Reuse and Reality: A Journey from Ideal Concept to Real World Implementation		Table of contents		Indexes		The gorge between the X-volution and the real world

A Travel-Related Case Study Using XML

Paul Spencer

Managing Consultant

Boynings Consulting East Boynings
29 Altwood Road
Maidenhead Berkshire SL6 4PB United Kingdom
Phone: +44 (0)1628 687010
Fax: +44 (0)1628 687011
Email: paul.spencer@boynings.co.uk Web: http://www.boynings.co.uk

Biographical notice:

Paul Spencer is the owner and managing consultant of Boynings Consulting. Boynings Consulting operates in two main areas. It advises companies on where and how to incorporate XML (Extensible Markup Language) into their products, reviewing existing product ranges and helping to manage implementations where necessary. It also advises companies on how to use intranets, extranets and the Internet to help manage relationships with staff, customers and suppliers, implementing solutions where appropriate. Many of the solutions developed use XML .

Before starting Boynings Consulting, Paul worked in travel distribution, running development and MIS departments. He has worked with many of the major tour operators, as well as with travel agents, airlines, technology providers and companies outside the travel industry.

Paul is the author of "XML Design and Implementation"

ABSTRACT:

Many applications that traditionally use HTML (Hypertext Markup Language) can be developed more simply and be easier to maintain if XML is used as well. The application described here uses XML as an intermediate format to separate database code from styling. In this case, styling is done on the client using both XSL (Extensible Stylesheet Language) and the DOM (Document Object Model) with scripting. This restricts the browser types that can be used, but the processing could alternatively have been done on the server, giving browser compatibility whilst retaining many of the benefits of XML .

The application also uses XML to provide a common language for transmitting data between systems, using a DTD to provide a level of error checking.

In this paper, I will discuss the use of XML in an application I have called Centaur. This was developed as a demonstration of XML in Internet Explorer 5 and as a case study for a book on implementing systems using XML . The application itself can be accessed (using Internet Explorer 5) on the Wrox Press web site athttp:// webdev.wrox.co.uk/books/2289 .

Centaur

My aim here is not to discuss the application itself, but a brief overview is useful. Centaur forms the middle tier in an environment for booking inclusive tours. By inclusive tours, I mean vacations supplied as complete entities by a single company that bulk books airline seats and accommodation, provides ground transfers between the resort airport and the holiday destination and provides representatives at the resort. In the UK, these are known as package holidays, and are usually booked either directly or through travel agents from descriptions in brochures.

This diagram shows how Centaur fits into an environment for booking such vacations over the Internet. Obviously, there is an opportunity to provide additional services such as travel insurance, car hire, and purchases of luggage, guide books and maps.

electronic brochure

In operation, the tour operators submit brochure-type information to the system, which stores this and allows searching based on several criteria. The user can then see descriptions of the tours and select some to store in an electronic brochure, which can be browsed off-line. It would be simple to allow this brochure to be written to a CD-ROM. Once a tour has been chosen, the client fills in a form to request a quotation. This information is sent to the tour operator system, and the reply is rendered to the browser. A similar mechanism is used for booking.

XML

Centaur comprises two main parts - a search system and a booking system. A fully functional system would have a third part to collect data from the tour operators. XML is used on both the client and the server, to a degree that would not be possible in a commercial system because of the need for browser independence.

XML

database

query

Once the user enters the system, she is presented with a search screen, in which she can enter some requirements for the vacation. This is done in a normal HTML form and results in a search of the local SQL Server database. This is entirely conventional. The database query reports its results as a simple piece of XML such as:

<results>
<result>
<tourid>6</tourid>
<tourOp>Argo</tourOp>
<name>Hotel Miramare</name>
<area>Cyprus</area>
</result>
<result>
<tourid>5</tourid>
<tourOp>Argo</tourOp>
<name>Hotel Mediterranean Beach</name>
<area>Cyprus</area>
</result>
</results>

DOM, Document Object Model

HTML, Hypertext Markup Language

JScript

XML

database

This XML is then manipulated in JScript with the DOM to provide, as HTML , a list of matching tours, any of which can be clicked on the display details of the tour. Traditionally, this would have been done by combining the database extraction and the formatting code. By providing the intermediate XML , we make it simple to modify the screen appearance without modifying the more complex code. In this case, this is of marginal benefit. Once we start to look at the details of the tour, it is of much more value.

Clicking on a tour from the list brings up brochure-style information:

HTML, Hypertext Markup Language

XML

database

As the vertical scroll bar in the diagram shows, there is a lot more description to follow, including a price table. This information is extracted on demand from the database, and formatted as XML . The XML is then rendered using an XSL stylesheet. This has many benefits over combining the database extraction and rendering in a single stage with HTML .

