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scalable and robust. If, on the other hand, you’re familiar with web-tier tools such as template engines and Object-Relational Mapping (ORM) tools or with frameworks such as Struts, Spring, or Tapestry, you’ll probably already know most of what we’re going to say here. In this case, you might like to skim over this section and pick up the MVC trail in chapter 4, where we discuss its use in a very different way.

3.4 Web server MVC

Web applications are no stranger to MVC, even the classic page-based variety that we spend so much time bashing in this book! The very nature of a web application enforces some degree of separation between the View and the Model, because they are on different machines. Does a web application inherently follow the MVC pattern then? Or, put another way, is it possible to write a web application that tangles the View and the Model together?

Unfortunately, it is. It’s very easy, and most web developers have probably done it at some point, the authors included.

Most proponents of MVC on the Web treat the generated HTML page, and the code that generates it, as the View, rather than what the user actually sees when that page renders. In the case of an Ajax application serving data to a JavaScript client, the View from this perspective is the XML document being returned to the client in the HTTP response. Separating the generated document from the business logic does require a little discipline, then.

3.4.1The Ajax web server tier without patterns

To illustrate our discussion, let’s develop an example web server tier for an Ajax application. We’ve already seen the fundamentals of the client-side Ajax code in chapter 2 and section 3.1.4, and we’ll return to them in chapter 4. Right now, we’ll concentrate on what goes on in the web server. We’ll begin by coding it in the simplest way possible and gradually refactor toward the MVC pattern to see how it benefits our application in terms of its ability to respond to change. First, let’s introduce the application.

We have a list of clothes in a clothing store, which are stored in a database, and we want to query this database and present the list of items to the user, showing an image, a title, a short description, and a price. Where the item is available in several colors or sizes, we want to provide a picker for that, too. Figure 3.6 shows the main components of this system, namely the database, a data structure representing a single product, and an XML document to be transmitted to our Ajax client, listing all the products that match a query.

Iterate through resultset

Tell client we are returning XML

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ORM			Template engine		Client-side parser

<?xml

version="1.0"?>

1. Database tables		4. Web browser
		3. XML stream
	2. Object model

Figure 3.6 Main components used to generate an XML feed of product data in our online shop example. In the process of generating the view, we extract a set of results from the database, use it to populate data structures representing individual garments, and then transmit that data to the client as an XML stream.

Let’s say that the user has just entered the store and is offered a choice between Menswear, Womenswear, and Children’s clothing. Each product is assigned to one of these categories by the Category column of the database table named Garments. A simple piece of SQL to retrieve all relevant items for a search under Menswear might be

SELECT * FROM garments WHERE CATEGORY = 'Menswear';

We need to fetch the results of this query and then send them to the Ajax application as XML. Let’s see how we can do that.

Generating XML data for the client

Listing 3.5 shows a quick-and-dirty solution to this particular requirement. This example uses PHP with a MySQL database, but the important thing to note is the general structure. An ASP or JSP page, or a Ruby script, could be constructed similarly.

Listing 3.5 Quick-and-dirty generation of an XML stream from a database query

<?php

header("Content-type: application/xml");

echo "<?xml version=\"1.0\" encoding=\"UTF-8\" ?>\n"; $db=mysql_connect("my_db_server","mysql_user"); mysql_select_db("mydb",$db);

$sql="SELECT id,title,description,price,colors,sizes"

."FROM garments WHERE category=\"{$cat}\""; $result=mysql_query($sql,$db);

echo "<garments>\n";

while ($myrow = mysql_fetch_row($result)) { printf("<garment id=\"%s\" title=\"%s\">\n"

Fetch the results from the database

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."<description>%s</description>\n<price>%s</price>\n", $myrow["id"],

$myrow["title"], $myrow["description"], $myrow["price"]);

if (!is_null($myrow["colors"])){

echo "<colors>{$myrow['colors']}</colors>\n";

}

if (!is_null($myrow["sizes"])){

echo "<sizes>{$myrow['sizes']}</sizes>\n";

}

echo "</garment>\n";

}

echo "</garments>\n"; ?>

The PHP page in listing 3.5 will generate an XML page for us, looking something like listing 3.6, in the case where we have two matching products in our database. Indentation has been added for readability. We’ve chosen XML as the communication medium between client and server because it is commonly used for this purpose and because we saw in chapter 2 how to consume an XML document generated by the server using the XMLHttpRequest object. In chapter 5, we’ll explore the various other options in more detail.

