In the paper we show how to model, with a (non contact-free) Contextual Condition Event net, both the control and the flow of data of the possible executions of a given shared memory system, representing the data directly with conditions and the operations with events. Contextual Condition Event nets are more general than classical Condition Event nets, since a new kind of relation between the set of the conditions and the set of the events is defined; however, the structure of the elements of the net is the classical one, without the additional labelling of tokens and arcs that can be found in higher order nets. We provide both a process and a functional semantics for the firing sequences of a net and we show that two firing sequences compute the same function, for every interpretation of the operations, if and only if they have the same process semantics. We apply this result to the serializability problem in the framework of the concurrency control. This problem concerns the monitoring of a set of activities (transactions) concurrently accessing the database in a multiuser database management system. We model a Transaction System by a Contextual Condition Event net; the operations of the transactions are events of the net, the objects of the database are conditions and each schedule corresponds to a firing sequence of the net.
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