Huibiao Zhu

Formalizing hybrid systems with event-b

This paper contains the development of hybrid systems in Event-B and the Rodin Platform. It follows the seminal approach introduced at the turn of the century in Action Systems. Many examples illustrate our approach.

Core Hybrid Event-B I

Faced with the increasing need for correctly designed hybrid and cyber-physical systems today, the problem of including provision for continuously varying behaviour as well as the usual discrete changes of state is considered in the context of Event-B. An extension of Event-B called Hybrid Event-B is presented, that accommodates continuous behaviours (called pliant events) in between familiar discrete transitions (called mode events in this context). The continuous state change can be specified by a combination of indirect specification via ordinary differential equations, or direct specification via assignment of variables to values that depend on time, or indirect specification by demanding that behaviour obeys a time dependent predicate. The syntactic elements of the extension are discussed, and the semantics is described in terms of the properties of time dependent valuations of variables. Refinement is examined in detail, with reference to the notion of refinement inherited from discrete Event-B. A full suite of proof obligations is presented, covering all aspects of the new framework. A selection of examples and case studies is presented. A particular challenge - bearing in mind the desirability of conforming to existing intuitions about discrete Event-B, and the impact on tool support (as embodied in tools for discrete Event-B like Rodin) - is to design the whole framework so as to disturb as little as possible the existing structures for handling discrete Event-B. Extends Event-B, as seamlessly as possible, to encompass continuous behaviours.Considers formal semantics.Considers refinement.Presents a full suite of proof obligations.Gives a selection of small case studies.

Modeling and Verifying Web Services Choreography Using Process Algebra

The Web Services Choreography Description Language (WS-CDL) is a newly developed specification for Web services composition to describe the observable behavior across multiple participants from a global perspective. However, this specification does not provide a formal semantics, whose informal description can lead to ambiguous understanding and different implementations. Hence, it causes difficulties for the engineering community to analyze the business behavior and ensure the correctness. In this paper, we present the semantics of WS-CDL in terms of process algebra CSP which has great advantages in designing and verifying concurrent processes. Therefore, all the properties we want to check within a WS-CDL document can be verified automatically in the CSP framework correspondingly. In addition, the exception and compensation handling mechanism, an important concept of long running transactions, is demonstrated clearly through our formalization work.

Tool Support for BPEL Verification in ActiveBPEL Engine

The BPEL is designed for integrating and orchestrating Web services and it provides the profound solution to model business process relying on Web service platform. ActiveBPEL is a commercial-grade open source implementation engine for BPEL. In this paper, we describe the work on tool support for the BPEL verification in ActiveBPEL. We implement the algorithm of the mapping from BPEL to timed automata, and integrate it into the ActiveBPEL. By using model checker UPPAAL engine, ActiveBPEL is enhanced and can verify the BPEL properties, such as deadlock and reachability. Moreover, those timed properties of BPEL specification can be checked in our framework as well. Some case studies are presented to show the usage of verification functionality in ActiveBPEL.

Modeling and Verifying the Code-Level OSEK/VDX Operating System with CSP

As an automotive industry standard of operating system specification, OSEK/VDX is widely applied in the process of designing and implementing the static operating system and the corresponding interfaces for automotive electronics. It is challenging to explore an effective method to support large-scale correctness verification of OSEK/VDX specification. In this paper, we employ process algebra CSP to describe and reason about a real code-level OSEK/VDX operating system. Thus the whole system is formally modeled as a CSP process which is encoded and implemented in process analysis toolkit (PAT). Furthermore, the expected properties are described and expressed in terms of the first-order logic. The properties are also established and verified in our framework. The result indicates that the whole system is deadlock-free and the scheduling scheme is sound with respect to the specification.

Towards the Semantics for Web Service Choreography Description Language

A choreography is a multi-part contract which describes peer to peer collaboration of services regardless of any specific programming language or supporting platform. WS-CDL, issued from W3C, is the first language for describing choreography. In this paper, we propose a language CDL0 to capture the important features of WS-CDL, including choreography composition, compensation and exception handling. An adjunctive concept role reference is introduced with the aim of distinguishing multiple participants which provide the same kind of service within a choreography model. The semantics is given by an operational approach to provide a formal base for the choreography language. We believe this formalism work helps to clear ambiguous points in the WS-CDL specification and promote the usage of choreography languages.

Specifying and Verifying Web Transactions

New evolving internet technologies are extending the role of the World Wide Web from a platform of information exhibition to a new environment for service interactions. While new business opportunities are brought in under this new era of internet, novel challenges are coming out at the same time. Current technologies have been found lacking efficient support for web transactions. Because transactions in the context of web services have distinct features, such as autonomous and interactive, the traditional automatic mechanisms of resource locking and rollback are proved to be inappropriate. For this reason, we suggest that web transactions are constructed through a series of compensable transactions, using the concept of compensation to ensure a relatively relaxed atomicity. This paper formally expresses the composition structures and behavioral dependencies of compensable transactions. Based on the formal description for a transaction model, we are able to further verify its transactional behavior according to the specified requirement of relaxed atomicity and more precise behavioral properties with temporal constraints.

Formalizing hybrid systems with Event-B and the Rodin Platform

This paper contains the development of hybrid systems with Event-B and the Rodin Platform. It follows the seminal approach introduced at the turn of the century in Action Systems. Many examples that have been entirely proved with the Rodin Platform illustrate our approach. We propose to complement the Event-B/Rodin Platform approach with the usage of Matlab, either to simulate examples with some correct as well as incorrect set of parameters, or to use the analytical power of Matlab to complement the usage of Event-B. This paper contains the development of hybrid systems with Event-B and the Rodin Platform.Many examples that have been entirely proved with the Rodin Platform illustrate our approach.Besides the formal development technique, we propose adding the usage of Matlab to verify systems.

Looking into Compensable Transactions

Transaction is a lasting debatable issue, no matter in database systems or in the new paradigm of web services. Particularly, in the context of service oriented computing, business transactions usually require long periods of time to complete. In case of failure, the traditional approaches, e.g., rollback, are not applicable to handle errors during long running transactions. Instead, compensation is suggested to be an error recovery mechanism. Hence, a business transaction is programmed as a composition of a set of compensable transactions. Sequence and parallel are two standard primitives to put compensable transactions together into a bigger compensable one. Besides, there are other useful compositional constructs, such as speculative choice, exception handling, alternative forwarding, and programmable compensation. These constructs cannot only improve the responsiveness to environment but also enhance the capability for dealing with errors. In this paper, we introduce a transactional calculus in which compensable transactions can be composed in a variety of ways. It is equipped with a trace model which is carefully presented to provide a clear meaning for each transactional construct. In addition, algebraic properties are investigated by giving corresponding equational laws.

Complementary methodologies for developing hybrid systems with event-b

This paper contains a further contribution to the handling of hybrid systems as presented in [3]. This time we insist on the usage of multiple methodologies involving not only refinements and proofs as in Event-B and the Rodin Platform, but also Matlab simulation, Animation, and Invariant discovery. We believe that a successful understanding of hybrid systems has to be done in this way by involving several distinct methodologies that are complementary. The paper also presents many examples illustrating the approach.