Khi một giao dịch đã hoàn thành tất cả các hoạt động, các tính chất ACID yêu cầu được thực hiện bền khi cam kết. Trong một hệ thống tập trung, đảm bảo cam kết là số lượng nguyên tử để đảm bảo rằng tất cả các bản cập nhật được viết để đăng nhập các tập tin và các hoạt động ghi vào đĩa, đánh dấu sự giao dịch hoàn chỉnh là nguyên tử. | Secure Database Systems 375 segment and the write privilege which allows a subject to modify all the objects in a segment. GemStone also supports the special privilege none which when granted to a subject deletes authorizations. Multilevel Security in Database Systems This section describes the multilevel security aspects of mandatory security for DB systems. The first part focuses mainly on relational systems. Then we provide an overview of secure object systems. Note that several other significant developments have been made in multilevel security for DB systems including inference problems secure concurrency control and recovery algorithms and multilevel security for distributed heterogeneous and federated DB systems. This chapter does not discuss all those developments. For details on inference problems we refer the reader to 25 for secure concurrency control 26 for secure distributed heterogeneous and federated DBs 27 . Multilevel Relational Data Model In a multilevel DB not all the data are assigned the same security level. If such a DB is based on the relational model the objects of classification may be the entire DB relations tuples attributes or data elements. Access to those objects is governed by the mandatory policy discussed in Section . A multilevel DBMS should protect the multilevel DB from unauthorized access or modification by subjects cleared at different security levels. A multilevel relational DB represents the multilevel DB as a set of relations. The corresponding model is called a multilevel relational data model. A goal of a multilevel relational DB designer is to represent multiple versions of the same entity action or event at different security levels without violating the integrity or security rules. One of the mechanisms being proposed to represent multiple versions of an entity at different security levels is polyinstantiation. Polyinstantiation enables two tuples with the same primary key to exist in a relational DB at
