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Software Design: What is meant by three tier architecture?

Three-tier architecture has been making waves for some time now ! But a lot of the people who talk about it are not aware of the details of this architecture. This article is going to try and explain this new beast called three-tier architecture. To understand the three tier system we first need to understand a more and earlier term i.e., client- server model. No doubt it is a type of computing model, and it plays the role of the distributed application among the clients and the servers. The basic role of the client server model, apart from providing a link between the client and server, is to divide the workload that exists among the servers (the computers the network that are responsible for providing resource and services to the clients) and the clients (one who initiates request from the server for resources and services and which interact with end users whereas the servers are never exposed to users).
The communication that exists between the clients and servers has been possible only because of the computer network. It is implemented on different hardware for both the client and server side. Even though the client and server might reside in different physical regions of the world, they are said to be in the same networking system. The server runs the server programs and is responsible for making available the services and resources that are requested by the clients. Because of this, a server is often called a host. There is only one way transfer of data from the server side. There is no file sharing from the client side. The client is responsible only for initiating the request. In such an architecture, the server should have a high capacity for handling multiple users at a time with heavy processing requests.
The relation between a server and a client is defined by the characteristics of that particular client server model. There are many web applications that implement the client server model in their architecture like:
1. Web access applications
2. Data base access applications and
3. Email
The client server model has been possible because of the presence of a strong network. It is not only software applications, but even many protocols also make use of the client server model like:
1. Telnet
2. DNS
3. SMTP
4. HTTP
Now let us get some details on the three-tier architecture. It consists of three layers and these layers are also called tiers.
1. 1st tier: Client front end: This end is responsible for generating the requests for the resources and the services.
2. 2nd tier: Business logic: Business logic is nothing but the protocols implemented during the processing of the requests. They can be implemented on both the client side as well as the server side. When it is implemented in server the server is called fat server. Similarly when it is implemented on the client side it is called fat client.
3. 3rd tier: Data base back end: This end is responsible for holding the data and responding back to the client side i.e., providing the service requested for.
Nowadays most web applications are using this three-tier architecture. The communication between the server and the client is established through the application programming interface or API. In a three-tier architecture, separate servers are used for the application and the data base. The middle tier is placed between the client and the server so as to control the excess burden on the server. Therefore it forms an intermediate junction between the client and the server and controls the incoming requests and the processed out going requests. Three-tier architecture is a means to enforce the separation between the 2 layers. Three-tier architecture has proved to be a cutting edge in networking technology as it has made the communication faster and more reliable, enabling the building of better and stronger web applications, capable of handling huge requests.

Computer Architecture, Fifth Edition Computer Organization and Design Digital Design and Computer Architecture

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