Ajax Advantages and problems

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) that has to be validated before reaching the business tier of your application. 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: 

• Google Suggest helps you with your Google searches. The functionality is pretty spectacular; check it out at http://www.google.com/webhp?complete=1. Similar functionality is offered by Yahoo! Instant Search, accessible at http://instant.search.yahoo.com/.
• GMail (http://www.gmail.com). GMail is very popular by now and doesn't need any introduction. Other web-based email services such as Yahoo! Mail and Hotmail have followed the trend and offer AJAX-based functionality.
• Google Maps (http://maps.google.com), Yahoo Maps (http://maps.yahoo.com), and Windows Live Local (http://local.live.com). 
• Other services, such as http://www.writely.com and http://www.basecamphq.com.

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. 

The technologies AJAX is made of are already implemented in all modern web browsers, such as Mozilla Firefox, Internet Explorer, or Opera, so the client doesn't need to install any extra modules to run an AJAX website. AJAX is made of the following: 

• JavaScript is the essential ingredient of AJAX, allowing you to build the client-side functionality. In your JavaScript functions you'll make heavy use of the Document Object Model (DOM) to manipulate parts of the HTML page.
• The XMLHttpRequest object enables JavaScript to access the server asynchronously, so that the user can continue working, while functionality is performed in the background. Accessing the server simply means making a simple HTTP request for a file or script located on the server. HTTP requests are easy to make and don't cause any firewall-related problems.
• A server-side technology is required to handle the requests that come from the JavaScript client. In this book we'll use PHP to perform the server-side part of the job.

For the client-server communication the parts need a way to pass data and understand that data. Passing the data is the simple part. The client script accessing the server (using the XMLHttpRequest object) can send name-value pairs using GET or POST. It's very simple to read these values with any server script. 

The server script simply sends back the response via HTTP, but unlike a usual website, the response will be in a format that can be simply parsed by the JavaScript code on the client. 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. But you can choose another format if you want (you can even send plain text), a popular alternative to XML being JavaScript Object Notation (JSON). 

This book assumes you already know the taste of the AJAX ingredients, except maybe the XMLHttpRequest object, which is less popular. 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. 

AJAX brings you the following potential benefits when building a new web application:

• It makes it possible to create better and more responsive websites and web applications.
• Because of its popularity, it encourages the development of patterns that help developers avoid reinventing the wheel when performing common tasks.
• It makes use of existing technologies.
• It makes use of existing developer skills.
• Features of AJAX integrate perfectly with existing functionality provided by web browsers (say, re-dimensioning the page, page navigation, etc).

Common scenarios where AJAX can be successfully used are:

• Enabling immediate server-side form validation, very useful in circumstances when it's unfeasible to transfer to the client all the data required to do the validation when the page initially loads.
• Creating simple online chat solutions that don't require external libraries such as the Java Runtime Machine or Flash.
• Building Google Suggest-like functionality
• More effectively using the power of other existing technologies.
• Coding responsive data grids that update the server-side database on the fly.
• Building applications that need real-time updates from various external sources.

Potential problems with AJAX are:

• Because the page address doesn't change while working, you can't easily bookmark AJAX-enabled pages. In the case of AJAX applications, bookmarking has different meanings depending on your specific application, usually meaning that you need to save state somehow (think about how this happens with desktop applications— there's no bookmarking there).
• Search engines may not be able to index all portions of your AJAX application site.
• The Back button in browsers, doesn't produce the same result as with classic web applications, because all actions happen inside the same page.
• JavaScript can be disabled at the client side, which makes the AJAX application nonfunctional, so it's good to have another plan in your site, whenever possible, to avoid losing visitors.

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