A database which resides all of its data centralized and on one machine or some machines which are connected together but looks one for the users looking from outside. And whoever wants to use data, he picks data from there and uses and saves again there. While Distributed databases can be defined as a collection of multiple, logically interrelated databases distributed over a computer network. And distributed database management system (DDBMS) manages the distributed databases and makes this distribution transparent to the user. All the database must be logically related that are managed by DDBMS (distributed database management system). The distributed databases are not just the ‗collection of files‘ stored individually at different network nodes. Rather to form DDBS (distributed databases) all the files should be logically related and there should be structures among those files. In the case of distributed databases, data must be physically distributed across the network nodes otherwise they will simply be separate databases not ‗distributed databases‘. Sometimes the multiprocessor system is also considered as distributed data processing systems but in fact it is not true. As multiprocessor system may use either ‗shared-nothing architecture‘ or ‗shared-everything architecture‘. Shared-nothing architecture system may work like distributed computing environment but it is not. because in distributed environment there may be heterogeneity of hardware as well as operating system at different sites in network which is not the case in multiprocessor systems. Thus for distributed databases data must be distributed over network nodes. While centralized databases are managed by DBMS, and no data distribution is done in this case.

Distributed data is defined as collection of logically distributed database which are connected with each other through a network. A distributed database management system is used for managing distributed database. Each side has its own database and operating system. A centralized database has all its data on one place. As it is totally different from distributed database which has data on different places. In centralized database as all the data reside on one place so problem of bottle-neck can occur, and data availability is not efficient as in distributed database. While in distributed databases users can issue commands from any location to access data and it does not affect the working of database. Distributed database allows us to store one copy of data at different locations. Its advantage is that if a user wants to access data then the nearest site (location) will provide data so it takes less time. There are multiple sites (computers) in a distributed database so if one site fails then system will not be useless, because other sites can do their job because the same copy of data is installed on every location. You will not find this thing in centralized database. Any time new nodes (computers) can be added to the network without any difficulty. Users do not know about the physical storage of data and it is known as distribution transparency, as we know that ideally, a DBMS must not show the details of where each file is stored or we can say that a DBMS should be distribution transparent.
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