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A CAD environment requires a significantly different model of transaction from that developed for typical data-processing applications. A viable model of CAD transaction must allow a group of cooperating designers to arrive at a design without being forced to wait over a long duration. It must also allow a group of designers to collaborate on a design with another group by assigning subtasks. In this paper, we provide both an intuitive description and a formal development of a new model of CAD transactions. collection of private systems connected to the public system [HASK82, KATZ84, LORI83, KlM84]. The public system manages the public database of stable design data and design control data. A private system manages the private database of a designer on a design workstation. A CAD transaction, initiated on a private system, consists of