Causality for debugging mobile agents

Mobile agents, i.e, pieces of programs that can be sent around networks of computers, appear more and more frequently on the Internet. These programs may be seen as an enrichment of traditional distributed computing, and stru cturing applications using mobile agents is destined to become the de facto way of constructing distributed systems in the near future. Since mobile a gents may carry communication links with them as they move across the netwo rk, they create very dynamic interconnection structures that can be extreme ly complex to analyse. In this paper we study an example of a system based on the mobile agent principle, written in the Facile programming language. We propose a Structural Operational Semantics (SOS) for Facile, giving a pr oved transition system that records encodings of the derivation trees of tr ansitions in their labels. This information enables us to easily recover no n-interleaving semantics for Facile by looking only at the labels of transi tions. We use the new semantics to debug an agent based system. This exampl e is a scaled down version of a system demonstrated at the European IT Conf erence Exhibition in Brussels, 1995. We also adopt our causal semantics to analyse the specification of a prefetch pipeline processor.