Listing 3.6 Sample XML output from listing 3.5

<garments>

<garment id="SCK001" title="Golfers' Socks"> <description>Garish diamond patterned socks. Real wool.

Real itchy.</description> <price>$5.99</price>

<colors>heather combo,hawaiian medley,wild turkey</colors> </garment>

<garment id="HAT056" title="Deerstalker Cap"> <description>Complete with big flappy bits.

As worn by the great detective Sherlock Holmes. Pipe is model's own.</description> <price>$79.99</price>

<sizes>S, M, L, XL, egghead</sizes> </garment>

</garments>

So, we have a web server application of sorts, assuming that there’s a nice Ajax front end to consume our XML. Let’s look to the future. Suppose that as our product range expands, we want to add subcategories (Smart, Casual, Outdoor, for

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example) and also a “search by season” function, maybe keyword searching, and a link to clearance items. All of these features could reasonably be served by a similar XML stream. Let’s look at how we might reuse our current code for these purposes and what the barriers might be.

Problems with reusability

There are several barriers to reusing our script as it stands. First, we have hardwired the SQL query into the page. If we wanted to search again by category or keyword, we would need to modify the SQL generation. We could end up with an ugly set of if statements accumulating over time as we add more search options, and a growing list of optional search parameters.

There is an even worse alternative: simply accepting a free-form WHERE clause in the CGI parameters, that is,

$sql="SELECT id,title,description,price,colors,sizes"

."FROM garments WHERE ".$sqlWhere;

which we can then call directly from the URL, for example:

garments.php?sqlWhere=CATEGORY="Menswear"

This solution confuses the Model and the View even further, exposing raw SQL in the presentation code. It also opens the door to malicious SQL injection attacks, and, although modern versions of PHP have some built-in defenses against these, it’s foolish to rely on them.

Second, we’ve hardwired the XML data format into the page—it’s been buried in there among the printf and echo statements somewhere. There are several reasons why we might want to change the data format. Maybe we want to show an original price alongside the sale price, to try to persuade some poor sap to buy all those itchy golfing socks that we ordered!

Third, the database result set itself is used to generate the XML. This may look like an efficient way to do things initially, but it has two potential problems. We’re keeping a database connection open all the time that we are generating the XML. In this case, we’re not doing anything very difficult during that while() loop, so the connection won’t be too lengthy, but eventually it may prove to be a bottleneck. Also, it works only if we treat our database as a flat data structure.

3.4.2Refactoring the domain model

We’re handling our lists of colors and sizes in a fairly inefficient manner at present, by storing comma-separated lists in fields in the Garments table. If we normalize our data in keeping with a good relational model, we ought to have a

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	Garments										Colors
	003	deerstalker									175		shocking pink
	004	fez									176		battleship gray
					Garments_to_Colors
	005	beret									177		lemon
						003	175
											178		blueberry
						003	178
						009	178
						017	183

Figure 3.7 A many-to-many relationship in a database model. The table Colors lists all available colors for all garments, and the table Garments no longer lists any color information.

separate table of all available colors, and a bridging table linking garments to colors (what the database wonks call a many-to-many relationship). Figure 3.7 illustrates the use of a many-to-many relationship of this sort.

