Section: New Results

Data driven systems

Web services

Participants : Blaise Genest, Loïc Hélouët.

This year, we considered transactional properties (ACID) for web services. In particular, we focused on the atomicity (A of ACID) property, obtained in case of a failure inside an atomic block through compensation of the executed actions of the block. To do so, logs need to be kept. We were interested in maintaining the maximal amount of privacy. We proposed modular algorithms [23] which maintain privacy between modules, with minimal information shared among modules, both in the logging and the compensation phases. Furthermore, each module logs a small number of information, such that the sum of all actions logged is guaranteed minimal. Last, modularity allows fast algorithms, as they need to consider only what happens in the module itself, and not the exact structure of its parent module nor of its sub-modules.

We also have extended the session system model originally proposed in  [55] . We have deisgned a mode for Web-based systems that allows to describe systems running an arbitrary number of transactions over an arbitrary number of agents. For these systems, syntactic restrictions allow to decide coverability properties, and then more elaborated business rules, such as conflict of interest (the fact that a participant to a system can be involved in two exclusive services), or the Chinese Wall Property (that prevents users of a system to use benefits or information right they may have obtained from a privileged role at later instant of any execution of the system. These results were obtained with M. Mukund and S. Akshay within the context of the DISTOL associated team, and should lead to a publication next year.

Implementation of scenarios

Participants : Loïc Hélouët, Rouwaida Abdallah.

We have revisited the problem of program synthesis from specifications described by High-level Message Sequence Charts. The main objective is to obtain a distributed implementation (for instance described with communicating automata) from a global specification given as High-level MSCS. In the general case, synthesis by a simple projection on each component of the system allows more behaviors in the implementation than in the specification. The differences arise from loss of ordering among messages, but we have shown that for a subclass of HMSCs (the local HMSCs) behaviors can be preserved by addition of communication controllers, that intercept messages to add stamping information before resending them, and deliver messages to processes in the order described by the specification. This work was published in [19] .

The second aspect of our work on scenarios implementability has considered implementation of requirements expressed as non-local HMSCs. We have proposed a new technique to transform an arbitrary HMSC specification into a local HMSC, hence allowing implementation. This transformation can be automated as a constraint optimization problem, and the impact of modifications brought to the original specification minimized w.r.t. a cost function. The approach was evaluated on a large number of randomly generated HMSCs, and the results show an average runtime of a few seconds, which demonstrates applicability of the technique. These results were published in [28] . Both results mentionned in this sections are part of the PhD thesis of Rouwaida Abdallah, defended this year [14] .

Attribute grammars

Participant : Éric Badouel.

Evaluation of attributes w.r.t. an attribute grammar can be obtained by inductively computing a function expressing the dependencies of the synthesized attributes on inherited attributes. This higher-order functional approach to attribute evaluation can straightforwardly be implemented in a higher-order lazy functional language like Haskell. The resulting evaluation functions are, however, not easily amenable to optimization when we want to compose two attribute grammars. In [21] , we present an alternative first-order functional interpretation of attribute grammars where the input tree is replaced by an extended cyclic tree each node of which is aware of its context viewed as an additional child tree. These cyclic representations of zippers (trees with their context) are natural generalizations of doubly-linked lists to trees over an arbitrary signature. Then we show that, up to that representation, descriptional composition of attribute grammars reduces to the composition of tree transducers.