Tools for Automated Verification of Web Services Tevfik Bultan Department of Computer Science University of California, Santa Barbara

Characteristics of Web Services Web services: Web accessible software applications which interact with each other through the Internet Goals Platform independent (.NET, J2EE) Dynamic service discovery Loosely coupled Tolerate pauses in availability and slow data transmission Approach Standardized data transmission: XML Interaction through standardized interfaces: WSDL Asynchronous messaging

Web Service Standards Data Type Service Composition Message BPEL4WS Web Service Standards Implementation Platforms Microsoft.Net, Sun J2EE WSDL SOAP XML Schema XML WSCI Interaction

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?

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 Collaborators: Xiang Fu, Jianwen Su, Rick Hull

An Example: Stock Analysis Service Three peers: Investor (Inv), Stock Broker (SB), and Research Department (RD) Inv initiates the stock analysis service by sending a register message to the SB The SB may accept or reject the registration If the registration is accepted, the SB sends an analysis request to the RD RD sends the results of the analysis directly to the Inv as a report After receiving a report the Inv can either send an ack to the SB or cancel the service Then, the SB either sends the bill for the services to the Inv, or continues the service with another analysis request

An Example: Stock Analysis Service (SAS) register ack, cancel accept, reject, bill request, terminate report Investor (Inv) Research Dept. (RD) Stock Broker (SB) SAS is a composite web service –a finite set of peers: Inv, SB, RD –and a finite set of message classes: register, ack, cancel, accept, reject, bill, request, terminate, report

Communication Model We assume that the messages among the peers are exchanged using reliable and asynchronous messaging –FIFO and unbounded message queues This model is similar to industry efforts such as –JMS (Java Message Service) –MSMQ (Microsoft Message Queuing Service) req Stock Broker (SB) Research Dept. (RD) req

!register ?reject ?accept ?report !ack !cancel ?bill Investor ?register !reject !accept !request ?ack ?cancel !bill Stock Broker Firm ?request ?terminate Research Dept. !report acc req reg rep ack Composite Web Service Execution !terminate bil ter

Conversations A virtual watcher records the messages as they are sent Watcher A conversation is a sequence of messages the watcher sees during an execution [Bultan, Fu, Hull, Su WWW’03] register accept request report Investor (Inv) Research Dept. (RD) Stock Broker (SB) ack rep acc bil regack reqter bill terminate

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

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 ( accept  F bill ) Model checking problem: Given an LTL property, does the conversation set satisfy the property?

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

Conversation Protocol A  B:msg1 B  A:msg2 B  C:msg3 C  B:msg4 B  C:msg5 G(msg1  F(msg3  msg5)) ? LTL property !msg1 ?msg2 Peer A ?msg1 !msg2 !msg5 !msg3 ?msg4 Peer B ?msg3 !msg4 Peer C Peer APeer BPeer C msg1 msg2, msg6 msg3, msg5 msg4 Conversation Schema Input Queue... Virtual Watcher ?msg6 B  A:msg6 !msg6 ?msg5 G(msg1  F(msg3  msg5)) ? LTL property

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

SAS Conversation Protocol register reject terminate accept request reportack request report ack cancel billcancel bill terminate This conversation protocol specifies the set of conversations for the SAS

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

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

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

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

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)

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

Bottom-Up Approach We know that analyzing conversations of composite web services is difficult due to asynchronous communication The question is: –Can we identify the composite web services where asynchronous communication does not create a problem?

Three Examples, Example 1 requesterserver !r 2 ?a 1 ?a 2 !e !r 1 Conversation set is regular: ( r 1 a 1 | r 2 a 2 )* eConversation set is regular: ( r 1 a 1 | r 2 a 2 )* e During all executions the message queues are bounded r 1, r 2 a 1, a 2 e ?r 1 !a 1 !a 2 ?r 2 ?e

Example 2 Conversation set is not regularConversation set is not regular Queues are not bounded requesterserver !r 2 ?a 1 ?a 2 !e !r 1 r 1, r 2 a 1, a 2 e ?r 1 !a 1 !a 2 ?r 2 ?e

