Distributed database system (DDBS) technology is the union of what appear to be two diametrically opposed approaches to data processing: database system and computer network technologies. Database systems have taken us from a paradigm of data processing in which each application defined and maintained its own data to one in which the data are defined and administered centrally. This new orientation results in data independence.
The working definition we use for a
distributed computing system or a distuributed processing database states that it is a number of autonomous processing elements (not necessarily homogeneous) that are interconnected by a computer network and that cooperate in performing their assigned tasks. The “processing element” referred to in this definition is a computing device that can execute a program on its own.
What is a Distributed Database System?
processing logic or processing elements are distributed. Another possible distribution is according to function. Various functions of a computer system could be delegated to various pieces of hardware or software. A third possible mode of distribution is according to data. Data used by a number of applications may be distributed to a number of processing sites. Finally, control can be distributed. The control of the execution of various tasks might be distributed instead of being performed by one computer system.
a distributed database as a collection of multiple, logically interrelated databases distributed over a computer network. A distributed database management system (distributed DBMS) is then defined as the software system that permits the management of the distributed database and makes the distribution transparent to the users.
1.3 Data Delivery Alternatives
In distributed databases, data are “delivered” from the sites where they are stored to where the query is posed. We characterize the data delivery alternatives along three orthogonal dimensions: delivery modes, frequency and communication methods. The combinations of alternatives along each of these dimensions (that we discuss next) provide a rich design space.
The alternative delivery modes are pull-only, push-only and hybrid. In the pullonly mode of data delivery, the transfer of data from servers to clients is initiated by a client pull.
In the push-only mode of data delivery, the transfer of data from servers to clients is initiated by a server push in the absence of any specific request from clients.
The hybrid mode of data delivery combines the client-pull and server-push mechanisms.
In periodic delivery, data are sent from the server to clients at regular intervals.
In conditional delivery, data are sent from servers whenever certain conditions installed by clients in their profiles are satisfied.
Ad-hoc delivery is irregular and is performed mostly in a pure pull-based system. Data are pulled from servers to clients in an ad-hoc fashion
1.4 Promises of DDBSs
The third component of the design space of information delivery alternatives is the communication method. These methods determine the various ways in which servers and clients communicate for delivering information to clients. The alternatives are unicast and one-to-many. In unicast, the communication from a server to a client is one-to-one: the server sends data to one client using a particular delivery mode with some frequency. In one-to-many, as the name implies, the server sends data to a number of clients.
1.4.1 Transparent Management of Distributed and Replicated Data
Transparency refers to separation of the higher-level semantics of a system from lower-level implementation issues. In other words, a transparent system “hides” the implementation details from users. The advantage of a fully transparent DBMS is the high level of support that it provides for the development of complex applications.
Data independence is a fundamental form of transparency that we look for...