Service substitution : a behavioral approach based on Petri Nets

Service-Oriented Computing is an emerging computing paradigm that supports the modular design of (software) systems. Complex systems are designed by composing less complex systems, called services. Such a (complex) system is a distributed application often involving several cooperating enterprises. As a system usually changes over time, individual services will be substituted by other services. Substituting one service by another one should not affect the correctness of the overall system. Assuring correctness becomes particularly challenging, as the services rely on each other, and each of the involved enterprises only oversees a part of the overall system. In addition, services communicate asynchronously which makes the analysis even more difficult. For this reason, formal methods to support service substitution are indispensable. In this thesis, we study service substitution at the level of service models. Thereby we restrict ourselves to service behavior. As a formalism to model service behavior, we use Petri nets. The first contribution of this thesis is the definition of several substitutability criteria that are suitable in the context of Service-Oriented Computing. Substituting a service S by a service S0 should preserve some behavioral properties of the overall system. For each set of behavioral properties and a given service S, there exists a set of behaviorally compatible services for S. A substitutability criterion defines which of these behaviorally compatible services of S have to be preserved by S0. We relate our substitutability criteria to preorders and equivalences known from process theory. The second contribution of this thesis is to present, for each substitutability criterion, a procedure to decide whether a service S0 can substitute a service S. The decision requires the comparison of the in general infinite sets of behaviorally compatible services for the services S and S0. Hence, we extend existing work on an abstract representation of all behaviorally compatible services for a given service. For each notion of behavioral compatibility, we present an algorithmic solution to represent all behaviorally compatible services. Based on these representations, we can decide substitutability of a service S by a service S0. The third contribution of this thesis is a method to support the design of a service S0 that can substitute a service S according to a substitutability criterion. Our approach is to derive a service S0 from the service S by stepwise transformation. To this end, we present several transformation rules. Finally, we formalize and we extend the equivalence notion for services specified in the language WS-BPEL. That way, we demonstrate the applicability of our work

Partner datenverarbeitender Services

Diese Arbeit untersucht den Einfluss von Daten auf das Verhalten und die Korrektheit eines verteilten Systems. Ein verteiltes System besteht aus mehreren Services. Ein Service ist eine selbständige, plattformunabhängige Einheit, die anderen Services eine bestimmte Funktionalität über eine wohldefinierte Schnittstelle zur Verfügung stellt. In dieser Arbeit betrachten wir die Interaktion von jeweils genau zwei Services miteinander. Zwei Services, die erfolgreich miteinander zusammenarbeiten können, nennen wir Partner. Ein Service heißt bedienbar, wenn er mindestens einen Partner hat. Ziel der Arbeit ist es, zu untersuchen, wann zwei Services Partner sind, und für einen Service zu entscheiden, ob dieser bedienbar ist. Aufgrund der Daten kann der Zustandsraum eines Service sehr groß oder sogar unendlich groß werden. Wir untersuchen zwei Klassen von Services mit unendlich vielen Zuständen. Für diese Klassen stellen wir Algorithmen vor, welche zu einem gegebenen Service einen Partner synthetisieren, falls ein solcher existiert. Auf diese Weise entscheiden wir konstruktiv die Bedienbarkeit eines Service. Weiterhin stellen wir Transformationsregeln für Partner vor und untersuchen, wie viel Speicherplatz ein Partner eines Services mindestens benötigt.This thesis studies the influence of data on the behavior and the correctness of a distributed system. A distributed system consists of several services. A service is a self-contained, platform-independent entity which provides a certain functionality to other services via a well-defined interface.In this thesis, we consider the interaction of exactly two services. Two services that can successfully cooperate with each other are called partners. We call a service controllable, if the service has at least one partner. The goal of this thesis is to study the conditions for which two services are partners and to decide whether a given service is controllable. Due to the data, the state space of a service may be very large or even infinite. We investigate two classes of services with infinitely many states. For these classes, we present algorithms that synthesize a partner of a service, if it exists. This allows us to decide the controllability of a service constructively. Furthermore, we present transformation rules for partners and investigate the minimum amount of memory that a partner of a service needs

