Build Your Own Ajax Web Applications
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Web applications evolve dreaming that one day they'll look and behave like their mature and powerful relatives, the desktop applications. The behavior of any computer software that interacts with humans is now even more important than it used to be, because nowadays the computer user base varies much more than in the past, when the users were technically sound as well.
Now you need to display good looking reports to Cindy, the sales department manager, and you need to provide easy-to-use data entry forms to Dave, the sales person. Because end-user satisfaction is all that matters, the software application you build must be satisfactory to all the users that interact with it. As far as web applications are concerned, their evolution-to-maturity process will be complete when the application's interface and behavior will not reveal whether the functionality is delivered by the local desktop or comes through fiber or air.
Delivering usable interfaces via the Web used to be problematic simply because features that people use with their desktop applications, such as drag and drop, and performing multiple tasks on the same window at the same time, were not possible. Another problem with building web applications is standardization. Yes, there are lots of headaches when trying to deliver functionality via the Web.
But why bother trying to do that in the first place, instead of building plain desktop applications? Well, even with the current problems that web applications have with being user-friendly, they have acquired extraordinary popularity because they offer a number of major technological advantages over desktop applications.
In this book, we'll further investigate how to use modern web technologies to build better web applications, to make the most out of the possibilities offered by the Web. But before getting into the details, let's take a short history lesson. In its first few initial versions, it didn't do much more than opening and closing connections.
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The later versions of HTTP version 1. HTTP is supported by all web browsers, and it does very well the job it was conceived for—retrieving simple web content. Whenever you request a web page using your favorite web browser, the HTTP protocol is assumed. So, for example, when you type www. HTML is a language that describes documents' formatting and content, which is basically composed of static text and images. HTML wasn't designed for building complex web applications with interactive content or user-friendly interfaces.
It's obvious that these restrictions don't really encourage building anything interesting. They are the foundation of the Internet as we know it today. Figure 1. To complement the lack of features, several technologies have been developed. While all web requests we'll talk about from now on still use the HTTP protocol for transferring data, the data itself can be built dynamically on the web server say, using information from a database , and this data can contain more than plain HTML allowing the client to perform some functionality rather than simply display static pages.
The technologies that enable the Web to act smarter are grouped in the following two main categories:. Server-side web technologies enable the web server to do much more than simply returning the requested HTML files, such as performing complex calculations, doing object-oriented programming, working with databases, and much more. Just imagine how much data processing Amazon must do to calculate personalized product recommendations for each visitor, or Google when it searches its enormous database to serve your request. Yes, server-side processing is the engine that caused the web revolution, and the reason for which Internet is so useful nowadays.
The important thing to remember is that no matter what happens on the server side, the response received by the client must be a language that the client understands obviously —such as HTML, which has many limits, as mentioned earlier. PHP is one of the technologies used to implement server-side logic. Each of these has its own way of allowing programmers to build server-side functionality.
PHP is not only a server-side technology but a scripting language as well, which programmers can use to create PHP scripts. This time, instead of sending back the contents of index. These results must be in HTML, or in another language that the client understands. On the server side, you'll usually need a database server as well to manage your data. In the case studies of this book, we'll work with MySQL, but the concepts are the same as any other server.
You'll learn the basics of working with databases and PHP in Chapter 3. However, even with PHP that can build custom-made database-driven responses, the browser still displays a static, boring, and not very smart web document. The need for smarter and more powerful functionality on the web client generated a separate set of technologies, called client-side technologies. Today's browsers know how to parse more than simple HTML. Let's see how. The various client-side technologies differ in many ways, starting with the way they get loaded and executed by the web client.
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Sometimes they even need long startup times, and are generally too heavy and powerful for the small requirements of simple web applications. Macromedia Flash has very powerful tools for creating animations and graphical effects, and it's the de-facto standard for delivering such kind of programs via the Web. Flash also requires the client to install a browser plug-in.
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Flash-based technologies have become increasingly powerful, and new ones keep appearing. Combining HTML with a server-side technology and a client-side technology, one can end up building very powerful web solutions. As pointed out in the beginning of the chapter, technology exists to serve existing market needs. And part of the market wants to deliver more powerful functionality to web clients without using Flash, Java applets, or other technologies that are considered either too flashy or heavy-weight for certain purposes.
The typical request with this scenario is shown in Figure 1.
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When put in perspective, AJAX is about reaching a better balance between client functionality and server functionality when executing the action requested by the user. Up until now, client-side functionality and server-side functionality were regarded as separate bits of functionality that work one at a time to respond to user's actions. AJAX comes with the solution to balance the load between the client and the server by allowing them to communicate in the background while the user is working on the page.
To explain with a simple example, consider web forms where the user is asked to write some data such as name, email address, password, credit card, etc. Without AJAX, there were two form validation techniques. The first was to let the user type all the required data, let him or her submit the page, and perform the validation on the server. In this scenario, the user experiences a dead time while waiting for the new page to load. The alternative was to do this verification at the client, but this wasn't always possible or feasible because it implied loading too much data on the client just think if you needed to validate that the entered city and the entered country match.
In the AJAX-enabled scenario, the web application can validate the entered data by making server calls in the background, while the user keeps typing. For example, after the user selects a country, the web browser calls the server to load on the fly the list of cities for that country, without interrupting the user from his or her current activity. The examples where AJAX can make a difference are endless. To get a better feeling and understanding of what AJAX can do for you, have a look at these live and popular examples:.
Just as with any other technology, AJAX can be overused , or used the wrong way. Just having AJAX on your website doesn't guarantee your website will be better. It depends on you to make good use of the technology. So AJAX is about creating more versatile and interactive web applications by enabling web pages to make asynchronous calls to the server transparently while the user is working. AJAX is a tool that web developers can use to create smarter web applications that behave better than traditional web applications when interacting with humans.
5 Interesting Things You Can Do With Ajax
The suggested format is XML , which has the advantage of being widely supported, and there are many libraries that make it easy to manipulate XML documents. However, to make sure we're all on the same page, we'll have a look together at how these pieces work, and how they work together, in Chapter 2 and Chapter 3. Until then, for the remainder of this chapter we'll focus on the big picture, and we will also write an AJAX program for the joy of the most impatient readers.