Centralized Database Systems
In database systems types the first category is the centralized Database system. In this category complete database is stored in one location. All the accesses to any subset of data will be to this centralized location. Centralized control is on retrieval and updating of data. This feature also poses the problem of vulnerability of data. Availability of resources is at the central site, so the Chances of failure of database system increases manifolds. The advantage is that the Database remains under control of DBA, preventing any un-monitored access on database.
An example of Centralized database system can be a bank which offers the transaction facilities to its customers on different geographical locations. For this the customer account information is stored in the home branch of the customer but must be stored at one centralized location and shared among other branches and ATM kiosks over a communications network
Distributed Database Systems
In a Distributed database the data is stored on multiple computers. These computers can be personal computers or mainframe computers. All the computers storing the data communicate with each another through various communication media, such as high speed networks or telephone lines.
This Database System has geographically separated sites and is managed separately administered and have a slower interconnection. The sites managing the data have a certain degree of control over the data they are responsible for. There must be coordination between the different sites storing and managing data.
An example of the Distributed database system is the railway reservation system. The railway reservation branches are scattered in various cities of the country and the railway database is distributed among these branches. The branches are connected through a communication network. Whenever a customer gets his ticket reservation, the update is done in all portions of database across all the branches to have real time status of any specific train.