To determine the available colors for our deerstalker hat, we look up the Garments_to_Colors table on the foreign key garment_id. Relating the color_id column back to the primary key in the Colors table, we can see that the hat is available in shocking pink and blueberry but not battleship gray. By running the query in reverse, we could also use the Garments_to_Colors table to list all garments that match a given color.

We’re making better use of our database now, but the SQL required to fetch all the information begins to get a little hairy. Rather than having to construct elaborate join queries by hand, it would be nice to be able to treat our garments as objects, containing an array of colors and sizes.

Object-relational Mapping tools

Fortunately, there are tools and libraries that can do that for us, known as ObjectRelational Mapping (ORM) tools. An ORM automatically translates between database data and in-memory objects, taking the burden of writing raw SQL off the developer. PHP programmers might like to take a look at PEAR DB_DataObject, Easy PHP Data Objects (EZPDO), or Metastorage. Java developers are relatively spoiled for choice, with Hibernate (also ported to .NET) currently a popular choice. ORM tools are a big topic, one that we’ll have to put aside for now.

Looking at our application in MVC terms, we can see that adopting an ORM has had a happy side effect, in that we have the beginnings of a genuine Model on

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our hands. We now can write our XML-generator routine to talk to the Garment object and leave the ORM to mess around with the database. We’re no longer bound to a particular database’s API (or its quirks). Listing 3.7 shows the change in our code after switching to an ORM.

In this case, we define the business objects (that is, the Model) for our store example in PHP, using the Pear::DB_DataObject, which requires our classes to extend a base DB_DataObject class. Different ORMs do it differently, but the point is that we’re creating a set of objects that we can talk to like regular code, abstracting away the complexities of SQL statements.

Listing 3.7 Object model for our garment store

require_once "DB/DataObject.php";

class	GarmentColor extends DB_DataObject {
var	$id;
var	$garment_id;
var	$color_id;
}
class	Color extends DB_DataObject {
var	$id;
var	$name;
}
class	Garment extends DB_DataObject {
var	$id;
var	$title;
var	$description;
var	$price;
var	$colors;
var	$category;

function getColors(){

if (!isset($this->colors)){ $linkObject=new GarmentColor(); $linkObject->garment_id = $this->id; $linkObject->find(); $colors=array();

while ($linkObject->fetch()){ $colorObject=new Color(); $colorObject->id=$linkObject->color_id; $colorObject->find();

while ($colorObject->fetch()){ $colors[] = clone($colorObject);

}

}

}

return $colors;

}

}

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As well as the central Garment object, we’ve defined a Color object and a method of the Garment for fetching all Colors that it is available in. Sizes could be implemented similarly but are omitted here for brevity. Because this library doesn’t directly support many-to-many relationships, we need to define an object type for the link table and iterate through these in the getColors() method. Nonetheless, it represents a fairly complete and readable object model. Let’s see how to make use of that model in our page.

Using the revised model

We’ve generated a data model from our cleaner database structure. Now we need to use it inside our PHP script. Listing 3.8 revises our main page to use the ORM- based objects.

Listing 3.8 Revised page using ORM to talk to the database

<?php

header("Content-type: application/xml");

echo "<?xml version=\"1.0\" encoding=\"UTF-8\" ?>\n"; include "garment_business_objects.inc"

$garment=new Garment; $garment->category = $_GET["cat"]; $number_of_rows = $garment->find(); echo "<garments>\n";

while ($garment->fetch()) {

printf("<garment id=\"%s\" title=\"%s\">\n"

."<description>%s</description>\n<price>%s</price>\n", $garment->id,

$garment->title, $garment->description, $garment->price);

$colors=$garment->getColors(); if (count($colors)>0){

echo "<colors>\n"; for($i=0;$i<count($colors);$i++){

echo "<color>{$colors[$i]}</color>\n";

}

echo "</colors>\n";

}

echo "</garment>\n";

}

echo "</garments>\n"; ?>

We include the object model definitions and then talk in terms of the object model. Rather than constructing some ad hoc SQL, we create an empty Garment