Section: New Results
Specific studies: active documents and web services
Participants : Albert Benveniste, Loïc Hélouët, Benoît Masson.
Active Documents have been introduced by the GEMO team at INRIA Futurs, headed by Serge Abiteboul, mainly through the language Active XML (or AXML for short). AXML is an extension of XML which allows to enrich documents with service calls or sc's for short. These sc's point to web services that, when triggered, access other documents; this materialization of sc's produces in turn AXML code that is included in the calling document. One therefore speaks of dynamic or intentional documents. In the past years, we have collaborated with the GEMO team to study a distributed version of their language.
This year, we have addressed the problem of distributed documents from a different point of view. Starting from our knowledge of distributed active XML (DAXML), we have first proposed a Petri Net semantics for a subset of DAXML  , and then considered compositionality issues  . Compositionality in services can be addressed in several ways: first one have to ensure that modules that provide services an modules that use them agree on the data that they exchange. This notion is called composability of modules. However, composability does not ensure that a service always terminates (i.e. it returns a result to the caller) when it is invoked with appropriate data. Composability plus termination of services is called compatibility. We have shown that under some restrictions on the recursion in active documents, on the data, and upon the assumption that services use positive guards, composability is decidable. This work has also helped us isolate the core idea behind active documents, and propose a model for them called Docnets. Docnets are dynamic Petri nets which places are typed, which transitions are guarded computable type transformations, and which can receive new tokens from their environment. Docnet modules compose well, and if their closure by type transformation is finite, their compatibility is decidable. This work led to a publication  .
Within the context of the DST associated team, we have proposed a new model, that combines arbitrary numbers of finite workflows, hence allowing for the definition of sessions. Sessions is a central paradigm in web-based systems. As messages exchange between two sites need not follow the same route over the net, a site can not rely on the identity of machines to uniquely define a transaction. This unique identification is essential, as commercial site, for instance, need to manage several interactions at a given time. The current trend, as in BPEL, is to associate a unique identifier to each session. Modeling realistic session hence often forces to include session counters, and hence render most of properties undecidable. The session formalism studied in 2011 can be seen as a mix of BPEL and ORC elements, but was designed to keep several properties decidable. The strength of this formalism is to allow designing systems that use sessions without the obligation to provide identifiers. The formalism has the expressive power of reset Petri nets for which coverability is decidable. This is sufficient to decide whether a set of agent can be found in some bad configuration during the lifetime of a system. This joint work with Ph. Darondeau from the S4 Team, and with M. Mukund from the Chennai Mathematical Institute led to a publication in the ATVA conference  .
Our last work on Web-services was the development of an experimental platform. During his post-doc, Benoît Masson has designed a distributed Active XML engine, which can be distributed over a network. We have built a lightweight experimentation platform, made of four linux machines, that run DAXML services and communicate with one another. Simultaneously, R. Abdallah has designed a synthesis tool to generate REST services from High-level Message Sequence Charts. These services were successfully tested on the platform.