Title of Presentation: Support of Asynchrony in Sensor Web

Primary (Corresponding) Author: Genong Yu

Organization of Primary Author: George Mason University

Co-Authors: Liping Di, Genong Yu, Nengcheng Chen, Min Min, Hulin Wang

Abstract: Sensor Web is a system of systems that consists of live sensor, data, and geospatial processing services. In a broad sense, it includes archived data and on-demand generated data, or virtual sensor. In such a system, asynchrony cannot be avoided because of differences in clock/time and long processing time of complex process. Asynchrony can occur at sensor planning stage, data collection phase, or data processing process. Support of asynchrony is one of the core characteristics of the Coordination and Event Notification Services (CENS), the central component of the Self-Adaptive Earth Predictive System (SEPS) that operates Sensor Webs. This paper reviews the different types of asynchrony in the Sensor Web system and their technical solutions. Technologies to support asynchronous mechanisms can be roughly grouped into two major categories: message-queuing and publish-subscribe. Representative technologies include Web Services Addressing (WS-Addressing) and Extensible Messaging and Presence Protocol (XMPP) in industry for message-queuing and publication-subscription respectively. In emerging geospatial standards, Web Notification (WNS) and Sensor Alert Service (SAS) are designed to deal with asynchronies correspondingly. There are generally two approaches in the coordination of systems of systems to work collaboratively. They are orchestration or choreography. Orchestration has a central director to guide the execution of each step such as a workflow. Choreography is a bottom-up approach and the coordination is achieved by defining each individual. In this project, the orchestration is used and Business Process Execution Language is adopted as the script language to describe the composite processes. No matter which approach is chosen, the composite process means syndication of all services and data as the final outcome of the complex composite process. This introduces problems similar to synchronize asynchronous threads in multi-thread programming, such as deadlock and reachability. To solve these problems, proper solutions to handle asynchrony in the Sensor Web system are crucial. With the acceptance of Representation State Transfer (REST), the new style for Web Service, a new paradigm for interoperations in the Web environment has emerged, called Resource Oriented Architecture (ROA). ROA is different from Service Oriented Architecture (ROA) in many aspects. Mechanisms in supporting the asynchronous are also emerging, e.g. HTTPEvents. Workflow standards specifically deals with RESTful services are emerging too, e.g. WfXML. Asynchronous prototypes for both resource-focused and service-focused workflow have been examined and demonstrated in two scenarios in this project. One is on the wild fire workflow and another is the georeferencing workflow. Open research issues are also pointed out, especially those to be studied in the emerging Web 3.0.