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XML Web Services


Web services are web application components.

Web services can be published, found, and used on the Web.

This tutorial introduces WSDL, SOAP, RDF, and RSS.


WSDL

  • WSDL stands for Web Services Description Language
  • WSDL is an XML-based language for describing Web services.
  • WSDL is a W3C recommendation

SOAP

  • SOAP stands for Simple Object Access Protocol
  • SOAP is an XML based protocol for accessing Web Services.
  • SOAP is based on XML
  • SOAP is a W3C recommendation

RDF

  • RDF stands for Resource Description Framework
  • RDF is a framework for describing resources on the web
  • RDF is written in XML
  • RDF is a W3C Recommendation

RSS

  • RSS stands for Really Simple Syndication
  • RSS allows you to syndicate your site content
  • RSS defines an easy way to share and view headlines and content
  • RSS files can be automatically updated
  • RSS allows personalized views for different sites
  • RSS is written in XML

What You Should Already Know

Before you study web services you should have a basic understanding of XML and XML Namespaces.

If you want to study these subjects first, please read our XML Tutorial.


Web Services

  • Web services are application components
  • Web services communicate using open protocols
  • Web services are self-contained and self-describing
  • Web services can be discovered using UDDI
  • Web services can be used by other applications
  • HTTP and XML is the basis for Web services

Interoperability has Highest Priority

When all major platforms could access the Web using Web browsers, different platforms couldn't interact. For these platforms to work together, Web-applications were developed.

Web-applications are simply applications that run on the web. These are built around the Web browser standards and can be used by any browser on any platform.


Web Services take Web-applications to the Next Level

By using Web services, your application can publish its function or message to the rest of the world.

Web services use XML to code and to decode data, and SOAP to transport it (using open protocols).

With Web services, your accounting department's Win 2k server's billing system can connect with your IT supplier's UNIX server.


Web Services have Two Types of Uses

Reusable application-components.

There are things applications need very often. So why make these over and over again?

Web services can offer application-components like: currency conversion, weather reports, or even language translation as services.

Connect existing software.

Web services can help to solve the interoperability problem by giving different applications a way to link their data.

With Web services you can exchange data between different applications and different platforms.

Any application can have a Web Service component.

Web Services can be created regardless of programming language.


A Web Service Example

In the following example we will use ASP.NET to create a simple Web Service that converts the temperature from Fahrenheit to Celsius, and vice versa:

<%@ WebService Language="VBScript" Class="TempConvert" %>

Imports System
Imports System.Web.Services

Public Class TempConvert :Inherits WebService

<WebMethod()> Public Function FahrenheitToCelsius
(ByVal Fahrenheit As String) As String
  dim fahr
  fahr=trim(replace(Fahrenheit,",","."))
  if fahr="" or IsNumeric(fahr)=false then return "Error"
  return ((((fahr) - 32) / 9) * 5)
end function

<WebMethod()> Public Function CelsiusToFahrenheit
(ByVal Celsius As String) As String
  dim cel
  cel=trim(replace(Celsius,",","."))
  if cel="" or IsNumeric(cel)=false then return "Error"
  return ((((cel) * 9) / 5) + 32)
end function

end class

This document is saved as an .asmx file. This is the ASP.NET file extension for XML Web Services.


Example Explained

Note: To run this example, you will need a .NET server.

The first line in the example states that this is a Web Service, written in VBScript, and has the class name "TempConvert":

<%@ WebService Language="VBScript" Class="TempConvert" %>

The next lines import the namespace "System.Web.Services" from the .NET framework:

Imports System
Imports System.Web.Services

The next line defines that the "TempConvert" class is a WebService class type:

Public Class TempConvert :Inherits WebService

The next steps are basic VB programming. This application has two functions. One to convert from Fahrenheit to Celsius, and one to convert from Celsius to Fahrenheit.

The only difference from a normal application is that this function is defined as a "WebMethod()".

Use "WebMethod()" to convert the functions in your application into web services:

<WebMethod()> Public Function FahrenheitToCelsius
(ByVal Fahrenheit As String) As String
  dim fahr
  fahr=trim(replace(Fahrenheit,",","."))
  if fahr="" or IsNumeric(fahr)=false then return "Error"
  return ((((fahr) - 32) / 9) * 5)
end function

<WebMethod()> Public Function CelsiusToFahrenheit
(ByVal Celsius As String) As String
  dim cel
  cel=trim(replace(Celsius,",","."))
  if cel="" or IsNumeric(cel)=false then return "Error"
  return ((((cel) * 9) / 5) + 32)
end function

Then, end the class:

end class

Publish the .asmx file on a server with .NET support, and you will have your first working Web Service.

Look at our example Web Service


ASP.NET Automates the Process

With ASP.NET, you do not have to write your own WSDL and SOAP documents.

If you look closer at our example Web Service, you will see that ASP.NET has automatically created a WSDL and SOAP request.

Using the Web Service Example

In the previous page we created a Web service.

The FahrenheitToCelsius() function can be tested here: FahrenheitToCelsius

The CelsiusToFahrenheit() function can be tested here: CelsiusToFahrenheit

These functions will send an XML response like this:

<?xml version="1.0" encoding="utf-8" ?>
<string xmlns="http://tempuri.org/">38</string>

Put the Web Service on Your Web Site

Using a form and the HTTP POST method, you can put the web service on your site, like this:

Fahrenheit to Celsius:

Celsius to Fahrenheit:

How To Do It

Here is the code to add the Web Service to a web page:

<form action='tempconvert.asmx/FahrenheitToCelsius'
method="post" target="_blank">
<table>
  <tr>
    <td>Fahrenheit to Celsius:</td>
    <td>
    <input class="frmInput" type="text" size="30" name="Fahrenheit">
    </td>
  </tr>
  <tr>
    <td></td>
    <td align="right">
     <input type="submit" value="Submit" class="button">
     </td>
  </tr>
</table>
</form>

<form action='tempconvert.asmx/CelsiusToFahrenheit'
method="post" target="_blank">
<table>
  <tr>
    <td>Celsius to Fahrenheit:</td>
    <td>
    <input class="frmInput" type="text" size="30" name="Celsius">
    </td>
  </tr>
  <tr>
    <td></td>
    <td align="right">
    <input type="submit" value="Submit" class="button">
    </td>
  </tr>
</table>
</form>

Substitute the "tempconvert.asmx" with the address of your web service like:

http://www.example.com/xml/tempconvert.asmx