An Event Calculus Treatment of the Invoicing of Orders

minimise this tendency, for example by supplying orders as a set at the system interface, and only arranging them internally as a sequence. This allows us to regard the iterative precess used to process orders one at a time as a description rather than a prescription. If the implementor has his own procedure for processing orders he should be able to insert this at the appropriate place. The results achieved will not be observably diierent from what is described here. The Event Calculus allows is to model information systems in terms of communications state machines. For the Invoicing of Orders case study, we have proposed a solution which consists of three machines, which deal respectively with the invoicing of orders, the maintainance of a stock inventory, and the cancellation of orders. A number of choices have been made in the construction of this speciic-ation, and these will now be reviewed.. The system operates in a cycle, inputting details of orders and stock delivery and outputting requests for dispatch on each cycle. Orders which cannot be met from existing stock are retained and retried on the following cycle, when the receive priority over any new orders. We do not generate any partial orders, i.e. orders which include as much of the ordered item as is currently available. We do not distinguish between an item which is normally held in stock but which is currently out of stock, and an item which is not part of our catalogue. This simpliies our speciication by allowing us to describe stock levels and stock deliveries as total functions, but would be an over simpliication for a real system. We only allow the input of new stock and new orders at a certain point in the cycle. We made this choice partly show that the calculus can express such constraints. The could be relaxed by adding additional state machines. For example we could add a machine dedicated to the input of orders, available to perform such input at any time, which passes accumulated orders to the current system on request. We have also taken a meta level design decision to work with a simple calculus which has no in-built notion of fairness or priority. It is possible to replace the Event Calculus rules, which are assumed as a preamble to our speciication, by alternative rules which embody the notion of priority and fairness at the primitive …