PETINA is a PErsonalized Tourist INformation Advisor system about Greece aiming to help tourists to construct tours satisfying speci ed constraints PETINA is going to be implemented in the ElipSys language a parallel logic programming language under develop ment As it tries to solve a combinatorial searching problem various ElipSys features such as data parallelism data driven computation and constraint satisfaction techniques can be exploited e ciently Moreover as it consults a large database containing tourist data the external database connection facility of ElipSys is exploitable as well

The authors present a new synchronization strategy for real-time tasks executed in a distributed environment. This strategy makes the timing properties of the system predictable since one is able to determine analytically whether the timing requirements of a task set will be met, and if not, which task timing requirements will fail. In particular, every task is allocated on a different node of a distributed system and can require the use of non-preemptable resources or access shared data. The proposed synchronization protocol is based on the on-demand paradigm where resources are assigned only when actually required so that the system never wastes unused assignments. Simple formulae for the worst-case determination of task blocking durations as well as for the schedulability analysis of a set of tasks are described. A schedulability analysis example is also presented that illustrates the concepts of scheduling and resource allocation of a set of time critical tasks