Static, pure-HTML files like Welcome.htm make perfectly serviceable web pages. We can even spruce up the presentation and usability of such pages by adding more HTML to create frames and tables. However, there's only so much we can achieve by writing pure HTML, precisely because their content is completely determined before the page is ever requested.

As you type in the HTML, you'll soon realize the problem – you know that the user will request the page sometime, but you don't know what the time will be when they do so! Hard-coding the time into your HTML will result in a page that will lways claim that the time is the same (and will almost always display the wrong time).

In other words, trying to write pure HTML for a web page that displays the time – but you can't be sure of the exact time that the web page should display until the time the page is requested. It can't be done using HTML alone.

Also HTML offers no features for personalizing your web pages, each web page that is served is the same for every user. There's also no security with HTML, the code is there for everybody to view, and there's nothing to stop you from copying somebody else's HTML code and using it in your own web page. Static pages might be very fast, as quick as copying a small file over a network, but they are quite limited without any dynamic features.

Since we can't create our page by saving our hard-coded HTML into a file before the page is requested, what we need is a way to generate the HTML after the page is requested. There are two ways of doing this; we'll look at them both now. Before we go any further, we need to make sure everybody is up to speed on the terminology we've introduced here.

While there are many web servers available (the commonest ones being Apache, IIS and Iplanet's Enterprise server) we're only going to talk about one in this book Microsoft's IIS 5. This is because it is the only web server that will run ASP.NET. The web server comes as part of the installation for both Windows 2000 and Windows XP. IIS version 5.0 comes with Windows 2000, and IIS version 5.1 with Windows XP; however, there is very little to distinguish the two, and we shall treat them in this chapter as the same product. We'll look at how we go about installing IIS shortly; however first let's take a look at its role in helping to create dynamic web pages.

So, in this model, our set of five steps now becomes six:

1. A web author writes a set of instructions for creating HTML, and saves it within an .htm file. The author also writes a set of instructions in a different language. This might be contained within the .htm file, or within a separate file.

2. Sometime later, a user types a page request into their browser, and the request is passed from the browser to the web server.

3. The web server locates the .htm page, and may also have to locate a second file that contains the instructions.

4. The web server sends both the newly created HTML stream and instructions back across the network to the browser.

5. A module within the browser processes the instructions and returns it as HTML within the .htm page – only one page is returned, even if two were requested.

6. The HTML is then processed by the browser which displays the page

Client-side technologies have fallen out of favor in recent times, as they take a long time to download, especially if we have to download a second file with a separate set of instructions. In some cases, we might have to download several files of separate instructions. A second drawback is that each browser interprets these instructions in different ways, so we have no way of guaranteeing that if Internet Explorer understands them, whether Netscape Navigator or Opera will. An other major drawbacks are that it is a problem to write client-side code that uses server-side resources  such as databases, because it is interpreted at client-side. Also all code for client-side scripting is available to everybody, which can be undesirable.

1. A web author writes a set of instructions for creating HTML, and saves these instructions within a file

2. Sometime later, a user types a page request into their browser, and the request is passed from the browser to the web server

3. The web server locates the file of instructions

4. The web server follows the instructions in order to create a stream of HTML

5. The web server sends the newly created HTML stream back across the network to the browser

6. The browser processes the HTML and displays the page

In fact either process of serving a dynamic web page is only slightly different from the process of serving a static web page – there's just one extra step involved (Step 5 on the client or Step 4 on the server). But in both cases this difference is crucial – the HTML that defines the web page is not generated until after the web page has been requested. For example, we can use either technique to write a set of instructions for creating a page that displays the current time:

Now we're going to look at the various different technologies, including ASP.NET and see how the logic is supported in each.

An Overview of the Technologies
    We just seen that there are also two distinct models for providing dynamic content. ASP.NET falls into the server-side model. However, we're going to look at what we consider to be the most important technologies in both models, as we will reference some of the client-side models in later chapters, particularly if we mention old style ASP. Not all of the technologies work in the same way as ASP.NET, but they all allow the user to achieve the same end-result – that of dynamic web applications. If ASP.NET is not an ideal solution to your problems, then you might want to consider these following technologies, taking into account the following questions:

• Are they supported on the platform you use?
  
