WSMX Execution Semantics Executable Software Specification Eyal Oren DERI

WSMX execution semantics2 Background information WSMX: execution environment for dynamic discovery, selection, mediation and invocation of semantic web services WSMX is built on WSMO: services, ontologies, goals and mediators WSMX: manages repository, achieves user’s goal by selecting matching service, mediating and invoking

WSMX execution semantics3 Designing WSMX Conceptual model: what are we talking about Architecture: components, how they interact Execution semantics: system behaviour

WSMX execution semantics4 What did we do? Design of WSMX includes formal specification of system behaviour Reasons: –enlarging developers’ understanding of the system –proving properties of the system –enabling model-driven component execution We present the execution semantics of WSMX Did our approach address the objectives?

WSMX execution semantics5 What are execution semantics? Four phases in software development: requirements, design, implementation, testing Formal methods can be used during design: to improve the design process and the result Formal method: a mathematically-based language for specifying systems Execution semantics: a formal definition of operational behaviour of a system.

WSMX execution semantics6 Why model execution semantics? Formal methods do not make correct programs! Increase understanding and agreement between project members (by explicit specification process) To check properties of the constructed specification: discover future properties of the modelled system

WSMX execution semantics7 Why model execution semantics? Execute the specification –general idea: specification is input for execution engine; engine executes model, informs components to perform a task (workflow management, business process management) –main argument in favour: flexibility, not hard- coding this interplay, no need to translate specification into programming code –main argument against: inefficient implementation; might not be important: if composition is simple and components themselves are implemented efficiently

WSMX execution semantics8 Requirements Enlarge developers’ understanding: technique must be easily readable yet unambiguous Check properties of the system: technique must be formal and have a proof system Automate coordination between components: technique must have operationalisation, and execution procedure for a model

WSMX execution semantics9 How to model execution semantics? We want to automate control flow between components Components treated as black-boxes: we know their functionality but not how they operate Using Petri nets –graphical, supposedly easily understood –formalised, well analysable –executable

WSMX execution semantics10 WSMX execution semantics (see paper)

WSMX execution semantics11 Using execution semantics The goal was: –understandability –model checking –model-driven execution Did this work?

WSMX execution semantics12 Did it work? Understandability –not easily readable: syntax is very simple, but model gets complex –hard to write and time-consuming to maintain –simulation is very helpful Executing specification –technical problems: synchronous invocation from the engine –work in progress – the general idea is ok

WSMX execution semantics13 Did it work? Model checking –large amount of work on problems in Petri nets (algorithms, tools, decidability & complexity results) –however, we did not use this possibility: –during implementation developers had to manually translate the Petri net specification into Java code. –but model is not easily readable: implementation did not strictly follow model! –model checking not very useful

WSMX execution semantics14 Conclusion Specifying execution semantics as part of software development Three objectives: understandability, model checking, component execution Used Petri nets to specify the behaviour of WSMX Petri nets are not a useful technique for this specification: –not easily readable (and not easily writable) –model checking and executability in principle possible

WSMX execution semantics15 Future Investigate other techniques that overcome these problems –take into account: we want to do model checking and execute the specification –UML Activity diagrams: easily readable, more intuitive, no model execution –YAWL: builds on Petri nets, designed for usability, no good tools yet Separate techniques: –use one model for proving and executing –use second model for clarification and understanding –(second would be generated from the first)

WSMX execution semantics16 Questions?