A Top-Down Approach to Modeling Global Behaviors of Web Services

1. A Top-Down Approach to Modeling Global Behaviors of Web Services Xiang Fu, Tevfik Bultan and Jianwen Su Department of Computer Science University of California, Santa Barbara {fuxiang,bultan,su}@cs.ucsb.edu

2. Web Services • Loosely coupled • Standardized data transmission via XML • Asynchronous messaging • Platform independent (.NET, J2EE) Interaction BPEL4WS, WSCI WSDL Service SOAP Implementation Platforms Message Microsoft .Net, Sun J2EE XML Schema Type XML Data Web Service Standards

3. Motivation • Challenges in both specification and verification • Distributed nature, no central control • How do we model the global behavior? • How do we specify the global properties? • Asynchronous messaging introduces undecidability in analysis • How do we check the global behavior? • How do we enforce the global behavior?

4. Outline • Web Service Composition Model • Capturing Global Behavior: Conversations • Bottom-Up vs. Top-Down Specification and Verification • Realizability Problem • Current and Future Work

5. Web Service Example • A user accesses to the UpToTheMinuteNews.com (U2MNews) web service using a Browser • To access the U2MNews, user’s Browser has to traverse a web proxy on the corporate firewall • A company called Acme Web Speedup Services (AWS) provides a caching proxy web service to speedup web accesses • The corporate IT starts using AWS caching service • The user starts receiving stale pages from U2MNews • We want to specify/model the following global property • User’s Browser always receives the webpages (which have U2MNews as the source) from the U2MNews web service, not from the AWS

6. Web Service Composition • A composite web service is a tripleS = (M, P, C ) [Bultan, Fu, Hull, Su WWW’03] • P : finite set of peers(web services) Req1 Browser IT • M : finite set of message classes Data1 Req3 Req2 Data2 • C : finite set of channels Data3 U2M News AWS Cache

7. Communication Channels R1 D1 • Reliable channels • Asynchronous communication • Queues are FIFO and unbounded IT Browser Req1 • Similar to industry efforts • JMS (Java Message Service) • MSMQ (Microsoft Message Queuing Service)

8. Message Classes • Messages are classified into classes • Each message class is associated with one channel • Modeling messages: Approach 1 • No message content • Can model messages with content as long as domains are finite Req1 IT Browser

9. Message Contents • Modeling messages: Approach 2 • Message classes have attributes which carry the contents of the messages message class Message Req { string url; … } Message Data { string url; string src; bool nocache; string data; … } original source message content actual source nocache tag

10. Finite State Peers • Peer: Finite state automaton + one FIFO queue • Reactive services (infinite behavior): Büchi automata • Message contents: Guarded Büchi automata outputmessages inputmessages Data3 Req3 [ Data3.src = “U2M”…] U2M News

11. Executing Web Service Composition Execution is a complete run if • Each sent message is eventually consumed • Each peer visits its final states infinitely often ! Req1 ? Data1 D1 ? Req1 Browser ! Req2 ! Data1 ! Data1 ? Req2 R1 D3 D2 ! Data2 R2 ? Data3 ? Data2 AWS Cache ! Req3 ? Req3 IT R3 ! Data3 U2M News

12. Outline • Web Service Composition Model • Capturing Global Behavior: Conversations • Bottom-Up vs. Top-Down Specification and Verification • Realizability Problem • Current and Future Work

13. Conversations Req3 Data2 Req2 Data3 • Watcher: “records” the messages as they are sent Req1 IT Browser Data1 Watcher R1 R2 R3 D3 D1 D2 U2M News AWS Cache • A conversation is a sequence of messages the watcher sees in a completerun • Conversation Set: the set of all possible conversations of a serviceS: C(S)

14. Specify System Goal Using LTL • The notion of conversation enables us to reason about temporal properties of the web service composition • LTL framework extends naturally to conversations • LTL temporal operators X (neXt), U (Until), G (Globally), F (Future) • Atomic properties Predicates on message classes or contents Example: The news is always up to date G (Data1 Data1.src = Data1.url = “U2M”) • Model checking problem: Given an LTL property, does the conversation set C(S) satisfy the property?

