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

On the verification of EPCs using T-invariants

To verify a (business) process model, for example expressed in terms of an Event-driven Process Chain (EPC), most of the approaches described in literature require the construction of its state space. Unfortunately, for complex business processes the state space can be extremely large (if at all finite) and, as a result, constructing the state space may require excessive time. Moreover, semi-formal modeling languages such as the EPC language require a rather lenient interpretation of their semantics. To circumvent both the state-explosion problem and the semantics-related problems of EPCs, we propose an alternative approach based on transition invariants (T-invariants). T-invariants are well-known in the Petri-net community. They do not require the construction of the state space and can be computed efficiently. Moreover, we will show that our interpretation of T-invariants in this context can be used to deal effectively with the semantics-related problems of EPCs. To demonstrate our approach we will use two case studies: one is based on the reference model of SAP R/3 while the other one is based on a trade execution process within a large Dutch bank. We will also argue that the approach can be applied to other (informal or formal) modeling techniques

On the verification of EPCs using T-invariants

To verify a (business) process model, for example expressed in terms of an Event-driven Process Chain (EPC), most of the approaches described in literature require the construction of its state space. Unfortunately, for complex business processes the state space can be extremely large (if at all finite) and, as a result, constructing the state space may require excessive time. Moreover, semi-formal modeling languages such as the EPC language require a rather lenient interpretation of their semantics. To circumvent both the state-explosion problem and the semantics-related problems of EPCs, we propose an alternative approach based on transition invariants (T-invariants). T-invariants are well-known in the Petri-net community. They do not require the construction of the state space and can be computed efficiently. Moreover, we will show that our interpretation of T-invariants in this context can be used to deal effectively with the semantics-related problems of EPCs. To demonstrate our approach we will use two case studies: one is based on the reference model of SAP R/3 while the other one is based on a trade execution process within a large Dutch bank. We will also argue that the approach can be applied to other (informal or formal) modeling techniques