OODBMS History and Concepts in Java

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OODBMS History and Concepts
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traversal paths, one for each direction of traversal of the relationship. In this particular case, relationship is_married_to is its own inverse.
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class Person:Object { attribute short age; attribute string name; attribute enum gender {male, female}; attribute Address home_address; attribute set<Phone_no> phones; attribute Department dept; relationship Person is_married_to inverse Person::is_married_to; void get_married(in Person p ) ; };
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Some OODBMSs allow the specification of class features that cannot be seen as attributes of its instances, such as the number of class instances present in each moment in the database or the average value of an attribute. An example of an operation that is invoked on classes rather than on objects is the new operation for creating new instances. In those systems, properties and methods characterizing the class as an object can be defined, which are not inherited by the class instances. These features usually are referred to as class properties and class methods. In the object model, different notions are related to the description of the common features of a set of objects: types, classes, and interfaces. The differences among those concepts are confusing and the terms are employed with different meanings in different contexts and systems. The type is a programming language concept, and provides the specification of a set of objects and values; that is, the operations that can be invoked on them. This specification is used (usually at compile time) to check program correctness. A class, by contrast, provides the implementation (i.e., the definition of object properties and the implementations of operations) for a set of objects of the same type. It also provides primitives for object creation. An interface provides the specification of the external behavior of a set of objects; a class can implement an interface but it cannot be instantiated directly. Both classes and interfaces are supported by the ODMG object model. Classes include the specification of object properties and operations; however, they do not include the implementation of operations, which is supposed to be given through a language for which the model provides a binding, whereas interfaces include only the specification of object operations. Extents and Persistence Mechanisms Besides being a template, a class also acts as an object factory since it has the capability of generating instances (usually through a new operation). The collection of all instances generated from a class is referred to as a class extent. Extents are the basis on which queries are formulated. In some OODBMSs extents automatically are associated with each class, whereas in others they must be maintained by applications. In those systems in which a class defines only the specification and the implementation of
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objects, such as ObjectStore, GemStone, and O2, explicit collections of objects of the corresponding types must be defined. Objects must be inserted in those collections upon creation, queries issued against those collections, and indexes built on those collections. The automatic association of an extent with each class has the advantage of simplifying the management of classes and their instances. By contrast, systems in which extents are handled manually provide greater flexibility at the price of increased complexity in managing class extents. An important issue concerns the persistence of class instances, and the mechanisms by which objects are made persistent (inserted into the database) or deleted (removed from the database). In relational databases, explicit statements (INSERT and DELETE in SQL) are provided to insert and delete data from the database. In OODBMSs, two different approaches can be adopted with respect to object persistence: Persistence is an implicit property of all class instances; the creation (through the new operation) of an instance also has the effect of inserting the instance in the database, thus the creation of an instance automatically implies its persistence. This approach usually is adopted in systems in which classes also have an extensional function. Some systems provide two different new operations: one for creating persistent instances of a class, the other one for creating temporary instances of that class. Persistence is an orthogonal property of objects; the creation of an instance does not have the effect of inserting the instance in the database. Rather, if an instance has to survive the program that created it, it must explicitly be made persistent for example, by assigning it a name, or by inserting it in a persistent collection of objects. This approach usually is adopted in systems in which classes do not have the extensional function. With respect to object deletion, two different approaches are possible: The system provides an explicit delete operation. Explicitly deleting objects poses the problem of referential integrity; if an object is deleted and there are other objects referring to it, these references are no longer valid (such references are called dangling references). The system does not provide an explicit delete operation. A persistent object is deleted only if all references to it have been removed (a periodic garbage collection is performed). This approach ensures referential integrity.
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