Example 3 Conversation set is regular: ( r 1 | r 2 | ra )* eConversation set is regular: ( r 1 | r 2 | ra )* e Queues are not bounded requesterserver !r 2 ?a!r !e !r 1 r 1, r 2 a 1, a 2 e ?r 1 ?r 2 ?e ?r !a

State Spaces of the Three Examples queue length # of states in thousands 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

Synchronizability Analysis A composite web service is synchronizable, if its conversation set does not change –when asynchronous communication is replaced with synchronous communication A composite web service is synchronizable, if it satisfies the synchronous compatible and autonomous conditions [Fu, Bultan, Su WWW’04]

Are These Conditions Too Restrictive? Problem SetSizeSynchronizable? SourceName#msg#states#trans. ISSTA’04SAS91215yes IBM Conv. Support Project CvSetup444yes MetaConv446no Chat245yes Buy556yes Haggle858no AMAB81015yes BPEL spec shipping233yes Loan666yes Auction9910yes Collaxa. com StarLoan677yes Cauction576yes

BPEL to GFSA Guarded automata GFSA to Promela (bounded queue) BPEL Web Services Promela Synchronizability Analysis GFSA to Promela (synchronous communication) Intermediate Representation Conversation Protocol Front End Realizability Analysis Guarded automaton skip GFSA parser success fail GFSA to Promela (single process, no communication) success fail AnalysisBack End (bottom-up) (top-down) Verification Languages Web Service Analysis Tool (WSAT) [Fu, Bultan, Su CAV’04]

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

The Guarded Automata Model //type declaration request [ id [int] ] // message declaration r2: request // local variable declaration last: request Guard{ a2/id = last/id => r2/id := last/id + 1, last/id := last/id + 1 } !r 2 ?a 1 ?a 2 !e !r 1

XML (eXtensible Markup Language) XML is a markup language like HTML Similar to HTML, XML tags are written as followed by HTML vs. XML –In HTML, tags are used to describe the appearance of the data... –In XML, tags are used to describe the content of the data rather than the appearance

An XML Document and Its Tree VIP investorID Register VIP01 requestList payment accountNum 0425 stockID XML documents can be modeled as trees where each internal node corresponds to a tag and leaf nodes correspond to basic types

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

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

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 g 1, g 2 denotes an ordered sequence where the first member is of type g 1 and the second member is of type g 2 g 1 & g 2 denotes an unordered sequence where one member is of type g 1 and the other member is of type g 2 g 1 | g 2 denotes a choice between type g 1 and type g 2, i.e., either type g 1 or type g 2, but not both

An MSL Type Declaration and an Instance Register[ investorID[string], requestList[ stockID[int]{1,3} ], payment[ creditCardNum[int] | accountNum[int] ] VIP

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

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

Mapping MSL type constructors to Promela t [ g ] is translated to a typedef declaration g { m, n } is translated to an array declaration g 1, g 2 is translated to a sequence of type declarations g 1 | g 2 is translated to a sequence of type declarations and an enumerated variable which is used to record which type is chosen g 1 & g 2 is not handled! We do not handle unordered type sequence (it can cause state-space explosion)

Example Register[ investorID[string], requestList[ stockID[int]{1,3} ], payment[ creditCardNum[int] | accountNum[int] ] 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; }

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

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

XPath Semantics Given an XML tree and a node n as a context node. returns n.. returns the parent of n Given an XML tree and a set of nodes * returns all the nodes b returns the nodes that are of basic type b t returns the nodes which are labeled with tag t

XPath Semantics Contd. Starting at the context node / q returns the nodes that match q // q returns the nodes that match q starting at any descendant q 1 / q 2 returns each node which matches q 2 starting at a child of a node which matches q 1 q 1 // q 2 returns each node which matches q 2 starting at a descendant of a node which matches q 1 q 1 [ q 2 ] applies q 2 to the children of the nodes which match q 1 q [ exp ] returns the nodes that match q and for children of which the expression exp evaluates to true

Examples //payment/* returns the node labeled accountNum /Register/requestList/stockID/int returns the nodes labeled 0001 and 0002 //stockID[int > 1]/int returns the node labeled 0002 investorID Register VIP01 requestList payment accountNum 0425 stockID

