Analyzing Conversations of Web Services

1. Analyzing Conversations of Web Services Tevfik Bultan Department of Computer Science University of California, Santa Barbara bultan@cs.ucsb.edu http://www.cs.ucsb.edu/~bultan Joint work with Xiang Fu, Georgia Southwestern State University Jianwen Su, University of California, Santa Barbara

2. Going to Lunch at UCSB • Before Xiang graduated from UCSB, Xiang, Jianwen and I were using the following protocol for going to lunch: • Sometime around noon one of us would call another one by phone and tell him where and when we would meet for lunch. • The receiver of this first call would call the remaining peer and pass the information. • Let’s call this protocol the First Caller Decides (FCD) protocol.

3. Implementation of the FCD Protocol Tevfik Xiang Jianwen !tj1 ?jt2 !xj1 ?jx2 !jt1 ?tj2 !tx1 ?xt2 !xt1 ?tx2 !jx1 ?xj2 ?jt1 ?xt1 ?jx1 ?tx1 ?tj1 ?xj1 !tx2 !tj2 !xt2 !xj2 !jx2 !jt2 !send ?receive Message Labels: t x 1 to Xiang 1st message from Tevfik

4. FCD Protocol does not Work with Voicemail • When the university installed a voicemail system FCD protocol started causing problems • We were showing up at different restaurants at different times! • Example scenario: tx1, jx1, xj2 The messages jx1 and xj2 are not consumed • Note that this scenario is not possible without voicemail!

5. A Different Lunch Protocol • Jianwen suggested that we change our lunch protocol as follows: • As the most senior researcher among us Jianwen would make the first call to either Xiang or Tevfik and tell when and where we would meet for lunch. • Then, the receiver of this call would pass the information to the other peer. • Let’s call this protocol the Jianwen Decides (JD) protocol

6. Implementation of the JD Protocol • JD protocol works fine with voicemail! Tevfik Xiang Jianwen ?xt ?tx ?jt ?jx !jt !jx !tx !xt

7. Conversation Protocols • The FCD and JD protocols specify a set of conversations • The implementations I showed are supposed to generate the set of conversations specified by these protocols • We can specify the set of conversations without showing how the peers implement them, we call such a specification a conversation protocol

8. FCD and JD Conversation Protocols FCD Protocol JD Protocol jt jx tx1 tj1 xt1 xj1 jt1 jx1 xj2 jx2 tj2 jt2 tx2 xt2 tx xt Conversation set: {(tx1, xj2), (tj1, jx2), (xt1, tj2), (xj1, jt2), (jt1, tx2), (jx1, xt2)} Conversation set: {(jt, tx), (jx, xt)}

9. Observations & Questions • The implementation of the FCD protocol behaves differently with synchronous and asynchronous communication whereas the implementation of the JD protocol behaves the same. • Can we find a way to identify such implementations? • The implementation of the FCD protocol does not obey the FCD protocol if asynchronous communication is used whereas the implementation of the JD protocol obeys the JD protocol even if asynchronous communication used. • Given a conversation protocol can we figure out if there is an implementation which generates the same conversation set?

10. Synchronizability and Realizability Analyses • We formalized these observations and questions using synchronizability and realizability analyses • The implementation of the JD protocol is synchronizable but the implementation of the FCD protocol is not synchronizable • The JD protocol is realizable but the FCD protocol is not realizable

11. Outline • Web Service Composition Model • Capturing Global Behaviors • Conversations • Top-Down vs. Bottom-Up Specification and Verification • Realizability vs. Synchronizability • XML messaging • MSL, XPath • Translation to Promela • Web Service Analysis Tool • Conclusions and Future Work

12. Characteristics of Web Services • Loosely coupled, interaction through standardized interfaces • Standardized data transmission via XML • Asynchronous messaging • Platform independent (.NET, J2EE) WS-CDL Interaction BPEL4WS Behavior Interface WSDL Implementation Platforms Microsoft .Net, Sun J2EE SOAP Message XML Schema Type XML Data Web Service Standards

13. Challenges in Verification of Web Services • 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? • XML data manipulation • How do we specify the XML messages? • How do we verify properties related to data?

14. A Model for Composite Web Services • A composite web service consists of • a finite set of peers • Lunch example: T, X, J • and a finite set of message classes • Lunch example (JD protocol): jt, tx, jx, xt tx Peer T Peer X xt jt jx Peer J

15. Communication Model • We assume that the messages among the peers are exchanged using reliable and asynchronous messaging • FIFO and unbounded message queues Peer T Peer X tx tx • This model is similar to industry efforts such as • JMS (Java Message Service) • MSMQ (Microsoft Message Queuing Service)

16. Conversations tx jt • A virtual watcher records the messages as they are sent Peer T Peer X Watcher jt tx Peer J • A conversation is a sequence of messages the watcher sees during an execution [Bultan, Fu, Hull, Su WWW’03]

17. Effects of Asynchronous Communication • Question: Given a composite web service, is the set of conversations a regular set? • Even when messages do not have any content and the peers are finite state machines the conversation set may not be regular • Reason: asynchronous communication with unbounded queues • Bounded queues or synchronous communication Conversation set always regular

18. Properties of Conversations • The notion of conversation enables us to reason about temporal properties of the composite web services • 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: G ( payment F receipt) • Model checking problem: Given an LTL property, does the conversation set satisfy the property?