XML

database

look and feel
parallel development

The first is parallel development. Knowing the data I needed, I could specify the XML file structure for someone to develop the five hundred lines of database extraction code. While he was doing this, I could get on and separately develop the look and feel of the page. This separation of tasks is very common, and in the past has led to developers providing a basic look and feel, then graphic designers getting the appearance how they want separately. Since the graphic designers have to work with complex code, this often leads to errors. The total separation of the tasks with XML as an intermediate format not only allows parallel working, but eliminates these errors.

maintainability

Closely related to this is maintainability. If I want to change the appearance of the page (and I did so several times while developing the system), I just change a stylesheet. The complex database code is not touched.

I can also add variety. I could, as the operator of this system, develop different stylesheets for different tour operators. I could even charge for developing a more vibrant style than the standard. I could also vary the style according to other parameters. Perhaps I could use the internal reference of the tour to select one of several standard stylesheets to add variety. All this can be done while maintaining only a single version of the database extraction code - something that is dependent on the use of XML as an intermediate format.

HTML, Hypertext Markup Language

IE5

Internet Explorer 5

XSL

Since I am using XSL , I am totally dependent on the client being Internet Explorer 5. However, it is trivial to regain client independence by changing the code to do the stylesheet processing on the server and send HTML to the client. The only disadvantage in this case is the additional server load. In other applications there could be further disadvantages since delivering XML to the client allows significant client-side processing. Doing this processing on the server can mean delivering large HTML pages when only small changes are required.

JScript

XML

Once a tour has been selected, the user can do one of two things. The first is add it to a brochure. She can then select further tours and add these to the brochure as well. The brochure can be saved as a cookie and restored in a later session. This cookie is an XML string simply because it is simpler to manipulate parsed XML than simple strings in JScript.

The second option is to get a quotation. This could be derived from the price information in the database. However, there are two disadvantages of this. It would not allow availability information to be given with the price and it would not allow the tour operators to manage their inventory by changing prices without constantly updating the Centaur database. For this reason, the price table displayed in the tour details is for guidance only, just as a brochure price is. For a quotation, the information is requested from the tour operator system.

DTD, Document Type Definition

HTTP, Hypertext Transfer Protocol

MSXML

XML

XMLHTTP

schema

It is obviously a benefit to have a standard interface here, and so XML is used. The MSXML parser can be run on the server as well as the client, and includes an XMLHTTP component for sending and receiving XML messages over HTTP. Since we are dealing with systems outside our control, it is useful to have automated error checking, and so the messages reference an external DTD. All information is put in attributes rather than text nodes so that the DTD can do more checking. Using a schema rather than the DTD would provide even better checking, but the decision was made at this stage to stick with the fully standardised DTD, rather than the proposals for schemas.

A typical quotation request would look like:

<?xml version="1.0"?>
<!DOCTYPE centaur SYSTEM "http://www.centaur.co.uk/centaur.dtd">
<centaur>
<quoterequest>
<qttour supplier="argo" ref="1130" depdate="19990801" duration="14" />
<qttravellers adults="2" children="2" infants="1" />
</quoterequest>
</centaur>

The reply to this might be:

<?xml version="1.0"?>
<!DOCTYPE centaur SYSTEM "http://www.centaur.co.uk/centaur.dtd">
<centaur>
	<quoteresponse>
		<qrdetails availability="true" quote="2214" notes=" />
	</quoteresponse>
</centaur>

DOM, Document Object Model

HTML, Hypertext Markup Language

JScript

This information is then inserted into the resulting HTML page using JScript and the DOM .

XSL	The booking process is similar to this. The messages are more complex, making it more worthwhile to use XSL for the display. Again, this is done on the client, but could equally well be done on the server.

XML

As I mentioned at the beginning, this system was built as a demonstrator and as a case study for a book. A third purpose was to gain experience in a significant XML development at our own expense rather than our clients'. For that reason, it was important to review the project and its use of XML and the lessons we learnt.

XML

With the experience we already had, we knew XML was an appropriate technology for the project. However, once we started, we found continually that there were places where XML could make things a little easier than they would otherwise have been. An example is in saving the brochure as XML . Although this uses up additional cookie space, it makes processing that little bit easier. This sort of use is very appropriate in intranet (and possibly extranet) environments where there is control over the client.

XML

relational database

The area we would consider doing differently next time is the data storage. We stored all data in a relational database and converted it to XML on the fly each time the data was requested. In fact, much of this data could have been stored in XML . The only parts that need fully relational storage are those that are used for searching. Since the tour operators would be supplying information in XML format, this would save conversions to and from relational formats.

HTML, Hypertext Markup Language

XML

guidelines

This project is typical of those that would previously have used only HTML for markup. Building the system has shown us the scale of the benefits XML can bring, whether or not browser independence is an issue, and provided us with guidelines for its use.

Acknowledgments

I would like to thank the staff at Wrox Press and the external reviewers who helped with the writing of the book. I would also like to thank Darren Gill, who assisted with the programming of Centaur.

Bibliography

Spencer 99: Spencer, P., XML Design and Implementation, Wrox Press Ltd. 1999


Reuse and Reality: A Journey from Ideal Concept to Real World Implementation		Table of contents		Indexes		The gorge between the X-volution and the real world