Formal Methods for Service Composition

1. Formal Methods for Service Composition Maurice H. ter Beek (ISTI–CNR, Pisa, Italy) Saturday, December 1 SEEFM 2007 • joint work with: Antonio Bucchiarone (IMT Institute for Advanced Studies, Lucca, Italy and Nokia Siemens Networks, Lisbon, Portugal) Stefania Gnesi (ISTI–CNR, Pisa, Italy)

2. Background Service composition approaches Syntactic service composition Semantic service composition Service composition characteristics Connectivity, correctness and QoS Compare standardization approaches w.r.t. characteristics Formal methods for service composition Automata, Petri nets and process algebras Compare formal methods w.r.t. characteristics Conclusions Outline

3. Service-Oriented Computing (SOC) An emerging cross-disciplinary paradigm for distributed computing Changes the way in which software applications are designed, architected (SOA), delivered and consumed Web Services (WSs) Autonomous, platform-independent computational elements, possibly managed by different organizations Described, published, discovered, orchestrated and programmed to build networks of collaborating applications, distributed both within and across organizational boundaries Background • We survey and compare service composition approaches (both industrial and academic)

4. Service Orchestration (like BPEL4WS) Combines available services by adding a central coordinator This orchestrator is responsible for invoking + combining services Service Choreography (like WS-CDL) No central coordinator Complex tasks defined by conversations of participating services Composition of peer-to-peer interactions among the collaborating services Syntactic Service Composition Approaches

5. Both XML-based BPEL: coordination/composition of services (WSDL-based) Processes model the flow of services by connecting activities that communicate with external service providers WS-CDL: choreography description of services Interactions describe the information exchange by specifying participants, information and channel Exception handling and compensations supported through exception and finalizer work units Contrary to BPEL, WS-CDL describes a global view of the behavior of the message exchanges of all services(rather than behavior defined from viewpoint of one service) BPEL vs. WS-CDL

6. Aim: the automation of service discovery, invocation, composition, interoperation and execution monitoring Describe services by explicit, machine-understandable semantics Often rely on ontologies to formalize the domain concepts shared among services (like OWL-S and WSMO) The Internet is seen as a globally linked database in which web pages are marked with semantic annotations Semantic Service Composition Approaches

7. Both ontology-based OWL-S Defines a service ontology with four main elements: service concept, service profile, service model and service grounding No clear distinction between choreography and orchestration WSMO Defines a model to describe semantic web services with four main elements: ontologies, WSs, goals and mediators Conceptual design in WSMF, annotations in WSML, execution environment WSMX for dynamic discovery/selection/invocation OWL-S more mature in certain aspects (choreography), while WSMO provides a more complete conceptual model OWL-S vs. WSMO

8. Connectivity: Reliability The ability to deliver responses continuously in time The ability to correctly deliver messages between two endpoints Accessibility The percentage of responses per service request Exception handling/Compensations What happens in case of an error and how to undo the already completed activities The ability to manage compensations of service invocations (in case of a failure) Service Composition Characteristics I

9. Correctness: Safety/Liveness The assertion that some bad event never happens in the course of a computation The assertion that some event does eventually happen in the course of a computation Security/Trust The ability of a service (composition) to provide proper authentication, authorization, confidentiality and data encryption The assurance that a service (composition) will perform as expected despite possible environmental disruptions, human and operators errors, hostile attacks and design and implementation errors Service Composition Characteristics II

10. Quality of Service (QoS): Accuracy The error rate of a service, measured as the number of errors generated by a service in a certain time interval Availability The probability that a service is available at any given time, measured as the percentage of time a service is available over an extended time period Performance Measured as the success rate of service requests: Maximum time needed to complete a request (response time) Number of completed requests over a period of time (throughput) Time needed by a service to process a request (latency) Service Composition Characteristics III

11. Comparing Standardization Approaches • Neither of these approaches offer any direct support for the verification of service compositions at design time • This is where formal methods come into play !

12. Automata Well-known model underlying formal specifications I/O automata, timed automata, team automata, etc. Their formal basis allows for automatic tool support Exemplary approaches (see paper for references) Frameworks to analyze and verify properties of service compositions of BPEL processes Translations from BPEL to Promela (finite automata) to use the SPIN model checker to verify LTL properties Translations from WS-CDL to timed automata to use the UPPAAL model checker to verify (timed) CTL properties Formal Methods for Service Composition I

13. Petri nets Well-known framework for modeling concurrent systems Their ease of conceptual modeling (graphical notation) has made Petri nets the model of choice in many applications Their formal basis allows for automatic tool support Exemplary approaches (see paper for references) Mapping of all BPEL control-flow constructs into labeled Petri nets (including the dead-path-elimination technique) Open-source tools BPEL2PNML and WofBPEL automatically transform BPEL processes in Petri nets and analyze them (including reachability analysis) Formal Methods for Service Composition II

14. Process Algebras Precise and well-studied set of formalisms CCS, π-calculus (which inspired BPEL to a certain extent), LOTOS, etc. Their formal basis allows automatic verification of behavioral properties Rich theory on bisimulation analysis for equivalence testing (to verify substitutivity + redundancy in service compositions) Exemplary approaches (see paper for references) Specify and compose services in CCS to use Concurrency Workbench to validate correctness properties Translations from BPEL to LOTOS to use CADP model-checking toolbox to verify temporal properties Formal Methods for Service Composition III

15. Paper provides a reference for service composition designers and developers willing to use formal methods and tools Comparing Approaches in Formal Methods

16. Most standardization approaches to service composition lack: Support to verify the (behavioral) correctness of service compositions Support to perform quantitative analysis of QoS aspects Formal Methods and tools allow one to simulate and verify the behavior of one’s model at design time Thus enable the detection and correction of errors as early as possible and in any case before implementation ! The use of formal methods can increase the confidence in the correctness of one’s (service composition) design Conclusions