defines the precondition and effects for the execution of each action. In the context of Web services, S 0 and G denote the initial state and the goal state specified in the requirement of Web service requesters. A is a set of available services. Γ denotes the state change function of each service, specified through the precondition and effect properties of the service model. Therefore, given a representation of services as OWL-S processes, AI planning techniques can be exploited for automatic service composition by treating this problem as a planning problem. Ideally, given a client objective and a set of Web Services, a planner would find a collection of Web Service requests that achieve the objective. Observe that the planning problems that will naturally occur are not classical, for the obvious reason that, due to the nature of the Web, we cannot assume complete information. Recently, much work on planning and reasoning about actions has studied how to generate (either conditional or conformant) plans in presence of incomplete information in the initial situation and on the effects of actions, possibly also dealing with partial state observability. Such techniques can therefore be profitably applied to the composition of Web services

23 1.2. Research Objectives and Contributions 5 when only a partial description of the component services is available, and to analyze the implications of partial state observability. 1.2 Research Objectives and Contributions This dissertation addresses the problem of engineering composite Web services, i.e., designing services that combine existing services to satisfy new requirements. Specifically, the research objective is to describe, simulate, (automatically) compose, verify, and develop Web services. Inspired by the Web service initiatives proposed both in industry and in academia, and motivated by the need for semantically well-founded and algorithmically manageable formalisms for tackling several issues of Web services, in this dissertation we advocate the use of two different types of formalisms, namely process algebras and action languages, for describing and reasoning about Web services, and investigate how they differ w.r.t. the research issues. Process Algebras. It is generally recognized (see for instance [Meredith & Bjorg, 2003]) that Web services and their interaction are best described by using process description languages, and it is clear that the standards for describing and composing Web services, like BPEL, are actually such languages. Abstract processes can be used to describe services at different level of expressiveness and to compose them in order to build more complicated services. Process algebras [Bergstra, Ponse, & Smolka, 2001] appeared to us as a natural formalism for providing solutions to a number of challenges raised by the Web service paradigm. Our interest in using process algebras is motivated by the fact that they are very well studied and have a formal foundation. Generally speaking, process calculi provide a simple and expressive framework in which to represent and reason about Web services. Moreover, they are equipped with tools that are effective at verifying that compositions of services conform their requirements and respect desired properties. Finally, abstract descriptions may also be used as a first step to develop certified Web services following a well-defined method. To this end, we suggest a general framework based on a mapping between abstract processes and executable services, and illustrate both the modelling of services by process algebras and the use of reasoning tools. Central to the process algebras approach is the definition of a two-way mapping between abstract processes and executable services (implementations and their associated interfaces). Our main contribution to this work can be identified in the definition of a methodology for encoding process algebraic description of Web services into executable code. In particular, starting from an initial proposal [Salaün, Ferrara, & Chirichiello, 2004; Salaün, Bordeaux, & Schaerf, 2006], we have defined [Chirichiello & Salaün, 2006; Chirichiello & Salaün, 2007] in a more precise way the correspondence between abstract processes and executable services and, based

24 6 1. Introduction on this correspondence, we have elaborated some systematic guidelines to enable developers to develop and deploy running Web services from abstract and validated descriptions of services-to-be. Regarding the Web service(s) to be implemented (compared to the other ones which are viewed as behavioral interfaces), we concentrate, at the concrete level, on WSDL interfaces and BPEL services. The proposed method has been elaborated empirically, via simulation experiments carried out using the Oracle BPEL Process Manager 2.0 [Oracle, BPEL Process Manager]. We have developed and deployed running Web services, thus our ideas have not remained at a conceptual level, as is most of the time the case. Furthermore, to illustrate the interest of the process algebras approach to Web services, we have considered an application to the problem of negotiation. This problem is a typical example of services involving both data (prices, goods, stocks, etc) and behaviors. Negotiation issues appear when several participants (clients and providers) have to interact to reach an agreement that is beneficial to all of them. In this context, we have experimented with the use of the process algebra LOTOS [ISO, 1989] and its tool CADP (Construction and Analysis of Distributed Processes) [VASY, CADP], to ensure trustworthy and automated negotiation steps. Summarizing all the results of our investigation, we have proved that the Web services community can benefit from the sophisticated languages developed in the process algebra area. In fact, process algebras can be used to tackle the composition of Web services because they are compositional languages (complex Web services can be built from basic ones easily by using process algebra constructs). In general, we can distinguish two notions of composition: choreography, that aims at finding a global model for services describing their interaction in a precise way; and orchestration, that aims at solving more complex tasks by building a new service (the orchestrator) using existing services by exchanging messages with them. Concerning the choreography issue, process algebras can be used to represent the behavior of all interacting Web services, and consequently perform on-the-fly compatibility checks in order to ensure that the component services can cooperate for solving the given tasks. Concerning the orchestration issue, process algebras can be used to verify that the interaction between the orchestrator and each of the selected partners works properly; to support the correct development of Web services. In particular, on one hand they can be used for encoding purposes during the design stage, i.e., for specifying abstractly the new service and its interactions with the other participants, and then translating it into an executable language. In this case, process algebras are especially worthy as a first description step because they enable us to analyze the problem at hand, to clarify some points, and to sketch a (first) solution using an abstract language (thus dealing only with essential concerns). From such a formal description of one or more services-to-be, verification tools can be used to validate their correct execution and, if necessary, to verify