19. 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

20. Conversation Schema tx Peer T Peer X xt Peer J jt jx jt jx Conversation Protocol LTL property ? GF(tx  xt)) tx xt Peer T Peer J Peer X Input Queue ?xt ?tx ?jt ?jx !jt !jx !tx !xt ... ? Virtual Watcher LTL property GF(tx  xt))

21. Conversation Protocols • Conversation Protocol: • An automaton that accepts the desired conversation set • A conversation protocol is a contract agreed by all peers • Each peer must act according to the protocol • For reactive protocols with infinite message sequences we use: • Büchi automata which accept infinite strings • For specifying message contents, we use: • Guarded automata • Guards are constraints on the message contents

22. 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

23. Interesting Question If this equality holds the conversation protocol is realizable Are there conditions which ensure the equivalence? ? Conversations specified by the conversation protocol  Conversations generated by the projected services

24. Realizability Problem !m2 ?m2 ?m1 !m1 Peer A Peer B Peer C Peer D Projection of the conversation protocol to the peers • Not all conversation protocols are realizable! AB: m1 CD: m2 Conversation protocol Conversation “m2 m1” will be generated by all peer implementations which follow the protocol

25. Another Non-Realizable Protocol m1 A B m2 A m2 m2 m3 C m1 m3 B m1 B A, C C BA:m2 AB:m1 m3 Watcher BA:m2 Generated conversation: m2 m1 m3 AB:m1 AC:m3

26. Realizability Conditions Three sufficient conditions for realizability (no message content) [Fu, Bultan, Su, CIAA’03, TCS’04] • 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 • At any state, each peer should be able to do only one of the following: send, receive or terminate (a peer can still choose among multiple messages)

27. Realizability Conditions • Following protocols fail one of the three conditions but satisfy the other two BA:m2 AB:m1 AB: m1 AB: m1 BA:m2 AB:m1 CD: m2 CA: m2 AC:m3 Not lossless join Not synchronous compatible Not autonomous

28. Bottom-Up Approach • We know that analyzing conversations of composite web services is difficult due to asynchronous communication • Model checking for conversation properties is undecidable even for finite state peers • The question is: • Can we identify the composite web services where asynchronous communication does not create a problem?

29. Three Examples, Example 1 • Conversation set is regular: (r1a1 | r2a2)* e • During all executions the message queues are bounded !a1 !a2 r1, r2 !e e ?r1 ?r2 ?a1 ?a2 ?e a1, a2 !r1 !r2 requester server

30. Example 2 • Conversation set is not regular • Queues are not bounded !a1 !a2 r1, r2 !e ?a1 ?a2 e ?r1 ?r2 ?e !r1 !r2 a1, a2 requester server

31. Example 3 r1, r2 !e !r1 !r2 ?r !a e ?r1 ?r2 ?a !r a1, a2 ?e requester server • Conversation set is regular: (r1 | r2 | ra)* e • Queues are not bounded

32. State Spaces of the Three Examples # of states in thousands queue length • Verification of Examples 2 and 3 are difficult even if we bound the queue length • How can we distinguish Examples 1 and 3 (with regular conversation sets) from 2? • Synchronizability Analysis

33. Synchronizability Analysis • A composite web service is synchronizable, if its conversation setdoes not change • when asynchronous communication is replaced with synchronous communication • If a composite web service is synchronizable we can check the properties about its conversations using synchronous communication semantics • For finite state peers this is a finite state model checking problem

34. Synchronizability Analysis • A composite web service is synchronizable, if it satisfies the synchronous compatible and autonomous conditions [Fu, Bultan, Su WWW’04, TSE] • Connection between realizability and synchronizability: • A conversation protocol is realizable if its projections to peers are synchronizable and the protocol itself satisfies the lossless join condition

35. Are These Conditions Too Restrictive?

36. Web Service Analysis Tool (WSAT) Verification Languages WebServices Front End Analysis Back End Intermediate Representation GFSA to Promela (synchronous communication) success BPEL to GFSA SynchronizabilityAnalysis Guarded automata BPEL fail (bottom-up) GFSA to Promela (bounded queue) Promela skip GFSA parser Conversation Protocol Guarded automaton success GFSA to Promela(single process, no communication) Realizability Analysis (top-down) fail http://www.cs.ucsb.edu/~su/WSAT/ [Fu, Bultan, Su CAV’04]

37. Guarded Automata Model • Uses XML messages • Uses MSL for declaring message types • MSL (Model Schema Language) is a compact formal model language which captures core features of XML Schema • Uses XPath expressions for guards • XPath is a language for writing expressions (queries) that navigate through XML trees and return a set of answer nodes