15. Question • Given a web service composition S, is the language C(S)always regular? If it is regular,finite statemodel checking techniques can be used for verification

16. Answer Conversation Sets are not always regular, even without message contents Example: C(S) = { w | w  (a | b) and for each prefix w’, |a|w’ |b|w’} a ?a !a ?b b !b P1 P2 • Causes: asynchronous communication with unbounded queues • Bounded queues or synchronous: Conversation Set always regular

17. Outline • Web Service Composition Model • Capturing Global Behavior: Conversations • Bottom-Up vs. Top-Down Specification and Verification • Realizability Problem • Current and Future Work

18. Bottom-Up vs. Top-Down Bottom-up approach • Specify the behavior of each peer • The global communication behavior (conversation set) is implicitly defined based on the composed behavior of the peers • Global communication behavior is hard to understand and analyze Top-down approach • Specify the global communication behavior (conversation set) explicitly as a protocol • Ensure that the conversations generated by the peers obey the protocol

19. Conversation Protocols • Conversation Protocol: • An automaton that accepts the desired conversation set • Reactive protocols, infinite message sequences • Büchi automaton • Accepts infinite strings • Message contents • Guarded automaton • Guards are constraints on the message contents • A conversation protocol is a contract agreed by all peers • Each peer must act according to the protocol

20. Req1Data1 Req2Data2 AWS System Goal Browser IT G (Data1 Data1.url = Data1.src= “U2M”) U2M Req3 Data3 IT  B: Data1 [ true  Data1’ = Data3 ] B  IT: Req1 [ true  Req1’.url = “U2M” ] IT  B: Data1 [ Data2.NoCache = false  Data1’ = Data2 ] U2M  IT: Data3 [ Req3.url = “U2M”  Data3’.url = Data3’.src = “U2M”  Data3’.NoCache = true] IT  AWS: Req2 [ true  Req2’ = Req1 ] AWS  IT: Data2 [ true  Data2’.url = Req2.url  Data2’.src = “AWS”] IT  U2M: Req3 [ Data2.NoCache = true  Req3’ = Req1 ]

21. Model Checking • Protocols without message contents • Finite state model checking techniques and tools • Protocols with finite domain message contents • Finite state model checking techniques and tools • Protocols with infinite domain message contents • Infinite state model checking techniques and tools

22. Req2,Data2 AWS Req1,Data1 Preset System Goal Browser IT G (Data1 Data1.url = Data1.src= “U2M”) Req3,Data3 U2M IT  B: Data1 [ true  Data1’ = Data3 ] B  IT: Req1 [ true  Req1’.url = “U2M” ] Signed(“U2M”, Data) Data.nocache = true IT  B: Data1 [ Data2.NoCache = false  Data1’ = Data2 ] U2M  IT: Data3 [ Req3.url = “U2M”  Data3’.url = Data3’.src = “U2M”  Data3’.NoCache = true] IT  AWS: Req2 [ true  Req2’ = Req1 ] Signed(Data2.url’, Data2’) AWS might falsify the NoCache tag AWS  IT: Data2 [ true  Data2’.url = Req2.url  Data2’.src = “AWS”] IT  U2M: Req3 [ Data2.NoCache = true  Req3’ = Req1 ]

23. Synthesize Peer Implementations • Conversation protocol specifies the global communication behavior • How do we implement the peers? • How do we obtain the contracts that peers have to obey from the global contract specified by the conversation protocol? • Project the global protocol to each peer • By dropping unrelated messages for each peer

