Database Reference
In-Depth Information
In large organizations, number of users is more and volume of data is high. This large
amount of data is difficult to manage by a single user. In this case data is integrated and
shared. Data is kept on various database servers and with the help of computer networks
data can be transmitted from one server to another. For example examination department
and accounts department of any college/ university may need address of a student. So,
same data is being shared by two departments. It is responsibility of DBMS to provide data
to various users at a time and ensures that no two users can modify same piece of data
at a same time. It may be possible that accounts department's user cannot view examina-
tion related data. They share data but may be different level of access to databases. Some
users may have access to view databases; others may have access to modify databases also.
These permissions to access databases are called privileges.
The second type of data that is also very important and keep information about data is
called Metadata or Data Dictionary. Data Dictionary keeps all kind of information about
users, internal structure of databases, privileges and rights etc. Database system may dis-
tributed or centralized. Earlier, main frames or minicomputers were used to keep central-
ized databases. This was called single tier system. Terminals ware used to access databases
through DBMS in single tier system.
In two-tier system is an example of distributed computing where different software is re-
quired for server machine and for the client machine. This system is called Client/Server
database system. A client-server environment was common in the preInternet days where a
transactional database serviced users within a single company. The number of users could
range from as little as one to thousands, depending on the size of the company. The crit-
ical factor was actually a mixture of both individual record change activity and modestly
sized reports. Client-server database models typically catered for low concurrency and low
throughput at the same time because the number of users was always manageable.
In three-tier system special software called middleware is used to connect client of one
DBMS to access another DBMS. In a distributed computing system there may numbers
of DBMS installed on different server machine. A distributed database system allows ap-
plications to access data from local and remote databases. It is two types homogenous and
heterogeneous distributed database system. A homogenous distributed database system is a
network of two or more Databases that reside on one or more machines. In a heterogeneous
distributed database system, at least one of the databases is of different type. Data is stored
geographically and resides to nearer where it will be
