In this paper, a net model for decentralized control of user accesses to a distributed database is proposed. It is developed in detail for the restricted case of updating distributed copies of a single database. Predicate/transition-nets, a first-order extension of Petri nets, are shown to provide suitable means for concise representation of complex decentralized systems and for their rigorous formal analysis. It will be demonstrated in the present paper how these net models can be constructed and interpreted in a quite natural manner and how they can be analyzed by linear algebraic methods. By this, it will be shown that the modeled distributed database system is deadlock-free and guarantees a consistent database as well as a fair and effective service to the users.