24. ? Req2 IT ! Data1 B ? Req1 B [ true  Data1’ = Data3 ] ! Data2IT • AWS [true  Data2’.src= “AWS” Data2’.url = Req2.url  Signed(Data2’.url,Data2’)] ! Req2AWS Signed(“U2M”,Data)  Data.NoCache = true [ true Req2’ = Req1] ? Data3 U2M ? Req3IT ! Data3IT [trueData3’.url= Data3’.src= “U2M”  Data3’. NoCache=true] ? Data2AWS • U2M ! Data1 B ! Req3U2M ! Req1IT [ Data2.NoCache = false  Data1’ = Data2 ] [ Data2.NoCache = true  Req3’ = Req1 ] [ true Req1’.url = “U2M” ] • IT ? Data1IT • Browser

25. Interesting Question Are there conditions which ensure the equivalence? ? Conversations generated by the composed behavior of the projected services Conversations specified by the conversation protocol 

26. Outline • Web Service Composition Model • Capturing Global Behavior: Conversations • Bottom-Up vs. Top-Down Specification and Verification • Realizability Problem • Current and Future Work

27. Realizability Problem • Not all conversation protocols are realizable! A  B: a ! a ! b ? b ? a C  D: b Peer A Peer B Peer C Peer D Conversation protocol Conversation “ba” will be generated by any legal peer implementation which follows the protocol

28. Realizability Problem • Three sufficient conditions for realizability (contentless messages) [Fu, Bultan, Su, CIAA’03] • Lossless join • Conversation set should be equivalent to the join of its projections to each peer • Synchronous compatible • When the projections are composed synchronously, there should not be a state where a peer is ready to send a message while the corresponding receiver is not ready to receive • Autonomous • Each peer should be able to make a deterministic decision on whether to wait or to send or to terminate

29. Realizability for Guarded Protocols • One natural conjecture: • Drop all guards and message contents to get the “skeleton” of the conversation protocol • Check realizability of the skeleton • Conjecture fails because there exists • Nonrealizable guarded protocols with realizable skeletons, and • Realizable guarded protocols with nonrealizable skeletons.

30. Examples Skeleton is realizable, but guarded protocol is not Guarded protocol is realizable, but its skeleton is not . D  B: d(1) D  A: e(2) A  B: a(1) C  D: c(2) D A: e(1) D  B: d(2) C  D: c(1) A  B: a(2) A  B: a B  A: b c (1) a(2) is a conversation of the projected peers

31. Realizability for Guarded Protocols • Skeleton Approach: The fourth condition • “unambiguous” property: for each word waccepted by a projected peer: there is one and only one w’ accepted by the conversation protocol whose projection to that peer is w • This extra condition might be too restrictive • Skeleton of U2M protocol does not satisfy the fourth condition • Exhaustive state space exploration • Treat each valuation of message contents as a new message class and get a standard conversation protocol without contents • Accurate, but costly

32. Current and Future Work • More analysis tools are necessary for guarded protocols with infinite domains • Symbolic analysis • Abstraction • Tying the formal model to the web service standards • Conversation protocols, peers: BPEL4WS • Message contents: XML, XML Schema • Guard conditions: XPath • Tools for model checking web services • Finite state vs. infinite-state • Message contents, local variables

33. Related Work • Conversation specification • IBM Conversation support project http://www.research.ibm.com/convsupport/ • Conversation support for business process integration [Hanson, Nandi, Kumaran EDOCC’02] • Orchestrating computations on the world-wide web [Choi, Garg, Rai, Misram, Vin EuroPar’02] • Verification of web services • Simulation, verification, composition of web services [Narayanan, McIlraith WWW’02] • Realizability problem • Realizability of Message Sequence Charts (MSC) [Alur, Etassami, Yannakakis ICSE’00, ICALP’01]

34. Our Previous Work • Verification of Vortex workflows [Fu, Bultan, Hull, Su TACAS’01] • Restrict to finite domains, use SMV • Formal verification of e-services and workflows [Fu, Bultan, Hull, Su WES’01] • Infinite domains, use Action Language Verifier • Conversations specification [Bultan, Fu, Hull, Su WWW’01] • A formalism for specification of composite e-services • Conversation protocols [Fu, Bultan, Su CIAA’03] • Realizability conditions, model checking