We consider the problem of synthesizing a fully controllable target behavior from a set of available partially controllable behaviors that are to execute within a shared partially predictable, but fully observable, environment. Behaviors are represented with a sort of nondeterministic transition systems, whose transitions are conditioned on the current state of the environment, also represented as a nondeterministic finite transition system. On the other hand, the target behavior is assumed to be fully deterministic and stands for the behavior that the system as a whole needs to guarantee. We formally define the problem within an abstract framework, characterize its computational complexity, and propose a solution by appealing to satisfiability in Propositional Dynamic Logic, which is indeed optimal with respect to computational complexity. We claim that this problem, while novel to the best of our knowledge, can be instantiated to multiple specific settings in different contexts and can thus be linked to different research areas of AI, including agent-oriented programming and cognitive robotics, control, multi-agent coordination, plan integration, and automatic web-service composition. 1

Abstract. Composition languages like BPEL and many enactment tools only support structured process models, while most composition approaches only consider unstructured models. In this paper, we outline a semi-automatic approach for composing a set of services with data flow dependencies into a structured process model. These data flow dependencies can be automatically derived from the input and output messages of each service, but some additional user input is needed to annotate dependencies with specific branching types. Heart of the approach is a fully automatic composition algorithm that given an annotated dependency graph constructs a structured composition. We illustrate the approach by applying it to an example case study from the CrossWork project, which studies the dynamic formation of cross-organisational workflows.

The promise of Web Service Computing is to use Web services as fundamental elements for realizing distributed applications/solutions. When no available service satisfies a desired specification, one might check whether (parts of) available services can be composed and orchestrated in order to realize the specification. The problem of automatic composition becomes especially interesting in the presence of conversational services. Among the various frameworks proposed in the literature, here we concentrate on the so called “Roman Model”, where: (i) each service is formally specified as a transition system that captures its possible conversations with a generic client; (ii) the desired specification is a target service, described itself as a transition system; (iii) the aim is to synthesize an orchestrator realizing the target service by exploiting execution fragments of available services. The Roman Model well exemplifies what can be achieved by composing conversational services and, also, uncovers relationships with automated synthesis of reactive processes in Verification and AI Planning. 1

In this paper we study the issue of service composition, for services that export a representation of their behavior in the form of a finite deterministic transition system. In particular, given a specification of the target service requested by the client as a finite deterministic transition system, the problem we face is how we can exploit the computations of the available services for realizing the computations of the target service. While ways to tackle such a problem are known, in this paper we present a new technique that is based on the notion of simulation, which is still optimal from the computational complexity point. Notably, such a technique, opens up the possibility of devising composition in a “just-in-time” fashion. Indeed, we show that, by exploiting simulation, it is actually possible to implicitly compute all possible compositions at once, and delay the choice of the actual composition to run-time.

The paper investigates fundamental decision problems and composition synthesis for Web services commonly found in practice. We propose a notion of synthesized Web services (SWS’s) to specify the behaviors of the services. Upon receiving a sequence of input messages, an SWS issues multiple queries to a database and generates actions, in parallel; it produces external messages and database updates by synthesizing the actions parallelly generated. In contrast to previous models for Web services, SWS’s advocate parallel processing and (deterministic) synthesis of actions. We classify SWS’s based on what queries an SWS can issue, how the synthesis of actions is expressed, and whether unbounded input sequences are allowed in a single interaction session. We show that the behaviors of Web services supported by various prior models, data-driven or not, can be specified by different SWS classes. For each of these classes we study the non-emptiness, validation and equivalence problems, and establish matching upper and lower bounds on these problems. We also provide complexity bounds on composition synthesis for these SWS classes, identifying decidable cases.