38. The Guarded Automata Model //type declaration request [ id [int] ] // message declaration r2: request // local variable declaration last: request !e ?a1 ?a2 !r1 !r2 Guard{ a2/id = last/id => r2/id := last/id + 1, last/id := last/id + 1 }

39. XML (eXtensible Markup Language) • XML is a markup language like HTML • Similar to HTML, XML tags are written as <tag> followed by </tag> • HTML vs. XML • In HTML, tags are used to describe the appearance of the data <b> </b> <i> </i> <br> <p> ... • In XML, tags are used to describe the content of the data rather than the appearance <date> </date> <address> </address>

40. An XML Document and Its Tree Register investorID requestList payment VIP01 stockID stockID accountNum 0001 0002 0425 <Register> <investorID> VIP01 </investorID> <requestList> <stockID> 0001 </stockID> <stockID> 0002 </stockID> </requestList> <payment> <accountNum> 0425 </accountNum> </payment> </Register> • XML documents can be modeled as trees • where each internal node corresponds to a • tag and leaf nodes correspond to basic types

41. XML Schema • XML provides a standard way to exchange data over the Internet. • However, the parties which exchange XML documents still have to agree on the type of the data • What are the tags that will appear in the document, in what order, etc. • XML Schema is a language for defining XML data types • MSL (Model Schema Language) is a compact formal model language which captures core features of XML Schema

42. MSL (Model Schema Language) • Basic MSL syntax g  | b | t[g ] | g{m ,n } | g,g | g&g | g|g g is an XML type (i.e., an MSL type expression)  is the empty sequence b is a basic type such as string, boolean, int, etc. t is a tag m and n are positive integers [ ] { } & , | are MSL type constructors

43. MSL Semantics • t[g ] denotes a type with root node labeled t with children of type g • g{m ,n } denotes a sequence of size at least m and at most n where each member is of type g • g1,g2 denotes an ordered sequence where the first member is of type g1 and the second member is of type g2 • g1&g2 denotes an unordered sequence where one member is of type g1 and the other member is of type g2 • g1|g2 denotes a choice between type g1 and type g2, i.e., either type g1 or type g2, but not both

44. An MSL Type Declaration and an Instance <Register> <investorID> VIP01 </investorID> <requestList> <stockID> 0001 </stockID> <stockID> 0002 </stockID> </requestList> <payment> <accountNum> 0425 </accountNum> </payment> </Register> Register[ investorID[string] , requestList[ stockID[int]{1,3} ] , payment[ creditCardNum[int] | accountNum[int] ] ]

45. Translating Guarded Automata to Promela • We used the SPIN model checker to verify the properties of conversations • SPIN is a finite state model checker • we restricted XML message contents to finite domains • We translate guarded automata models to Promela (input language of the SPIN model checker) • First, translate MSL type declarations to Promela type declarations • Then, translate XPath expressions to Promela code

46. Mapping MSL types to Promela • Basic types • integer and boolean types are mapped to Promela basic types int and bool • We only allow constant string values and strings are mapped to enumerated type (mtype) in Promela • Other type constructors are handled using • structured types (declared using typedef) in Promela • or arrays

47. Mapping MSL type constructors to Promela • t[g ] is translated to a typedef declaration • g{m ,n } is translated to an array declaration • g1,g2 is translated to a sequence of type declarations • g1|g2is translated to a sequence of type declarations and an enumerated variable which is used to record which type is chosen • g1&g2 is not handled! We do not handle unordered type sequence (it can cause state-space explosion)

48. Example typedef t1_investorID{ mtype stringvalue;} typedef t2_stockID{int intvalue;} typedef t3_requestList{ t2_stockID stockID [3]; int stockID_occ; } typedef t4_accountNum{int intvalue;} typedef t5_creditCard{int intvalue;} mtype {m_accountNum, m_creditCard} typedef t6_payment{ t4_accountNum accountNum; t5_creditCard creditCard; mtype choice; } typedef Register{ t1_investorID investorID; t3_requestList requestList; t6_payment payment; } Register[ investorID[string] , requestList[ stockID[int]{1,3} ] , payment[ creditCardNum[int] | accountNum[int] ] ]

49. XPath • In order to write specifications or programs that manipulate XML documents we need: • an expression language to access values and nodes in XML documents • XPath is a language for writing expressions (queries) that navigate through XML trees and return a set of answer nodes • An XPath query defines a function which • takes and XML tree and a context node (in the same tree) as input and • returns a set of nodes (in the same tree) as output

50. XPath Syntax Basic XPath syntax: q . | .. | b | t | * | /q | //q | q/q | q//q | q [ q ] | q [exp] q is an XPath query exp denotes a predicate on basic types, i.e., on the leaf nodes of the XML tree b denotes a basic type such as string, boolean, int, etc. t denotes a tag