Object Migration in Object-Oriented Databases

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Record oriented data models are not adequate to new applications of database systems. Object oriented data model is promising to be used in such applications. In this paper, we study updates on objects in object-oriented databases. Specifically, we study the effect of updating an object on its position in the class lattice. An update may affect the object such that it no longer satisfies conditions of its current class. We consider three types of updates on objects : (1) Adding instance variables, (2)Dropping instance variables, and (3)Modifying instance variables. An update may cause object migration. That is, the updated object may change its current class. As the new class may be different from the class before the update, some actions have to be taken. Our objective is to provide a procedure to automatically perform the migration. The idea is explained through an example. Consider the schema shown in Fig. 1. It models people in a university. The schema is represented by a rooted directed graph. Nodes represent classes in the schema. An edge from class Ci to class Cj means that Ci IS-A Cj. The root of the graph is a system defined class called OBJECT. Suppose that instances of class GOOD-GRAD students are defined to be graduate students with total marks exceeding certain value. From this, an update that modifies this value may cause an object to migrate from class GRAD to class GOOD-GRAD. Such object migration has some side effects that should be handled by the system. If class GOOD-GRAD has some instance variables not defined in class GRAD, values of these instance variables have to be provided by a user, or considered null (which sometimes may not be possible). Now, we can see that, as an update causes object migration the system has to take some actions. With respect to each update causing migration, we give a group of actions should be taken by the system (or in cooperation with a user).
一般社団法人情報処理学会の論文
1989-10-16

Object Migration in Object-Oriented Databases

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