• Are they difficult to learn?
  
• Are they easy to maintain?
  
• Do they have a long-term future?
  
• Do they have extra capabilities, such as being able to parse XML?
  
• Are a lot of people already using them – are there a lot of tools available?
  
• Are the support, skills, and knowledge required to use them readily available?
  

JavaScript
    JavaScript is the original browser scripting language, and is not to be confused with Java. Java is a complete application programming language in its own right. Netscape had originally developed a scripting language, known as LiveScript, to add interactivity to their web server and browser range. It was introduced in the release of the Netscape 2 browser, when Netscape joined forces with Sun and in the process, they changed its name to JavaScript. JavaScript borrows some of its syntax and basic structures from Java (which in turn borrowed ideas from C), but has a different purpose – and evolved from different origins (LiveScript was developed separately to Java).

For example, while JavaScript can control browser behavior and content, it isn't capable of controlling features such as file handling. In fact, JavaScript is actively prevented from doing this for security reasons. Think about it: you wouldn't want a web page capable of deleting files on your hard drive, now would you? Meanwhile, Java can't control the browser as a whole, but it can do graphics and perform network and threading functions.

Javascript is much easier to learn than Java. It is designed to create small, efficient, applications that can do many things, from performing repetitive tasks, to handling events generated by the user (such as mouse clicks, keyboard responses, and so on).

Microsoft introduced their own version of JavaScript, known as JScript, in Internet Explorer 3.0 and have supported it ever since right up to, and including IE6. It has only minor differences from the Netscape version of the language, although in older versions of both browsers, the differences were originally quite a lot wider.

The biggest drawback is that there isn't a single non-Microsoft browser that supports VBScript for client-side scripting. For a short while there were some proprietary plug-ins for Netscape that provided VBScript support, but these never took off. You'll find that JavaScript is much more widely used and supported. If you want to do client-side scripting of web pages on the Internet then JavaScript is the only language of choice. Indeed Microsoft themselves have replaced VBScript in their .NET framework, with VB.NET. VBScript should only be considered when working on Intranet pages where it is known that all clients are IE on Windows.

With both JavaScript and VBScript there is a module, known as a script engine, built into the browser that dynamically processes the instructions, or script, as it is known in this case.

There is a catch. ActiveX controls were developed by Microsoft, and despite being compatible with the HTML standard, they are not supported on any Netscape browser prior to version 6 (which, at time of writing, was still in beta) without an ActiveX plug-in. Without this, they will only function on Internet Explorer. Also, unlike VBScript, ActiveX is able to manipulate items on the user's machine such as the files or Windows registry. For this reason it is very often considered a security risk and is not even allowed through firewalls. Consequently, ActiveX controls still can't really be considered either a common or a cross-platform way of making your pages dynamic and are falling out of use.

Java enjoys better functionality than scripting languages, offering better capabilities in areas such as graphic functions and file handling. Java is able to provide these powerful features without compromising security because the applets run in what is known as a sandbox – which prevents a malicious program downloaded from the web from doing damage to your system. Java also boasts strong database support through JDBC.

Microsoft and Netscape browsers both have built-in Java support via something known as the Java Virtual Machine (JVM), and there are several standard <object> and non-standard <applet> tags that are used to add Java applets to a web page. These tags tell the browser to download a Java file from a server and execute it with the Java Virtual Machine built into the browser. Of course, this extra step in the web page building phase means that Java applets can take a little while to download, and can take even longer to process once on the browser. So, while smaller Java applets (that provide features such as drop-down menus and animations) are very popular on the Web, larger ones are still not as widespread as scripted pages.

Although the popularity of Java today isn't quite what some people expected, it makes an ideal teaching tool for people wishing to break out into more complex languages; and its versatility makes it well suited for programming web applications.

Curl is still in the very early stages of development, although the first version has been released, and more details can be obtained at http://www.curl.com.