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.

The phase that identifies an efficient execution plan for evaluating a query that has the least estimated cost is referred to as query optimization.

Proactive Update:
The updates that are applied to database before it becomes effective in real world .
Retroactive Update:
The updates that are applied to database after it becomes effective in real world .
Simulatneous Update:
the updates that are applied to database at the same time when it becomes effective in real world .

If one or a few processes have a high access rate to data on one track of a storage disk, then they may monopolize the device by repeated requests to that track. This generally happens with most common device scheduling algorithms (LIFO, SSTF, C-SCAN, etc). High-density multisurface disks are more likely to be affected by this than low density ones.

The JDBC Optional Package for CDC/Foundation Profile (JDBCOP for CDC/FP) is an API that enables mobile Java applications to communicate with relational database servers using a subset of J2SE's Java Database Connectivity. This optional package is a strict subset of JDBC 3.0 that excludes some of JDBC's advanced and server-oriented features, such as pooled connections and array types. It's meant for use with the Foundation Profile or its supersets.

nl2br() inserts a HTML tag <br> before all new line characters \n in a string.
echo nl2br("god bless \n you");
output:
god bless<br>
you