Jianwen Su

Analysis of interacting BPEL web services

This paper presents a set of tools and techniques for analyzing interactions of composite web services which are specified in BPEL and communicate through asynchronous XML messages. We model the interactions of composite web services as conversations, the global sequence of messages exchanged by the web services. As opposed to earlier work, our tool-set handles rich data manipulation via XPath expressions. This allows us to verify designs at a more detailed level and check properties about message content. We present a framework where BPEL specifications of web services are translated to an intermediate representation, followed by the translation of the intermediate representation to a verification language. As an intermediate representation we use guarded automata augmented with unbounded queues for incoming messages, where the guards are expressed as XPath expressions. As the target verification language we use Promela, input language of the model checker SPIN. Since SPIN model checker is a finite-state verification tool we can only achieve partial verification by fixing the sizes of the input queues in the translation. We propose the concept of synchronizability to address this problem. We show that if a composite web service is synchronizable, then its conversation set remains same when asynchronous communication is replaced with synchronous communication. We give a set of sufficient conditions that guarantee synchronizability and that can be checked statically. Based on our synchronizability results, we show that a large class of composite web services with unbounded input queues can be completely verified using a finite state model checker such as SPIN.

E-services: a look behind the curtain

The emerging paradigm of electronic services promises to bring to distributed computation and services the flexibility that the web has brought to the sharing of documents. An understanding of fundamental properties of e-service composition is required in order to take full advantage of the paradigm. This paper examines proposals and standards for e-services from the perspectives of XML, data management, workflow, and process models. Key areas for study are identified, including behavioral service signatures, verification and synthesis techniques for composite services, analysis of service data manipulation commands, and XML analysis applied to service specifications. We give a sample of the relevant results and techniques in each of these areas.

WSAT: A Tool for Formal Analysis of Web Services

This paper presents Web Service Analysis Tool (WSAT), a tool for analyzing and verifying composite web service designs, with the state of the art model checking techniques. Web services are loosely coupled distributed systems communicating via XML messages. Communication among web services is asynchronous, and it is supported by messaging platforms such as JMS which provide FIFO queues to store incoming messages. Data transmission among web services is standardized via XML, and the specification of web service itself (invocation interface and behavior signature) relies on a stack of XML based standards (e.g. WSDL, BPEL4WS, WSCI and etc.). The characteristics of web services, however, raise several challenges in the application of model checking: (1) Numerous competing web service standards, most of which lack formal semantics, complicate the formal specification of web service composition. (2) Asynchronous messaging makes most interesting verification problems undecidable, even when XML message contents are abstracted away [3]. (3) XML data and expressive XPath based manipulation are not supported by current model checkers.

Synchronizability of conversations among Web services

We present a framework for analyzing interactions among Web services that communicate with asynchronous messages. We model the interactions among the peers participating in a composite Web service as conversations, the global sequences of messages exchanged among the peers. This naturally leads to the following model checking problem: Given an LTL property and a composite Web service, do the conversations generated by the composite Web service satisfy the property? We show that asynchronous messaging leads to state space explosion for bounded message queues and undecidability of the model checking problem for unbounded message queues. We propose a technique called synchronizability analysis to tackle this problem. If a composite Web service is synchronizable, its conversation set remains the same when asynchronous communication is replaced with synchronous communication. We give a set of sufficient conditions that guarantee synchronizability and that can be checked statically. Based on our synchronizability results, we show that a large class of composite Web services with unbounded message queues can be verified completely using a finite state model checker such as SPIN. We also show that synchronizability analysis can be used to check the reliability of top-down conversation specifications and we contrast the conversation model with the Message Sequence Charts. We integrated synchronizability analysis to a tool we developed for analyzing composite Web services.

Specification and Verification of Artifact Behaviors in Business Process Models

SOA has influenced business process modeling and management. Recent business process models have elevated data representation to the same level as control flows, for example, the artifact-centric business process models allow the life cycle properties of artifacts (data objects) to be specified and analyzed. In this paper, we develop a specification language ABSL based on computation tree logic for artifact life cycle behaviors (e.g., reachability). We show that given a business model and starting configuration, it can be decided if an ABSL sentence is satisfied when the domains are bounded, and if an ABSL-core (sublanguage of ABSL) sentence is satisfied when the domains are totally ordered but unbounded. We also show that if the starting configuration is not given, ABSL(-core) is still decidable if the number of artifacts is bounded with bounded (resp. unbounded but ordered) domains.

