A CSP solution to the "trains" problem

The problem as stated leaves one in doubt as to what one must do. Is one meant to abstractly specify the safety and liveness properties one desires of the network? Is one meant to provide some plausible ~'implementation ~' which gaurantees the basic principle that two trains must not be simultaneously on the same piece of track? Perhaps we are even meant to enter ~he realm of inventing scheduling algorit~/ms for the flow of trains. Of course any complete solution to the problem must, in some sense, address all of these issues. The scheduling problem seems, however, to be outside the scope of the workshop. The solution presented here is a CSP program which addresses the second question above. It can be proved to satisfy certain abstract conditions, and can of course be used as a base (guaranteeing safety) upon which to build any scheduling programs. It is by no means the simplest program which guarantees safety: instead it shows how CSP can be used to create a control structure more realistic than if, say, the actual events of a train enter&ng or leaving a piece of track were "negotiable". The only approach we make towards scheduling is to allow trains to book line sections one step ahead, and to seek an alternative line if the first one they try to book is in use. The element of external control left in the network is in the hands of (notional) train drivers. It would of course be expected that the drivers be controlled and regulated by some signalling system; this higher level of detail is omitted. Of course the particular control structure chosen here is only one of many which have the desired safety properties. No special merit is claimed for the particular solution given, and the reader is invited to devise his own program to model his o,~nideas on how the network should be regulated and controlled. The main purpose of this solution is to illustrate the power of CaP in precisely specifying parallel interaction. As in all CSP programs, communication is achieved by the synchronisation of euent~, or actions, of the elements of the network. Each process has its own a l p ~ b e t of events; in the operation of the network, an event can only occur if all the processes in whose alphabets it is are willing to communicate it. This situation is easiest …