ALLU

J2EE 3-tier or n-tier architecture August 18, 2007

The J2EE platform is a multi-tiered system. A tier is a logical or functional partitioning of a system. When the developers are not disciplined, the display logic, business logic and database logic are muddled up and/or duplicated in a 2- tier client server system.

The advantages of the multi-tier architecture are:

·         Forced separation of user interface logic and business logic.

·         Business logic sits on small number of centralized machines (may be just one).

·         Easy to maintain, to manage, to scale, loosely coupled etc.

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.

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. Let us look at some of the key benefits:

·         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.