An Approach to Object Sharing in Distributed Datbase Systems (1983)

This paper describes DODM, a simple model for object sharing in distributed database systems. The model provides a small set of operations for object definition, manipulation, and retrieval in a distributed environment. Relationships among oojects can be established across database boundarres, objects are relocatable within the distributed environment, and mechanisms are provided for object sharing among individual databases. An object naming convention supports location transparent object references; that is, objects can be referenced by user-defined names rather than by address. The primitive operations introduced can be used as the basis for the specification and stepwise development of database models and database systems of increasing complexity. An example is provided to illustrate the use of DODM in the design of a distributed database system supporting a semantically expressive database model. 1. Int reduction Distributed computing systems are becoming increasingly common. This trend is largely caused by the decreasing cost of hardware: not only are powerful personal computers becoming so Inexpensive today that individuals can afford them for personal use, but the cost of computer networks that enable computer systems to exchange information at a very high rate is decreasing drastically. Decentralization overcomes many of the limitations and deficiencies of centralized systems. A network ‘This research was supported, in part, by the Joint Sewlees Electronics Program through the Air Force Office of ScientWic Research under contract F49820-El-C.0070. of computers simply provides a higher level of performance, availability, reliability, fault tolerance, and security than a centralized computer system. In addition to the technical advantages that make decentralized systems feasible, social attitudes tend to indicate that a collection of smaller, autonomous computer systems are preferable to large central systems. The growing popularity of distributed computing establishes a need for mechanisms that allow individual users to communicate with each other and share both hardware and software resources. Individual users also need access to the growing number of “public” databases, which contain a variety of information such as grocery prices, the values of stocks, and the histories of bank accounts. Of course, sharing mechanisms decrease the autonomy of the components of the distributed environment, and affect the performance, availability, reliability, fault tolerance, and security of the total system. Sharing and communication mechanisms alsc introduce data transmission and naming problems. Most current approaches to distributed database management system design fail to adequately address issues concerning location transparency (the ability to reference data by name rather than by address), logical decentralization, catalog management, and the uniform handling of meta-data and user-data. Logically centralized database systems [Rothnie 80, Stonebraker 77, Andler 821 provide the users with a single integrated database schema describing all the data in the physically centralized or distributed environment. Recent research has also resulted in approaches to support the integration of heterogeneous as well as homogeneous (preexisting) databases [Motro 81, Smith 81, Litwin 81, Kimbleton 791. However, a critical remaining problem is accommodating informalion sharing among individual, autonomous databases. Finally, existing distributed database system architectures that emphasize the autonomy of the individual databases [Heimbigner 82, Williams 81, Tsichritzis 821 require centralized or complex catalog management. The aim of the research described in this paper is to define a simple model for object sharing in distributed database systems. This is done by stepwise development of a series of object-oriented models. First, a simple model called ODM (for object-orienfed databsse model) is defined. ODM provides a