Object oriented databases are also called Object Database Management Systems (ODBMS). Object databases store objects rather than data such as integers, strings or real numbers. Objects are used in object oriented languages such as Smalltalk, C++, Java, and others. Objects basically consist of the following:
* Attributes - Attributes are data which defines the characteristics of an object. This data may be simple such as integers, strings, and real numbers or it may be a reference to a complex object.
* Methods - Methods define the behavior of an object and are what was formally called procedures or functions.
Therefore objects contain both executable code and data. There are other characteristics of objects such as whether methods or data can be accessed from outside the object. We don't consider this here, to keep the definition simple and to apply it to what an object database is. One other term worth mentioning is classes. Classes are used in object oriented programming to define the data and methods the object will contain. The class is like a template to the object. The class does not itself contain data or methods but defines the data and methods contained in the object. The class is used to create (instantiate) the object. Classes may be used in object databases to recreate parts of the object that may not actually be stored in the database. Methods may not be stored in the database and may be recreated by using a class.
Comparison to Relational Databases
Relational databases store data in tables that are two dimensional. The tables have rows and columns. Relational database tables are "normalized" so data is not repeated more often than necessary. All table columns depend on a primary key (a unique value in the column) to identify the column. Once the specific column is identified, data from one or more rows associated with that column may be obtained or changed.
To put objects into relational databases, they must be described in terms of simple string, integer, or real number data. For instance in the case of an airplane. The wing may be placed in one table with rows and columns describing its dimensions and characteristics. The fusalage may be in another table, the propeller in another table, tires, and so on.
Breaking complex information out into simple data takes time and is labor intensive. Code must be written to accomplish this task.
With traditional databases, data manipulated by the application is transient and data in the database is persisted (Stored on a permanent storage device). In object databases, the application can manipulate both transient and persisted data.
When to Use Object Databases
Object databases should be used when there is complex data and/or complex data relationships. This includes a many to many object relationship. Object databases should not be used when there would be few join tables and there are large volumes of simple transactional data.
Object databases work well with:
* CAS Applications (CASE-computer aided software engineering, CAD-computer aided design, CAM-computer aided manufacture)
* Multimedia Applications
* Object projects that change over time.
Object Database Advantages over RDBMS
* Objects don't require assembly and disassembly saving coding time and execution time to assemble or disassemble objects.
* Reduced paging
* Easier navigation
* Better concurrency control - A hierarchy of objects may be locked.
* Data model is based on the real world.
* Works well for distributed architectures.
* Less code required when applications are object oriented.
Object Database Disadvantages compared to RDBMS
* Lower efficiency when data is simple and relationships are simple.
* Relational tables are simpler.
* Late binding may slow access speed.
* More user tools exist for RDBMS.
* Standards for RDBMS are more stable.
* Support for RDBMS is more certain and change is less likely to be required.
* Object Data Management Group
* Object Database Standard ODM6.2.0
* Object Query Language
* OQL support of SQL92
How Data is Stored
Two basic methods are used to store objects by different database vendors.
* Each object has a unique ID and is defined as a subclass of a base class, using inheritance to determine attributes.
* Virtual memory mapping is used for object storage and management.
Data transfers are either done on a per object basis or on a per page (normally 4K) basis.