Reachability Problems in Piecewise FIFO Systems

Systems consisting of several finite components that communicate via unbounded perfect FIFO channels (i.e., FIFO systems) arise naturally in modeling distributed systems. Despite well-known difficulties in analyzing such systems, they are of significant interest as they can describe a wide range of communication protocols. In this article, we study the problem of computing the set of reachable states of a FIFO system composed of piecewise components. This problem is closely related to calculating the set of all possible channel contents, that is, the limit language, for each control location. We present an algorithm for calculating the limit language of a system with a single communication channel. For multichannel systems, we show that the limit language is piecewise if the initial language is piecewise. Our construction is not effective in general; however, we provide algorithms for calculating the limit language of a restricted class of multichannel systems in which messages are not passed around in cycles through different channels. We show that the worst case complexity of our algorithms for single-channel and important subclasses of multichannel systems is exponential in the size of the initial content of the channels.