Extending and Relating Semantic Models of Compensating CSP

Business transactions involve multiple partners coordinating and interacting with each other. These transactions have hierarchies of activities which need to be orchestrated. Usual database approaches (e.g.,checkpoint, rollback) are not applicable to handle faults in a long running transaction due to interaction with multiple partners. The compensation mechanism handles faults that can arise in a long running transaction. Based on the framework of Hoare's CSP process algebra, Butler et al introduced Compensating CSP (cCSP), a language to model long-running transactions. The language introduces a method to declare a transaction as a process and it has constructs for orchestration of compensation. Butler et al also defines a trace semantics for cCSP. In this thesis, the semantic models of compensating CSP are extended by defining an operational semantics, describing how the state of a program changes during its execution. The semantics is encoded into Prolog to animate the specification. The semantic models are further extended to define the synchronisation of processes. The notion of partial behaviour is defined to model the behaviour of deadlock that arises during process synchronisation. A correspondence relationship is then defined between the semantic models and proved by using structural induction. Proving the correspondence means that any of the presentation can be accepted as a primary definition of the meaning of the language and each definition can be used correctly at different times, and for different purposes. The semantic models and their relationships are mechanised by using the theorem prover PVS. The semantic models are embedded in PVS by using Shallow embedding. The relationships between semantic models are proved by mutual structural induction. The mechanisation overcomes the problems in hand proofs and improves the scalability of the approach

An Algebra of Hierarchical Graphs and its Application to Structural Encoding

We define an algebraic theory of hierarchical graphs, whose axioms characterise graph isomorphism: two terms are equated exactly when they represent the same graph. Our algebra can be understood as a high-level language for describing graphs with a node-sharing, embedding structure, and it is then well suited for defining graphical representations of software models where nesting and linking are key aspects. In particular, we propose the use of our graph formalism as a convenient way to describe configurations in process calculi equipped with inherently hierarchical features such as sessions, locations, transactions, membranes or ambients. The graph syntax can be seen as an intermediate representation language, that facilitates the encodings of algebraic specifications, since it provides primitives for nesting, name restriction and parallel composition. In addition, proving soundness and correctness of an encoding (i.e. proving that structurally equivalent processes are mapped to isomorphic graphs) becomes easier as it can be done by induction over the graph syntax

Hierarchical models for service-oriented systems

We present our approach to the denotation and representation of hierarchical graphs: a suitable algebra of hierarchical graphs and two domains of interpretations. Each domain of interpretation focuses on a particular perspective of the graph hierarchy: the top view (nested boxes) is based on a notion of embedded graphs while the side view (tree hierarchy) is based on gs-graphs. Our algebra can be understood as a high-level language for describing such graphical models, which are well suited for defining graphical representations of service-oriented systems where nesting (e.g. sessions, transactions, locations) and linking (e.g. shared channels, resources, names) are key aspects

Recovery within long running transactions

As computer systems continue to grow in complexity, the possibilities of failure increase. At the same time, the increase in computer system pervasiveness in day-to-day activities brought along increased expectations on their reliability. This has led to the need for effective and automatic error recovery techniques to resolve failures. Transactions enable the handling of failure propagation over concurrent systems due to dependencies, restoring the system to the point before the failure occurred. However, in various settings, especially when interacting with the real world, reversal is not possible. The notion of compensations has been long advocated as a way of addressing this issue, through the specification of activities which can be executed to undo partial transactions. Still, there is no accepted standard theory; the literature offers a plethora of distinct formalisms and approaches. In this survey, we review the compensations from a theoretical point of view by: (i) giving a historic account of the evolution of compensating transactions; (ii) delineating and describing a number of design options involved; (iii) presenting a number of formalisms found in the literature, exposing similarities and differences; (iv) comparing formal notions of compensation correctness; (v) giving insights regarding the application of compensations in practice; and (vi) discussing current and future research trends in the area.peer-reviewe

LTS Semantics for Compensation-based Processes

Business processes design is an error-prone task often relying on long-running transactions with compensations. Unambiguous formal semantics and flexible verification tools should be used for early validation of processes. To this aim, we define a small-step semantics for the Sagas calculus according to the so-called coordinated interruption policy. We show that it can be tuned via small changes to deal with other compensation policies and discuss possible enhancements

Activity diagrams: a formal framework to model business processes and code generation

Activity Diagram is an important component of the set of diagrams used in UML. The OMG document on UML 2.0 proposes a Petri net based semantics for Activity Diagrams. While Petri net based approach is useful and interesting, it does not exploit the underlying inherent reactive behaviour of activity diagrams. In the first part of the paper, we shall capture activity diagrams in synchronous language framework to arrive at executional models which will be useful in model based design of software. This also enables validated code generation using code generation mechanism of synchronous language environments such as Esterel and its programming environments. Further, the framework leads to scalable verification methods. The traditional semantics proposed in OMG standard need enrichment when the activities are prone to failure and need compensating actions. Such extensions are expected to have applications in modelling complex business processes. In the second part of the paper, we propose an enrichment of the UML Activity Diagrams that include compensable actions. We shall use some of the foundations on Compensable Transactions and Communicating Sequential Processes due to Tony Hoare. This enriched formalism allows UML Activity Diagrams to model business processes that can fail and require compensating actions

A compensating transaction example in twelve notations

The scenario of business computer systems changed with the advent of cross-entity computer interactions: computer systems no longer had the limited role of storing and processing data, but became themselves the players which actuated real-life actions. These advancements rendered the traditional transaction mechanism insufficient to deal with these new complexities of longer multi-party transactions. The concept of compensations has long been suggested as a solution, providing the possibility of executing “counter”-actions which semantically undo previously completed actions in case a transaction fails. There are numerous design options related to compensations particularly when deciding the strategy of ordering compensating actions. Along the years, various models which include compensations have emerged, each tackling in its own way these options. In this work, we review a number of notations which handle compensations by going through their syntax and semantics — highlighting the distinguishing features — and encoding a typical compensating transaction example in terms of each of these notations.peer-reviewe

Issues about the Adoption of Formal Methods for Dependable Composition of Web Services

Web Services provide interoperable mechanisms for describing, locating and invoking services over the Internet; composition further enables to build complex services out of simpler ones for complex B2B applications. While current studies on these topics are mostly focused - from the technical viewpoint - on standards and protocols, this paper investigates the adoption of formal methods, especially for composition. We logically classify and analyze three different (but interconnected) kinds of important issues towards this goal, namely foundations, verification and extensions. The aim of this work is to individuate the proper questions on the adoption of formal methods for dependable composition of Web Services, not necessarily to find the optimal answers. Nevertheless, we still try to propose some tentative answers based on our proposal for a composition calculus, which we hope can animate a proper discussion