In distributed system, multiple computers are connected on the network working together as a system. These computer are independent but their collection appears to it users as a single coherent system. Distributed system provides sharing of resources and information. Processes executed on these systems can communicate with one another by exchanging messages over communication channel.
The distributed processing refers to LAN designed so that a single program can run simultaneously at various sites. Most distributed processing systems contain sophisticated software that detects idle CPUs on the network and parcels out programs to utilize them.
Another form of distributed processing involves distributed database, databases in which the data stored across two or more computer systems. The database system keeps track of where the data is so that the distributed nature of the database is not apparent to users.
A distributed database consists of two or more data files located at different sites on a computer network. Because the database is distributed, different users can without interfering with one another. However, the DBMS must periodically synchronize the scattered database to make sure they all have consistent data.
The software system that facilitates the the management of a DDB in such a way that the distribution aspects are transparent to users.
A DDBMS running on a different computer at each site can handle local applications autonomously and participates in at least one global application requiring data from other sites. Communication between different sites via a network is essential for any global application.
The Functions of Such DDBMS are:
To extend communication services to provide access to remote sites and allow the transfer of queries and data across the network.
To extend the DD to store data distribution details.
To provide distributed query processing, including optimization and remote data access.
To extend accuracy control to maintain consistency of replicated data.
To extend recovery services to take account of failures either of sites or of communication links.
Distributed data storage:
There are two approaches for storing data in distributed database.
Replication: The system create different similar copy of any data and each copy is stored on different places. There are two types of replication strategy.
Complete Replication: The complete replication strategy consists of maintaining a complete copy of the database at each site. This maximizes accessibility and reliability, but costs of storage and the communications needed for updates may be high.