In this paper we look at the problem of composing services that export their behavior in terms of a transition system, characterizing the choices of actions given to a client at each point in time. The composition consists of synthesizing an orchestrator that coordinates the available services so as to mimic the desired target service asked by the client. Specifically, in this paper we study the “conformant form ” of the problem, where available services are partially controllable and partially observable, and hence, the orchestrator has to make its decisions exploiting the observations made so far only. We give a sound and complete procedure to synthesize the orchestrator in such case, and characterize the computational complexity of the problem. The procedure is based on working with belief (or knowledge) states, a standard technique to tackle conformant planning. Moreover we show that, although in general unavoidable, the powerset construction at the base of the belief state approach can be delegated to the symbolic manipulations of the game-structure model checking tool (TLV), which can be used to efficiently implement the orchestrator synthesis procedure.

Service Oriented Computing is emerging as the main approach to build distributed enterprise applications on the Web. The widespread use of Web services is hindered by the lack of adequate security and privacy support. In this paper, we present a novel framework for enforcing access control in conversation-based Web services. Our approach takes into account the conversational nature of Web services. This is in contrast with existing approaches to access control enforcement that assume a Web service as a set of independent operations. Furthermore, our approach achieves a tradeoff between the need to protect Web service’s access control policies and the need to disclose to clients the portion of access control policies related to the conversations they are interested in. This is important to avoid situations where the client cannot progress in the conversation due to the lack of required security requirements. We introduce the concept of k-trustworthiness that defines the conversations for which a client can provide credentials maximizing the likelihood that it will eventually hit a final state. Categories and Subject Descriptors

The promise of Web Service Computing is to utilize Web services as fundamental elements for realizing distributed applications/solutions. In particular, when no available service can satisfy client request, (parts of) available services can be composed and orchestrated in order to satisfy such a request. In this paper, we address the automatic composition when the behavior of the available services is nondeterministic, and hence it is not fully controllable by an orchestrator. The service behavior is modeled by the possible conversations the service can have with its clients. The presence of nondeterministic conversations stems naturally when modeling services in which the result of each interaction with its client can not be foreseen. The behavior of the component services is thus only partially controllable, and an orchestrator needs to cope with such partial controllability. We propose an automatic composition synthesis technique, based on reduction to satisfiability in Propositional Dynamic Logic, that is sound, complete and decidable. Moreover, we will characterize the computational complexity of the problem and show that the proposed technique is optimal wrt computational complexity.

Typical approaches to service composition seek to realize a goal service specification, described using a labeled transition system (LTS) provided by a service developer, by constructing a structurally equivalent LTS using a set of available component services (also described using LTSs) that match the input and output requirements of the transitions. As such, existing composition approaches fail to realize the goal LTS whenever available component service LTSs cannot be used to “mimic ” the structure of the goal LTS. This failure requires that the service developer formulates an alternate goal LTS and re-iterates the composition step. However, the process of manual reformulation of the goal LTS is both laborious and error prone. In this setting, we describe an efficient data structure and algorithms for analyzing data and control flow dependencies implicit in a user-supplied goal LTS specification to automatically generate alternate LTS specifications that capture the same overall functionality with respect to the data and control dependencies, and determine whether any of the alternatives can lead to a feasible composition. The result is a significant reduction in the need for the tedious manual intervention in reformulating LTS specifications of the goal service. 1

start The paper formulates and investigates the aggregation problem for synthesized mediators of Web services (SWMs). An SWM is a finite-state transducer defined in terms of templates for component services. Upon receiving an artifact, an SWM selects a set of available services from a library to realize its templates, and invokes those services to operate on the artifact, in parallel; it produces a numeric value as output (e.g., the total price of a package) by applying synthesis rules. Given an SWM, a library and an input artifact, the aggregation problem is to find a mapping from the component templates of the SWM to available services in the library that maximizes (or minimizes) the output. As opposed to the composition syntheses of Web services, the aggregation problem aims to optimize the realization of a given mediator, to best serve the users ’ need. We analyze this problem, and show that its complexity depends on the underlying graph structure of the mediator: while it is undecidable when such graphs contain even very simple cycles, it is solvable in single-exponential time (in the size of the specification) for SWMs whose underlying graphs are acyclic. We prove several results of this kind, with matching lower bounds (NP and PSPACE), and analyze restrictions that lead to polynomial-time solutions.