Object-based Scripting Languages

The primary focus of this book has been on programming languages, specifically Java and the .NET languages. However, these object-oriented languages are not the only domains for programming with objects. We have already mentioned that C++ is not a true object-oriented programming language, but is actually an object-based programming language. Remember that C++ is considered to be object-based. Object-oriented concepts are not enforced. You can write a non-object-oriented C program using a C++ compiler. There is also a class of languages called scripting languages. JavaScript, VBScript and ASP all fall into this category.

Overall Model

Many technologies are used to create Web pages. Programming languages, scripting languages, and markup languages all have a place in the model. Although this book focuses primarily on object-oriented programming languages, it is important to understand that programming languages are just part of the puzzle.

At this point, let's take a brief pause to cover a few of the Internet- related topics that form the basis for our discussion on the Web. First, it is important to review the concepts of a client-server model. Figure 13.1 shows a typical client-server model.

Figure 13.1. The client-server model.

It is important to understand that there really are two sides to the client-server story. As the name implies, the two parts of the model are the client side, which in many cases is the browser, and the server side, which is the physical Web server. A simple e-commerce example serves as a good study for this discussion.

Suppose you are creating a simple Web page that will request the following information from the user :

• Date

• First name

• Last name

• Age

When invoked, the HTML is rendered in the browser, which is considered the client, as shown in Figure 13.2.

Figure 13.2. Rendering an HTML document.

This is obviously a very simple HTML document; however, it illustrates the concept of form validation quite well. One of the major issues we must address when developing a client-server system is whether we will do client validation, server validation, or both.

For example, suppose we want to verify that the date entered by the user is valid. We also want the age to be within a valid range ”we certainly don't want someone to enter an age of -5. The question is whether to validate on the client side or the server side. Let's explore why this is an important discussion and how it relates to objects.

First, let's address the issue of the Age field. In most business systems, the customer information would be stored in a database that resides with the server. For security reasons, the client is not permitted to access the database directly.

Client Security

Because anyone can bring up a Web browser, it would be very foolish to let the client (browser) access the database directly. Thus, when the client needs to inspect or update the database, it must request the operation from the server. This is a basic security issue.

The reason why this example is so interesting is because it's a perfect example of the interface/implementation paradigm stressed throughout this book. In this case, the client is requesting a service from the server. The software system provides an interface through which the client can literally send messages and request specific services from the server.

In the example relating to the Age field in the HTML document in Figure 13.2, suppose a user named Mary wants to update her age in the database. After bringing up the Web page, the user enters the appropriate information on the form (including her age in the Age field) and then clicks on the Register button. In the simplest scenario, the information in the form is sent to the server, which then processes the information and updates the database.

How is the information entered in the Date field verified ? If no validation is done, the software on the server accesses the Age field in Mary's record and makes the update. If the age that Mary enters is incorrect, the inappropriate age is entered in the database.

If the validation is done on the server, the software on the server checks to make sure that the Age value falls into appropriate ranges. It is also possible that the database itself does some checking to ensure that the age is within proper limits.

However, there is one major limitation to server-side validation ”the information must be sent to the server. This might seem counter-intuitive, but you can ask this simple question: Why validate something on the server that can be validated on the client?

There are several points that address this question:

• Sending things to the server takes more time.

• Sending things to the server increases network traffic.

• Sending things to the server takes up server resources.

• Sending things to the server increases the potential for error.

For these reasons, as well as other possible issues, our goal is to do as much of the validation on the client as possible. This is where the scripting languages come into play.