1. A-Priori Verification of Web Services with Abduction Marco Alberti 1 Federico Chesani 2 Marco Gavanelli 1 Evelina Lamma 1 Paola Mello 2 Marco Montali 2 1 ENDIF, Università di Ferrara 2 DEIS, Università di Bologna

2. CILC'06, Bari, 27 giugno 2006 A-Priori Verification of Web Services with Abduction 2 Motivation: Service Oriented Computing Web services: – Defined by their behavioural interface – Simple stand-alone applications, or – Potential components of complex applications, by Composition of web services: – As specified by a choreography Will the web services comply to the choreography?

3. CILC'06, Bari, 27 giugno 2006 A-Priori Verification of Web Services with Abduction 3 Choreographies vs. MAS protocols Similarities (Baldoni et al., LNCS 3670): – Both define rules for collaboration among peers – Both express dependencies and constraints in interactions – Both abstract away from internals, focusing on interface Wide literature on MAS protocol verification Approach: apply MAS techniques to SOC verification

4. CILC'06, Bari, 27 giugno 2006 A-Priori Verification of Web Services with Abduction 4 In this talk: The A l LoWS framework Abductive Logic Web-service Specification Language to specify choreographies and web service interfaces (originally: interaction protocols and reactive agents), with an abductive semantics Definition of conformance of a web service to a choreography Abductive proof procedures to verify conformance

5. CILC'06, Bari, 27 giugno 2006 A-Priori Verification of Web Services with Abduction 5 Representation of interactions Interaction as exchange of messages. Messages format: – m x (Sender, Receiver, Content) Events: actual message exchanges – H(Message[,Time]) Expectations: desired message exchanges – E(Event[,Time])

6. CILC'06, Bari, 27 giugno 2006 A-Priori Verification of Web Services with Abduction 6 Choreography Specification Abductive triple ε : abducible signatures (events H and expectations E chor ) Choreography Knowledge Base: logic program with expectations in clause bodies Choreography Integrity Constraints: implications relating events and expectations

7. CILC'06, Bari, 27 giugno 2006 A-Priori Verification of Web Services with Abduction 7 Choreography example KB chor deadline(10). IC chor H(m 1 (X,Y),T 1 ) /\ deadline(T d ) E chor (m 2 (Y,X),T 2 ) /\ T 2 < T 1 + T d \/ E chor (m 3 (Y,X),T 2 ) /\ T 2 < T 1 + T d

8. CILC'06, Bari, 27 giugno 2006 A-Priori Verification of Web Services with Abduction 8 Web Service specification Abductive triple ε : abducible signatures (events H and expectations E ws ) Web service Knowledge Base: logic program with expectations in clause bodies Web service Integrity Constraints: implications relating events and expectations

9. CILC'06, Bari, 27 giugno 2006 A-Priori Verification of Web Services with Abduction 9 Conformance Our idea of conformance: a web service will comply to a choreography,provided that its peers comply too. Feeble conformance: all expected events happen. Strong conformance: all happened events are expected.

10. CILC'06, Bari, 27 giugno 2006 A-Priori Verification of Web Services with Abduction 10 Possible interaction A pair (HAP, EXP) that, together with the KB, entails the following: – a Goal (usually defines a needed event) – The integrity constraints – The web service being discussed follows its own expectations about itself – The other peers follow the choreography expectations about themselves

11. CILC'06, Bari, 27 giugno 2006 A-Priori Verification of Web Services with Abduction 11 Feeble conformance A possible history HAP is feeble conformant if there exists EXP that, together with joined KBs, entails: - A goal - The integrity constraints (IC chor U IC ws ) - E ws (X) H(X) - E chor (X) H(X) - is a feeble conformant interaction if EXP is minimal w.r.t set inclusion - A web service is feeble conformant if all the possible histories are feeble conformant

12. CILC'06, Bari, 27 giugno 2006 A-Priori Verification of Web Services with Abduction 12 Strong conformance A feeble conformant interaction is also strong conformant if it entails the following: H(m x (ws,R,C)) E chor (m x (ws,R,C)) H(m x (S,ws,C)) E ws (m x (S,ws,C)) A web service is strong conformant if all possible histories are strong conformant.

13. CILC'06, Bari, 27 giugno 2006 A-Priori Verification of Web Services with Abduction 13 The SCIFF abductive proof procedure Derived from Fung and Kowalskis IFF – Originally applied to compliance verification in MAS Properties: – Termination for acyclic programs – Soundness – Completeness Generative version: g-SCIFF (events are abducible) – Applied to verification of protocol properties

14. CILC'06, Bari, 27 giugno 2006 A-Priori Verification of Web Services with Abduction 14 Conformance verification g-SCIFF SCIFF /\ Feeble Conformance Possible Interaction HAP,EXP Is HAP feeble conformant? Strong Conformance Are all events expected?

15. CILC'06, Bari, 27 giugno 2006 A-Priori Verification of Web Services with Abduction 15 Example: more capabilities Choreography – H(m 1 (ws,X,C))E chor (m 2 (X,ws,D)) Web service – H(m 1 (ws,X,C)) E ws (m 2 (X,ws,D)) \/ E ws (m 3 (X,ws,D)) Result: feeble and strong conformant

16. CILC'06, Bari, 27 giugno 2006 A-Priori Verification of Web Services with Abduction 16 Example: Missing Capability Choreography – H(m 1 (ws,X,C)) E chor (m 2 (X,ws,D)) \/ E chor (m 3 (X,ws,D)) Web service – H(m 1 (ws,X,C)) E ws (m 2 (X,ws,D)) Result: not feeble conformant Counterexample: HAP = {H(m 1 (ws,a,c)),H(m 3 (a,ws,d))}

17. CILC'06, Bari, 27 giugno 2006 A-Priori Verification of Web Services with Abduction 17 Example: Wrong reply Choreography – H(m 1 (X,ws,C)) E ws (m 2 (ws,X,D)) \/ E ws (m 3 (ws,X,D)) Web service – H(m 1 (X,ws,C)) E chor (m 2 (ws,X,D)) Result: non conformant Counterexample HAP = {H(m 1 (a,ws,c)),H(m 3 (ws,a,d))}

18. CILC'06, Bari, 27 giugno 2006 A-Priori Verification of Web Services with Abduction 18 Example: Predefined answer Choreography – H(m 1 (X,ws,C)) E chor (m 2 (ws,X,D)) \/ E chor (m 3 (ws,X,D)) Web service – H(m 1 (X,ws,C)) E ws (m 2 (ws,X,D)) Result: feeble and strong conformant

19. CILC'06, Bari, 27 giugno 2006 A-Priori Verification of Web Services with Abduction 19 Ongoing work Is a uniform language good w.r.t expressiveness? Automatic translation from graphical notations Experimentation and (performance) comparison with other frameworks

20. A-Priori Verification of Web Services with Abduction Marco Alberti 1 Federico Chesani 2 Marco Gavanelli 1 Evelina Lamma 1 Paola Mello 2 Marco Montali 2 1 ENDIF, Università di Ferrara 2 DEIS, Università di Bologna