On the expressive power of database queries with intermediate types

Abstract The set-height of a complex object type is defined to be its level of nesting of the set construct. In a query of the complex object calculus which maps a database D to an output type T , an intermediate type is a type which is used by some variable of the query, but which is not present in D or T . For each k , i ⩾ 0 we define CALC k, i to be the family of calculus queries mapping from and to types with set-height ⩽ k and using intermediate types with set-height ⩽ i . In particular, CALC 0.0 is the classical relational calculus, and CALC 0.1 is equivalent to the family of second-order (relational) queries. Several results concerning these families of languages are obtained. A primary focus is on the families CALC 0, i , which map relations to relations. Upper and lower bounds in terms of hyper-exponential time and space on the complexity of these families are provided. The CALC 0, i hierarchy does not collapse with respect to expressive power. The union ∪ 0 ⩽ i CALC 0, i is exactly the family of elementary queries , i.e., queries with hyper-exponential complexity. The expressive power of queries from the complex object calculus interpreted using semantics based on the use of arbitrarily large finite or infinite set of invented values is studied. Under these semantics, the expressive power of the relational calculus is not increased, and the CALC 0, i hierarchy collapses at CALC 0.1 . In general, queries with these semantics may not be computable. We also consider an alternative semantics which yields a family of queries equivalent to the computable queries.

Binary Reachability Analysis of Discrete Pushdown Timed Automata

We introduce discrete pushdown timed automata that are timed automata with integer-valued clocks augmented with a pushdown stack. A configuration of a discrete pushdown timed automaton includes a control state, finitely many clock values and a stack word. Using a pure automata-theoretic approach, we show that the binary reachability (i.e., the set of all pairs of configurations (α,β), encoded as strings, such that α can reach β through 0 or more transitions) can be accepted by a nondeterministic pushdown machine augmented with reversal-bounded counters (NPCM). Since discrete timed automata with integer-valued clocks can be treated as discrete pushdown timed automata without the pushdown stack, we can show that the binary reachability of a discrete timed automaton can be accepted by a nondeterministic reversal-bounded multicounter machine. Thus, the binary reachability is Presburger. By using the known fact that the emptiness problem is decidable for reversal-bounded NPCMs, the results can be used to verify a number of properties that can not be expressed by timed temporal logics for discrete timed automata and CTL* for pushdown systems.

Formal Verification of e-Services and Workflows

We study the verification problem for e-service (and workflow) specifications, aiming at efficient techniques for guiding the construction of composite e-services to guarantee desired properties (e.g., deadlock avoidance, bounds on resource usage, response times). Based on e-service frameworks such as AZTEC and e-FLow, decision flow language Vortex, we introduce a very simple e-service model for our investigation of verification issues. We first show how three different model checking techniques are applied when the number of processes is limited to a predetermined number. We then introduce pid quantified constraint, a new symbolic representation that can encode infinite many system states, to verify systems with unbounded and dynamic process instantiations. We think that it is a versatile technique and more suitable for verification of e-service specifications. If this is combined with other techniques such as abstraction and widening, it is possible to solve a large category of interesting verification problems for e-services.

Untyped sets, invention, and computable queries

Conventional database query languages are considered in the context of untyped sets. The algebra without while has the expressive power of the typed complex object algebra. The algebra plus while, and COL with untyped sets (under stratified semantics or inflationary semantics) have the power of the computable queries. The calculus has power beyond the computable queries; and is characterized using the typed complex object calculus with invention. The Bancilhon-Khoshafian calculus is also discussed. A technical tool, called “generic Turing machine”, is introduced and used in several of the proofs.

Efficient index-based KNN join processing for high-dimensional data

In many advanced database applications (e.g., multimedia databases), data objects are transformed into high-dimensional points and manipulated in high-dimensional space. One of the most important but costly operations is the similarity join that combines similar points from multiple datasets. In this paper, we examine the problem of processing K-nearest neighbor similarity join (KNN join). KNN join between two datasets, R and S, returns for each point in R its K most similar points in S. We propose a new index-based KNN join approach using the iDistance as the underlying index structure. We first present its basic algorithm and then propose two different enhancements. In the first enhancement, we optimize the original KNN join algorithm by using approximation bounding cubes. In the second enhancement, we exploit the reduced dimensions of data space. We conducted an extensive experimental study using both synthetic and real datasets, and the results verify the performance advantage of our schemes over existing KNN join algorithms.