XPath to Promela Generate code that evaluates the XPath expression [Fu, Bultan, Su ISSTA’04] Traverse the XPath expression from left to right –Code generated in each step is inserted into the BLANK spaces left in the code from the previous step –A tree representation of the MSL type is used to keep track of the context of the generated code Uses two data structures –Type tree shows the structure of the corresponding MSL type –Abstract statements which are mapped to Promela code

IF(v) if :: v -> BLANK :: else -> skip fi v = l – 1 do :: v BLANK v++ :: else -> break od BLANK FOR(v,l,h) EMPTY INC(v) SET(v,a) v++ v = a StatementPromela Code

investorID Register string requestList int payment creditCard int stockID (idx: i1) accountNum int Register[ investorID[string] & requestList[ stockID[int]{1,3} ] & payment[ creditCardNum[int] | accountNum[int] ] Type Tree

FOR (i1,1,3) EMPTY IF (cond) SET (bRes1,0) IF (bRes1) IF (i2==i3) IF (bRes2) EMPTY SET (bRes2,0) SET (bRes2,1) SET (bRes1,1) $register // stockID / [int()>5] / [position() = = last()]/ int() cond  v_register.requestlist.stockID[i1] > 5 Sequence Insert Generated Statements

$request//stockID=$register//stockID[int()>5][position()=last()] /* result of the XPath expression */ bool bResult = false; /* results of the predicates 1, 2, and 1 resp. */ bool bRes1, bRes2, bRes3; /* index, position(), last(), index, position() */ int i1, i2, i3, i4, i5; i2=1; /* pre-calculate the value of last(), store in i3 */ i4=0; i5=1; i3=0; do :: i4 < v_register.requestList.stockID_occ -> /* compute first predicate */ bRes3 = false; if :: v_register.requestList.stockID[i4].intvalue>5 -> bRes3 = true :: else -> skip fi; if :: bRes3 -> i5++; i3++; :: else -> skip fi; i4++; :: else -> break; od;

$request//stockID=$register//stockID[int()>5][position()=last()] i1=0; do :: i1 bRes1 = false; if :: v_register.requestList.stockID[i1].intvalue>5 -> bRes1 = true :: else -> skip fi; if :: bRes1 -> bRes2 = false; if :: (i2 == i3) -> bRes2 = true; :: else -> skip fi; if :: bRes2 -> if :: (v_request.stockID.intvalue == v_register.requestList.stockID[i1].intvalue) -> bResult = true; :: else -> skip fi :: else -> skip fi; i2++; :: else -> skip fi; i1++; :: else -> break; od;

Model Checking Using Promela Found subtle errors in an example –SAS: Stock Analysis Service [Fu, Bultan, Su ISSTA’04] –3 peers: Investor, Broker, ResearchDept. –Investor  Broker: a registerList of stockIDs –Broker  ResearchDept.: relay request (1 stockID per request) find the stockID in the latest request, send its subsequent stockID in registerList –Repeating stockID will cause error. –Only discoverable by analysis of XPath expressions

Related Work Conversation specification –IBM Conversation support project –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] Realizability problem –Realizability of Message Sequence Charts (MSC) [Alur, Etassami, Yannakakis ICSE’00, ICALP’01]

Related Work Verification of web services –Simulation, verification, composition of web services using a Petri net model [Narayanan, McIlraith WWW’02] –BPEL verification using a process algebra model and Concurrency Workbench [Koshkina, van Breugel TAV- WEB’03] –Using MSC to model BPEL web services which are translated to labeled transition systems and verified using model checking [Foster, Uchitel, Magee, Kramer ASE’03] –Model checking Web Service Flow Language specifications using SPIN [Nakajima ICWE’04]

Current and Future Work Extending the source and target languages Symbolic analysis [Fu, Bultan, Su ICWS’04] Abstraction Design for verification for web services [Betin-Can, Bultan ’04]

Translator for bottom-up specifications Guarded automata Translation with bounded queue Synchronizability Analysis Translation with synchronous communication Intermediate Representation Conversation Protocols Front End Realizability Analysis Guarded automaton skip Translator for top-down specifications success fail Translation with single process, no communication success fail AnalysisBack End BPEL Web Service Specification Languages DAML-S WSCI Promela SMV Action Language Verification Languages... Automated Abstraction Current and Future Work