Behavioral service substitution

Serviceevolution erlaubt es, einen Service durch einen anderen Service zu verfeinern oder zu ersetzen. Der Austausch durch einen anderen Service sollte garantieren, dass alle oder ausgewählte Partner des Originalservices erhalten bleiben. In dieser Arbeit entwickeln wir einen Ansatz welcher einem Serviceentwickler helfen soll, Analyse- und Syntheseaufgaben für den Serviceaustausch so durchzuführen, dass jeder Partner eines gegebenen Services beim Austausch erhalten bleibt. Wir modellieren einen Kontrollfluss eines Services als Beschreibung der Reihenfolge von asynchron kommunizierenden Ereignissen mittels eines impliziten ungeordneten Nachrichtenspeichers. Weiterhin studieren wir den Verhaltensaspekt von korrekter Interaktion zwischen Services und konzentrieren uns auf zwei Varianten von Verklemmungsfreiheit als Korrektheitskriterien von Serviceersetzung. Der wichtigste Beitrag ist ein Ansatz zur Charakterisierung jedes möglichen Austausches eines gegebenen Services. Die zentrale Idee dieses Ansatzes ist eine systematische Untersuchung der Verbindung zwischen einem Service und all seiner Partner bzgl. eines gegebenen Korrektheitskriteriums. Wir nutzen diese Verbindung um von einem gegebenen Service einen kanonischen Partner und einen kanonischen Austausch bzgl. aller Partner zu synthetisieren. Ein Service welcher den kanonischen Austausch eines gegebenen Services verfeinert wird als Austausch des gegebenen Services angesehen, wenn die Menge all seiner Partner jeden Partner des gegebenen Services enthält. Mit dem kanonischen Austausch eines gegebenen Services identifizieren wir die Menge der möglichen austauschenden Services eines gegebenen Services bei der jeder exakt die gleichen Partner wie der gegebene Service hat. Einige Ergebnisse dieser Arbeit fundieren auf früheren Arbeiten zu Austausch und Korrektheit von Services und können daher mit diesen verbunden werden um schwierigere Analyse- und Syntheseaufgaben für den Serviceaustausch durchzuführen.Service evolution allows one service to be refined into or substituted by another service. Substituting one service by another service should guarantee to preserve all or selected partners of the original service. In this thesis, we develop an approach that shall assist a service designer, such as a domain expert, to perform analysis and synthesis tasks on service substitution. We model a control flow of services that describes the ordering of asynchronously communicating events over an implicit unordered message buffer. We study the behavioral aspect of correct interaction between services and concentrate on two variants of deadlock freedom as correctness criteria of service substitution. The major contribution of this thesis is an approach for characterizing the set of all substitutes for a given service. We systematically investigate the relationship between a service and all its partners under a given correctness criterion and employ this relationship to synthesize from a given service its canonical partner and its canonical substitute with respect to all partners. A service that refines the canonical substitute for a given service is regarded as a substitute for the given service if the set of all its partners includes every partner of the given service. With the canonical substitute of a given service, we identify a specific subset of the set of all substitutes for the given service, each of which has exactly the same set of partners as that of the given service. Parts of the results in this thesis have been established upon previous works on service substitution and correctness of services. Consequently, we can also combine our results with the related existing techniques to perform more sophisticated analysis and synthesis tasks on service substitution

Behavioral service substitution

Service-oriented design supports system evolution and encourages reuse and modularization. A key ingredient of service orientation is the ability to substitute one service by another without reconfiguring the overall system. This chapter aims to give an overview of the state of the art and open challenges in the area of service substitution. Thereby, we restrict ourselves to changes of the service behavior. We present a formal model of service behavior, formalize service substitution, study algorithms to decide service substitution, and provide rules to construct services that are correct by design. Beside analysis at design time, we also investigate analysis at runtime, where we measure the deviation of a running service (or collection of services) from its specification based on recorded event data (e.g., message or transaction logs)

Behavioral service substitution

Service-oriented design supports system evolution and encourages reuse and modularization. A key ingredient of service orientation is the ability to substitute one service by another without reconfiguring the overall system. This chapter aims to give an overview of the state of the art and open challenges in the area of service substitution. Thereby, we restrict ourselves to changes of the service behavior. We present a formal model of service behavior, formalize service substitution, study algorithms to decide service substitution, and provide rules to construct services that are correct by design. Beside analysis at design time, we also investigate analysis at runtime, where we measure the deviation of a running service (or collection of services) from its specification based on recorded event data (e.g., message or transaction logs)