How The Distributed Database Different From Centralized Database?

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. Let me define some advantages of distributed database, it will clear the difference between centralized and distributed database.

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 as I earlier said that 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.

Definition of a Distributed database
www.webopedia.com

"A database that consists of two or more data files located at
different sites on a computer network. Because the database is
distributed, different users can access it without interfering with
one another. However, the DBMS must periodically synchronize the
scattered databases to make sure that they all have consistent data"

Here you can find a really good text about Distributed Databases:

Scale Up in Distributed Databases
www.cs.rpi.edu

I have extracted some key phrases on Why use a distributed database
over a centralized one:

"A big advantage of distributed DBMSs over centralized ones is that of
scalability. Growth can be sustained more gracefully in a distributed
system."

"Local autonomy is another reason for a business to implement a DDBMS.
Since the nature of today's applications usually require data in
geographic areas that are often decentralized, it often makes sense to
implement a distributed system. In this way, data can physically
reside nearest to where it is most often accessed, thus providing
users with local control of data that they interact with."

"Another reason why one might want to consider a parallel architecture
is to improve reliability and availability of the data in a scalable
system. In a distributed system, with some careful tact, it is
possible to access some, or possibly all of the data in a failure mode
if there is sufficient data replication."

Another good paper about Distributed Database Systems:

Emerging Technologies for Developing Distributed Database Systems
bridge.ecn.purdue.edu

And here are the solutions to a test about distributed Database
Systems, very useful to say in a few words what a DDBMS is:

Distributed Database Systems - Solutions
www.cs.jcu.edu.au

Major features of a DDB are:
    * Data stored at a number of sites, each site logically single
processor
    * Sites are interconnected by a network rather than a
multiprocessor configuration
    * DDB is logically a single database (although each site is a
database site)
    * DDBMS has full functionality of a DBMS
    * To the user, the distributed database system should appear
exactly like a non-distributed database system.

Advantages of distributed database systems are:
    * local autonomy (in enterprises that are distributed already)
    * improved performance (since data is stored close to where needed
and a query may be split over several sites and executed in parallel)
    * improved reliability/availability (should one site go down)
    * economics
    * expandability
    * shareability

Disadvantages of distributed database systems are:
    * complexity (greater potential for bugs in software)
    * cost (software development can be much more
      complex and therefore costly. Also, exchange
      of messages and additional computations
      involve increased overheads)
    * distribution of control (no single database administrator
controls the DDB)
    * security (since the system is distributed the chances of
security lapses are greater)
    * difficult to change (since all sites have control of their own
sites)
    * lack of experience (enough experience is not available in
developing distributed systems)

"The database is stored on several computers.The computers in a distributed system communicate with another through various communication media,such as high speed networks or telephone lines and they do not share main memory or disks."

"Centralized system a modern,general purpose computer system consists of one to a few CPUs and a number of device controllers that are connected through a common bus that provides access to shared memory and the CPUs have local cache memories."

Centralized database controlled by dba and importants secure informations are stored in the database.
Whereas in a distributed database replica of data available in multiple sites.So loss of data in one site could not be affect the  total system.

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.

1. Differentiate between features of the centralized database and distributed
Databases

Test