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Real-time update of access control policies, that is, updating policies while they are in effect and enforcing the changes immediately, is necessary for many security-critical applications. In this paper, we consider real-time update of access control policies in a database system. Updating policies while they are in effect can lead to potential security problems, such as, access to database objects by unauthorized users. In this paper, we propose several algorithms that not only prevent such security breaches but also ensure the correctness of execution. The algorithms differ from each other in the degree of concurrency provided and the semantic knowledge used. Of the algorithms presented, the most concurrency is achieved when transactions are decomposed into atomic steps. Once transactions are decomposed, the atomicity, consistency, and isolation properties no longer hold. Since the traditional transaction processing model can no longer be used to ensure the correctness of the execution, we use an alternate semantic-based transaction processing model. To ensure correct behavior, our model requires an application to satisfy a set of necessary properties, namely, semantic atomicity, consistent execution, sensitive transaction isolation, and policy-compliant. We show how one can verify an application statically to check for the existence of these properties.