Compose Presence for Composite Web Services by Timed AND-OR Tree

Both presence and web service composition are very important to web based real-time communication systems. Although there is some work to combine presence with individual web services, there still lacks an efficient presence composition method to determine presence for composite web services, which is a collection of web services arranged by a process. This paper proposes a distributed computation model, called TAO-Tree (Timed AND-OR tree) to map discrete presence states to continuous time domain such that presence compositions can be carried recursively by arithmetic operations. Furthermore, the paper describes a method to translate the presence delays back to discrete presence states with minimal errors. We implement the TAO-Tree module in a prototype system and the experiments show the approach is feasible and satisfactory.