In n-tier architecture the entire application is divided in several pieces. These pieces can be logical or physical. Each piece performs a specific task such as displaying user interface or data access. There can be any number of tiers or layers of such pieces. Hence, the name n-tier (Note that many times the terms tier and layer are used interchangeably). However, most commonly applications have 3 distinct tiers or layers. They are:

• Presentation Layer
• Business Logic Layer
• Data Access Layer

As you can guess, presentation layer is nothing but a piece of software that deals with user interface of your application. Displaying data to the end user and allow them to interface with it is the core functionality of this layer.

In most of the cases the data entered by the end user needs some kind of validation or further processing. This is the responsibility of Business Logic Layer.

Finally, your application data needs to be stored and retrieved in some data store (RDBMS, XML etc.). This task is handled by Data Access Layer.

In short, the process works like this:

• User requests for some application data.
• The data access layer retrieves the data and is forwarded to the presentation layer via business logic layer. Sometimes data access layer gives this data directly to presentation layer.
• Presentation layer receives the data to be displayed via business logic layer.
• The user changes the data and initiates the appropriate action (such as insert, or update).
• The business logic layer validates the data submitted by the user.
• If the data is valid it is handed over to data access layer for updating into the database.

Advantages of N-Tier ArchitectureAt first glance this division of tasks may seem to be unnecessary. However, it gives following benefits:

• The applications gets divided in logically isolated pieces reducing tight coupling between the UI, business processes and database.
• Change in the underlying database and data access methods do not have any effect on the presentation layer or client application.
• Client application no longer has SQL statements embedded in it. This makes it de-coupled from rest of the application.
• Table and column names can be effectively eliminated from the client-side code.
• The client application is unaware from where data comes (location transparency).
• It becomes easier to modify or extend your application, without breaking or recompiling the client-side code.

The downside of n-tier architecture is that you need to create many isolated classes and pieces of software. However, benefits of n-tier applications will far outweigh its disadvantage.

Software that provides services to access the Internet, an intranet, or an extranet. A Web server hosts Web sites, provides support for HTTP and other protocols, and executes server-side programs (such as CGI scripts or servlets) that perform certain functions. In the J2EE architecture, a Web server provides services to a Web container. For example, a Web container typically relies on a Web server to provide HTTP message handling. The J2EE architecture assumes that a Web container is hosted by a Web server from the same vendor, so it does not specify the contract between these two entities. A Web server can host one or more Web containers.

A container that implements the Web component contract of the J2EE architecture. This contract specifies a runtime environment for Web components that includes security, concurrency, life-cycle management, transaction, deployment, and other services. A Web container provides the same services as a JSP container as well as a federated view of the J2EE platform APIs. A Web container is provided by a Web or J2EE server.

Two important pieces of RDBMS architecture are the kernel, which is the software, and the data dictionary, which consists of the system-level data structures used by the kernel to manage the database

You might think of an RDBMS as an operating system (or set of subsystems), designed specifically for controlling data access; its primary functions are storing, retrieving, and securing data. An RDBMS maintains its own list of authorized users and their associated privileges; manages memory caches and paging; controls locking for concurrent resource usage; dispatches and schedules user requests; and manages space usage within its table-space structures

There are three major sets of files on disk that compose a database. All the files are binary. These are

Ø Database files

Ø Control files

Ø Redo logs

The most important of these are the database files where the actual data resides. The control files and the redo logs support the functioning of the architecture itself.

All three sets of files must be present, open, and available to Oracle for any data on the database to be useable. Without these files, you cannot access the database, and the database administrator might have to recover some or all of the database using a backup, if there is one.

A component architecture for the development and deployment of object-oriented, distributed, enterprise-level applications.

Applications written using the Enterprise JavaBeans architecture are scalable, transactional, and secure.