DISTRIBUTED DATA PROCESSING (CLIENT/SERVER)

In this data processing method, data process is distributed between client and server. Server takes care of managing data. Client interacts with user. For example, if you assume a process where we need to draw a graph to show the number of students in a given month for each subject, the following steps will take place:

• First, client interacts with user and takes input (month name) from user and then passes it to server.

• Server then will query the database to get data related to the month, which is sent to server, and will send data back to client. 
• The client will then use the data retrieved from database to draw a graph.  

If you look at the above process, the client and server are equally participating in the process. That is the reason this type of data processing is called as distributed. The process is evenly distributed between client and server. Client is a program written in one of the font-end tools such as Visual basic or Delphi. Server is a database management system such as Oracle, SQL Server etc. The language used to send commands from client to server is SQL .
This is also called as two-tier client/server architecture. In this we have only two tiers (layers) one is server and another is client.
The following is an example of 3-tier client server, where client interacts with user on one side and interacts with application server on another side. Application, which processes and validates data, takes the request from client and sends the request in the language understood by database server. Application servers are generally object oriented. They expose a set of object, whose methods are to be invoked by client to perform the required operation. Application server takes some burden from database server and some burden from client.

In 3-tier client/server architecture, database server and application server may reside on different machines or on the same machine. Since the advent of web application we are also seeing more than 3-tiers, which is called as n-tier architecture. For example, the following is the sequence in a typical web application.

• Client- web browser, sends request to web server. 
• Web server executes the request page, which may be an ASP or JSP. 
• ASP or JSP will access application server. 
• Application server then will access database server.    

Summary :

A DBMS is used to store and manipulate data. A DBMS based on relational model is RDBMS. Primary key is used for unique identification of rows and foreign key to join tables. Relational algebra is a collection of operators used to operate on tables. We will see how to practically use these operators in later chapter. SQL is a language commonly used in RDBMS to store and retrieve data. In my opinion, SQL is one of the most important languages if you are dealing with an RDBMS because total data access is done using SQL.