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Oracle XE + AJAX: Asynchronous XML in Action

Building a real application

This procedure will handle all the processing work for us by setting the content type to text/xml (required by the XmlHttpRequest object for DOM processing), setting null handling to output empty tags for null values, and disabling pretty printing for smaller response size. Also, when something goes wrong at the query level, e.g., empty result set or invalid query, the procedure manually outputs the hard-coded "No data found" message to indicate its state.

The AJAXE procedure will be a core dispatcher of responses. It will accept two parameters: q - the type of action requested, w - the extra parameter required for certain types of action (optional) (See Listing 3).

The functionality of the sample application will be divided into three basic use cases:

  • Filling the SELECT list with elements obtained through the AJAX request and hooking the event handler to it
  • Obtaining the results set and rendering it into a HTML table dynamically
  • Using simple auto-complete on the HTML input field
Note that both these procedures have to be created in the HR schema to work. At this point you can test whether the procedure is working for you by accessing http://localhost:8080/apex/hr.ajaxe?q=count (remember to adjust the URL to reflect your system's configuration). A valid XML document should be outputted at this address. You're halfway through.

Hands-on Document Object Model (DOM)
AJAX extensively relies on a set of JavaScript programming techniques once referred to as DHTML. But now they're much more powerful thanks to the unification of the DOM implementation among Web browser vendors. Back in the days of Netscape Navigator 4 and Microsoft Internet Explorer 4 you had to use a different syntax for each browser to manipulate page content dynamically. No need to do that anymore.

The Document Object Model is a tree-like hierarchical representation of nodes/HTML elements (see Table 1). The root node of an HTML document is the <html> tag that usually has two children: <head> and <body> - the manipulations happen in the second. Since DOM is a vast subject, we'll focus only on the most useful properties and methods of JavaScript XML objects. For the complete DOM specification, see the W3C Web site at http://w3.org.

Again, remember that there are a number of other JavaScript methods for working with the DOM object. For purposes of this article, an ajaxTable function was created: it builds up the HTML table from the XML response obtained through the AJAX request. By default DBMS_XMLGEN returns a XML tree of the following structure:

<?xml version="1.0"?>
<ROWSET>
<ROW>
<COLUMN1></COLUMN1>
<COLUMN2></COLUMN2>
...
</ROW>
...
</ROWSET>

The code in Listing 4 represents the body of the ajaxTable function.

This function appends the DOM tree in Listing 5 to the DIV element of ID="q" (the TBODY element is required here to render properly under Internet Explorer).

AJAX = Asynchronous JavaScript and XML
Technologically, AJAX is nothing new. It owes its recent popularity to the unification of the JavaScript standards that allow access to the XmlHttpRequest object. XML isn't new either. It now plays a key role in data and information interchange and is universally adopted by both the enterprise and the open source community. The biggest problem with the Web was that applications needed a full-page refresh after each user action on the page. It didn't matter whether it was just sorting table elements or downloading a whole new page.

There are a number of AJAX frameworks on the market, including the Dojo Toolkit, Prototype, or Microsoft Atlas to name a few. However, writing basic handlers alone helps enable a deeper understanding of what's happening under AJAX's hood. First, we need an XmlHttpRequest object that unfortunately still depends on the browser - Microsoft Internet Explorer continues handling it using an ActiveX control so the appropriate requests object creation code is presented in Listing 6. This is the only part that requires browser-compatibility code. Everything else here is cross-browser.

As for the handler, a dedicated function to handle all the asynchronous operations was created. It takes a request method, a URL, and callback functions as parameters. After the response is obtained the callback is immediately called with the fetched XML passed as an argument.

function async(method, url, callback) {
    request.open(method, url, true);
    request.onreadystatechange = function() {
       if (request.readyState == 4 && request.status == 200) {
       callback(request.responseXML);
          }
    }
    request.send(null);
}

The callback function will do all the DOM processing based on the XML content from the response. The callback works directly on the XMLDocument JavaScript object.

Putting It All Together
Now that we have all the building blocks of the solution, let's take a look at the logic behind it. Figure 1 separates client side and server side into three separate blocks: Oracle XE, Web page, and browser client. Notice that the Web page has been separated here because of its state-dependent nature - thanks to AJAX it now incorporates the logic to handle user actions and transform them into HTTP requests; the standard Web model assumes that it's the browser that dispatches HTTP requests to the Web server. That is the key concept in the solution. Take a look at Figure 1 to see the whole interaction process.

1.  The browser client sends a direct HTTP request to the Web server (here: the Embedded PL/SQL Gateway of Oracle XE); the URL of the request has been supplied in the location bar or through a link.
2.  Oracle XE serves a static HTML page that includes AJAX scripts. This static page is then rendered by the browser and now all the requests and interaction happen indirectly, as AJAX requests.
3.  When the user triggers an event on the Web page by interacting with page elements such as forms and links, an event handler catches it and sends the appropriate HTTP request through the AJAX engine.
4.  The XMLHttpRequest object (Table 2) requests a remote resource from the Embedded PL/SQL gateway. At this point, the user doesn't have to wait for the script to finish fetching responses from the server; it's handled in an asynchronous manner. The script takes as much time as needed to fetch the response while the user can further interact with the Web page.
5.  The script performs DOM modifications in the background while the user can trigger the next events on the Web page.

You can now upload the attached HTML file (hr-ajax-demo.html) into your XE instance through FTP or WebDAV, which contains the complete code for the sample application. After putting it into the /public/ folder of XE, open http://localhost:8080/public/hr-ajax-demo.html with your Web browser. After carefully following all the steps of this tutorial the demo page should pre-load the list of departments and respond to user interaction almost immediately.

Advantages and Potential Problems
Although programming AJAX isn't necessarily difficult, the question is whether asynchronous querying techniques are safe and production-ready. AJAX isn't always a sure choice over standard development methodologies. There are at least several viewpoints to consider.

Advantages
•  Better user experience
•  Greater responsiveness
•  Smaller server load, increased performance
•  Network applications can be updated on-the-fly without redistributing updates
•  Separation of content from presentation (easier maintenance)

Potential Problems
•  Cross-browser incompatibilities
•  Client-side security concerns

The Real World
Today, AJAX applications are gaining a lot of steam. With Google leading this trend together and Yahoo! and Microsoft trailing, the desktop experience is delivered to users through increasingly dynamic Web pages. Since the early adoption of AJAX it has matured to be a full-blown solution often chosen by enterprises for commercial solutions.

After the unification of DOM implementations and the adoption of World Wide Web Consortium (W3C) standards among developers of Web browsers, it's now possible to create cross-platform Web applications more easily than ever before. With a wide range of available AJAX toolkits, developers can now upgrade Web applications to an asynchronous architecture at a low cost. AJAX is flexible enough to be plugged into existing solutions and greatly enhances usability. The movement of desktop applications to the Web is a matter of time and represents the next step in software evolution.

More Stories By Przemek Piotrowski

Przemek Piotrowski has been following Oracle 10g Express Edition (Oracle XE) development since its early beta stages back in 2005 and can easily distinguish new features of this first database from Oracle, free to develop, deploy and distribute. Oracle XE's small footprint doesn't stop him from taking advantage of many of its advanced features and plugging them into existing Web architectures.

More Stories By Mark Townsend

Mark Townsend is the vice president of database product management in Oracle's Server Technology Division. His responsibilities include requirement analysis, release planning, co-ordination of database product management activities, communication with analysts and press on database topics, and development and delivery of field technical training. He was also the product boss for Oracle XE. Mark has been with Oracle since 1991 and has specialized in the Oracle database for over 15 years.

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