Event Composition in Time-dependent Distributed Systems

Many interesting application systems, ranging from workflow management and CSCW to air traffic control, are eventdriven and time-dependent and must interact with heterogeneous components in the real world. Event services are used to glue together distributed components. They assume a virtual global time base to trigger actions and to order events. The notion of a global time that is provided by synchronized local clocks in distributed systems has a fundamental impact on the semantics of event-driven systems, especially the composition of events. The well studied 2g-precedence model, which assumes that the granularity of global time-base g can be derived from a priori known and bounded precision of local clocks may not be suitable for the Internet where the accuracy and external synchronization of local clocks is best effort and cannot be guaranteed because of large transmission delay variations and phases of disconnection. In this paper we introduce a mechanism based on NTP synchronized local clocks with global reference time injected by GPS time servers. We argue that timestamps of events can be related to global reference time with bounded accuracy and propose that event timestamps are modeled using accuracy intervals. We present algorithms for event composition and event consumption which make use of accuracy interval based timestamping and illustrate the problems that arise due to inaccuracy and message transmission delays.