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Explain about 3-Tier architecture ?

Each Tier is assigned a unique responsibility in a 3-Tier system. Each Tier is logically separated and loosely coupled from each other, and may be distributed.

1. Client Tier : It represents Web browser, a Java or other application, Applet, WAP phone etc. The Client Tier makes requests to the Web Server who will be serving the request by either returning static content if it is present in the Web Server or forwards the request to either Servlet or JSP in the Application Server for either static or dynamic content.

2. Presentation Tier : It encapsulates the presentation logic required to serve clients. A Servlet or JSP in the Presentation Tier intercepts client requests, manages logons, sessions, accesses the business services, and finally constructs a response, which gets delivered to client.

3. Business Tier : It provides the business services. This tier contains the business logic and the business data. All the business logic is centralised into this tier as opposed to 2-Tier systems where the business logic is scattered between the front end and the backend. The benefit of having a centralised business tier is that same business logic can support different types of clients like browser, WAP, other stand-alone applications etc.

4. Integration Tier : It is responsible for communicating with external resources such as databases, legacy systems, ERP systems, messaging systems like MQSeries etc. The components in this tier use JDBC, JMS, J2EE
Connector Architecture (JCA) and some proprietary middleware to access the resource tier.

5. Resource Tier : It is the external resource such as a database, ERP system, Mainframe system etc responsible for storing the data. This tier is also known as Data Tier or EIS (Enterprise Information System) Tier.