DISTRIBUTED VERSUS CENTRALIZED DATABASE

DISTRIBUTED DATABASE VS CENTRALIZED DATABASE
I have extracted some key phrases on Why we use a distributed databaseover a centralized one:
"A big advantage of distributed DBMSs over centralized ones is that ofscalability. 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 ingeographic 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 architectureis to improve reliability and availability of the data in a scalablesystem. 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."

Major features of a DDB are:
* Data stored at a number of sites, each site logically singleprocessor
* Sites are interconnected by a network rather than a multiprocessor configuration
* DDB is logically a single database (although each site is adatabase site)
* DDBMS has full functionality of a DBMS
* To the user, the distributed database system should appearexactly 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 neededand 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 costlAlso,exchangeof messages and additional computations involve increased overheads)
* distribution of control (no single database administratorcontrols the DDB)
* security (since the system is distributed the chances ofsecurity lapses are greater)
* difficult to change (since all sites have control of their ownsites)
* lack of experience (enough experience is not available indeveloping distributed systems)