Three-Tier Architecture

Professor Ortiz presents a little of the theory behind the three-tier architecture and shows how it may be applied using Linux, Java and MiniSQL.
Running MiniSQL

MiniSQL, developed by Hughes Technologies, is an exceptionally fast DBMS with very low system requirements. It supports a fairly useful subset of the Structured Query Language (SQL). Using it for commercial purposes requires purchasing a license, although free licenses are provided for academic and charity organizations.

The software is distributed in source code form, all bundled up in a gzipped tar file (currently, the latest distribution file is msql-2.0.11.tar.gz). It may be downloaded from the Hughes Technology web site (see Resources). The MiniSQL manual, with all the necessary installation and usage information, is contained in the files msql-2.0.11/doc/manual.ps.gz and msql- 2.0.11/doc/manual-html/manual.html, once the distribution file is extracted. The reader is encouraged to carefully review and follow the instructions contained there. However, it must be noted that two important details are missing from the MiniSQL manual:

  • The “system” section contained in the /usr/local/Hughes/msql.conf file has a parameter called Remote_Access that has a default value of false. It must be changed to true in order to allow access to the database from remote systems.

  • Like other server dæmons (for example, the HTTP web server), the MiniSQL 2.0 server, called msql2d, should be run as a background process. Executing the following command as root should achieve this: /usr/local/Hughes/bin/msql2d &

In addition to the database server, MiniSQL comes with some other useful utilities: a server administration program, an interactive SQL monitor, a schema viewer, a data dumper and a table-data exporter and importer. The server administration program is required to create the Hangman database that will contain the mystery words. The following command must be executed as root:

/usr/local/Hughes/bin/msqladmin create hangman
Afterward, a mystery-words table needs to be created. Only two columns will be contained in this table: word (the mystery word or sentence) and category (a classification for the mystery word: computers, animals, movies, etc.), both of them being character strings. Also, a few rows should be inserted. The interactive SQL monitor may be used for both purposes. Executing the command
/usr/local/Hughes/bin/msql hangman
enters the interactive monitor with the “hangman” database. The MiniSQL prompt should appear. SQL queries can now be issued, followed by “\g”(GO) to indicate that the query should be sent to the database server. Here are the SQL commands for the Hangman application:
create table mystery (word char(40), category char(15))\g
insert into mystery values ('elephant', 'animals')\g
insert into mystery values ('rhinoceros', 'animals')\g
insert into mystery values ('gone with the wind', 'movies')\g

Accessing the MiniSQL from a Java Program

The application's middle tier uses Blackdown's Linux Port Java Development Kit 1.2.2, release candidate 4, and CIE's mSQL-JDBC driver for JDBC 2.0. The Java tutorial is one of many excellent places to learn how to access databases from within a Java program; that's why only the specific issues on accessing MiniSQL will be dealt with here.

Before attempting to access the MiniSQL server from a Java application, the corresponding JDBC driver must be installed. The driver may be freely downloaded from The Center for Imaginary Environments web site (see Resources). The distribution file comes with many things, but the most important part is the JAR file that contains the driver itself (currently, the file is msql-jdbc-2.0b5.jar). The easiest way to install the driver is to copy the JAR file to the /usr/local/jdk1.2.2/jre/lib/ext directory (root privileges are required to copy files to this directory).

In order to load the driver from the Java program, the following statement should be executed:

Class.forName("com.imaginary.sql.msql.MsqlDriver");

The connection to the database server is established when executing this statement (ignore line wrap):

Connection con = DriverManager.getConnection
 ('jdbc:msql://localhost:1114/hangman');
Inside the JDBC URL, the URL of a remote system should replace “localhost” if the MiniSQL server is not running in the same machine. 1114 is the default port number to which the MiniSQL server is listening. The msql.conf file can be modified in order to specify another port number.

Conclusions

The three-tier architecture is a versatile and modular infrastructure intended to improve usability, flexibility, interoperability and scalability. Linux, Java and MiniSQL result in an interesting combination for learning how to build three-tier architecture systems. Nevertheless, more convenient implementations than the one presented here may be produced using component technology in the middle tier, such as CORBA (Common Object Request Broker Architecture), EJB (Enterprise Java Beans) and DCOM (Distributed Component Object Model). The interested reader should review these topics to get a better understanding of the current three-tier architecture capabilities.

Resources

email: aortiz@campus.cem.itesm.mx

Ariel Ortiz Ramirez (aortiz@campus.cem.itesm.mx) is a faculty member in the Computer Science Department of the Monterey Institute of Technology and Higher Education, Campus Estado de Mexico. Although he has taught several different programming languages for almost a decade, he personally has much more fun when programming in Scheme and Java (in that order). He can be reached at aortiz@campus.cem.itesm.mx.

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awesome article

amila's picture

This is quite simple and completed, understandable awesome article.
thanks for publishing. I have removed my one of my misunderstanding about 3 tier architecture reading this. (actually this article + wikipedia done that)
Thanks again and keep it up

thanx so much for the simple

Anonymous's picture

thanx so much for the simple yet valuable info

firoz, orionis

3-tier Architecture Comment

Purushottam Choudhary's picture

Very Good Article on 3 tier Architecture.

Good Article on 3 tier

Anonymous's picture

Good Article on 3 tier Architecture.

Comment

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Anonymous's picture

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Comment

Rita dsouza's picture

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