Distributed Database - Why?

Why Distributed Database?

We are interested in Distributed database for various reasons. Some of them are;

1. Data are always available to end users, i.e., they are easily accessible. The availability makes the total system reliable.
2. Distributed database increases the performance of the overall system. Because, the servers are available near the place where it is very much needed.
3. Support organizational growth. Because, the distributed database structure would not cause stopping of all ongoing services. Only new distributed server may need to be established to handle the new details. 
4. Handling addition of any server, modification of existing modules etc. are easy.
5. Distributed data handling increases the parallelism. That is, a number of queries can be handled simultaneously over multiple distributed server when compared to the central server approach.

Let us consider the scenario of XYZ bank which is headquartered in New Delhi. Also, assume that the bank maintains its server in its head office. Now, all the bank transactions done at all the branches of XYZ bank must reach the central server to access the data. For example, consider a customer who is trying to withdraw the money from his account through an ATM located in Chennai. His withdrawal request must be sent to the central server, processed in central server, and money will be disbursed in the ATM.  The following image shows the Central Server approach for any database for any organization. The requests initiated are shown in thick lines.

The following image shows the Distributed Server approach for the above given scenario. Now assume that, XYZ bank established several servers which are distributed throughout the country, say 10 different servers. Now, any request generated from the ATM from any part of the country will be forwarded to the server available in that part of the country. For any reason, if the requested data is not available with the local server, the server searches for the actual location of the requested data and forwards the request to that server, and routes the answer to the initiator.
The image shown below depicts the distributed server concept. It shows a set of DSs(Distributed Servers), a set of Nodes (not all are labeled), and a set of links which shows the request generated from node to the DS. The dashed line shows that the request generated by a node which is local to some other DS and the received DS forwarded to other DS where the intended data would be available. Here, the main advantage is consumption of network bandwidth is controlled, .i.e., network traffic reduced. Availability of the data and the server increased, as they are very close and accessible.