n-tiered application

• The J2EE platform is a multi-tiered system

A tier is a logical or functional partitioning of a system

Client tier 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.

Presentation tier 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.

Business tier provides the business services. This tier contains the business logic and the business data. All the business logic is centralized 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 centralized business tier is that same business logic can support different types of clients like browser, WAP, other stand-alone applications etc.

Integration tier 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.

Resource tier 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.



The advantages of a 3-tiered or n-tiered application:
3-tier or multi-tier architectures force separation among presentation logic, business logic and database logic

Manageability: Each tier can be monitored, tuned and upgraded independently and different people can have clearly defined responsibilities.

·         Scalability: More hardware can be added and allows clustering (i.e. horizontal scaling).

·         Maintainability: Changes and upgrades can be performed without affecting other components.

·         Availability: Clustering and load balancing can provide availability.

·         Extensibility: Additional features can be easily added.
http://allu.wordpress.com/2007/08/18/j2ee-3-tier-or-n-tier-architecture/

• In software engineering, multi-tier architecture (often referred to as n-tier architecture) is a client–server architecture in which presentation, application processing, and data management functions are logically separated. For example, an application that uses middleware to service data requests between a user and a database employs multi-tier architecture. The most widespread use of multi-tier architecture is the three-tier architecture.

http://en.wikipedia.org/wiki/Multitier_architecture

• Using J2EE to develop n-tier applications involves breaking apart the different layers in the two-tier architecture into multiple tiers. An n-tier application could provide separate layers for each of the following services:

    Presentation: In a typical Web application, a browser running on the client machine handles presentation.
    Dynamically generated presentation: Although a browser could handle some dynamically generated presentation, for the widest support of different browsers much of the action should be done on the Web server using JSPs, servlets, or XML (Extensible Markup Language) and XSL (Extensible Stylesheet Language).
    Business logic: Business logic is best implemented in Session EJBs (described later).
    Data access: Data access is best implemented in Entity EJBs (described later) and using JDBC.
    Backend system integration: Integration with backend systems may use a variety of technologies. The best choice will depend upon the exact nature of the backend system.
http://www.javaworld.com/jw-12-2000/jw-1201-weblogic.html