Introduction to Object-Oriented Concepts and Features
The term object-oriented—abbreviated OO or O-O—has its origins in OO programming languages, or OOPLs. Today OO concepts are applied in the areas of databases, software engineering, knowledge bases, artificial intelligence, and computer systems in general. OOPLs have their roots in the SIMULA language, which was proposed in the late 1960s. The programming language Smalltalk, developed at Xerox PARC in the 1970s, was one of the first languages to explicitly incorporate additional OO concepts, such as message passing and inheritance. It is known as a pure OO programming language, meaning that it was explicitly designed to be object-oriented. This contrasts with hybrid OO programming languages, which incorporate OO concepts into an already existing language. An example of the latter is C++, which incorporates OO concepts into the popular C programming language.
An object typically has two components: state (value) and behavior (operations). It can have a complex data structure as well as specific operations defined by the programmer. Objects in an OOPL exist only during program execution; therefore, they are called transient objects. An OO database can extend the existence of objects so that they are stored permanently in a database, and hence the objects become persistent objects that exist beyond program termination and can be retrieved later and shared by other programs. In other words, OO databases store persistent objects permanently in secondary storage, and allow the sharing of these objects among multiple programs and applications. This requires the incorporation of other well-known features of database management systems, such as indexing mechanisms to efficiently locate the objects, concurrency control to allow object sharing among concurrent programs, and recovery from failures. An OO database system will typically interface with one or more OO programming languages to provide persistent and shared object capabilities.
The internal structure of an object in OOPLs includes the specification of instance variables, which hold the values that define the internal state of the object. An instance variable is similar to the concept of an attribute in the relational model, except that instance variables may be encapsulated within the object and thus are not necessarily visible to external users. Instance variables may also be of arbitrarily complex data types. Object-oriented systems allow definition of the operations or functions (behavior) that can be applied to objects of a particular type. In fact, some OO models insist that all operations a user can apply to an object must be predefined. This forces a complete encapsulation of objects. This rigid approach has been relaxed in most OO data models for two reasons. First, database users often need to know the attribute names so they can specify selection conditions on the attributes to retrieve specific objects. Second, complete encapsulation implies that any simple retrieval requires a predefined operation, thus making ad hoc queries difficult to specify on the fly.
To encourage encapsulation, an operation is defined in two parts. The first part, called the signature or interface of the operation, specifies the operation name and arguments (or parameters). The second part, called the method or body, specifies the implementation of the operation, usually written in some general-purpose programming language. Operations can be invoked by passing a message to an object, which includes the operation name and the parameters. The object then executes the method for that operation. This encapsulation permits modification of the internal structure of an object, as well as the implementation of its operations, with-out the need to disturb the external programs that invoke these operations. Hence, encapsulation provides a form of data and operation independence (see Chapter 2).
Another key concept in OO systems is that of type and class hierarchies and inheritance. This permits specification of new types or classes that inherit much of their structure and/or operations from previously defined types or classes. This makes it easier to develop the data types of a system incrementally, and to reuse existing type definitions when creating new types of objects.
One problem in early OO database systems involved representing relationships among objects. The insistence on complete encapsulation in early OO data models led to the argument that relationships should not be explicitly represented, but should instead be described by defining appropriate methods that locate related objects. However, this approach does not work very well for complex databases with many relationships because it is useful to identify these relationships and make them visible to users. The ODMG object database standard has recognized this need and it explicitly represents binary relationships via a pair of inverse references, as we will describe in Section 11.3.
Another OO concept is operator overloading, which refers to an operation’s ability to be applied to different types of objects; in such a situation, an operation name may refer to several distinct implementations, depending on the type of object it is applied to. This feature is also called operator polymorphism. For example, an operation to calculate the area of a geometric object may differ in its method (implemen-tation), depending on whether the object is of type triangle, circle, or rectangle. This may require the use of late binding of the operation name to the appropriate method at runtime, when the type of object to which the operation is applied becomes known.
In the next several sections, we discuss in some detail the main characteristics of object databases. Section 11.1.2 discusses object identity; Section 11.1.3 shows how the types for complex-structured objects are specified via type constructors; Section 11.1.4 discusses encapsulation and persistence; and Section 11.1.5 presents inheri-tance concepts. Section 11.1.6 discusses some additional OO concepts, and Section 11.1.7 gives a summary of all the OO concepts that we introduced. In Section 11.2, we show how some of these concepts have been incorporated into the SQL:2008 standard for relational databases. Then in Section 11.3, we show how these concepts are realized in the ODMG 3.0 object database standard.