Web Services

4.4.1. What is a Web Service?

A Web Service is a web server providing one or more methods that can be invoked by some external site.
In the .NET Framework, the code of each method can be written in any .NET language.
The external site can pass arguments to the method, and the method returns a result to the external site.
Web Services are an alternative to CORBA and Java's RMI.

The most flexible way is for the external site to send its message using HTTP POST where the body of the HTTP POST is in XML coded using SOAP:

POST /barry.cornelius/moving/WebServerConvert/Service1.asmx HTTP/1.1
Host: localhost
Content-Type: text/xml; charset=utf-8
Content-Length: XXXX
SOAPAction: "http://www.dur.ac.uk/barry.cornelius/webservices/ToFahrenheit"

<?xml version="1.0" encoding="utf-8"?>
<soap:Envelope xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/">
  <soap:Body>
    <ToFahrenheit xmlns="http://www.dur.ac.uk/barry.cornelius/webservices/">
      <pCentigrade>0</pCentigrade>
    </ToFahrenheit>
  </soap:Body>
</soap:Envelope>

The response is also sent in XML coded using SOAP:

HTTP/1.1 200 OK
Content-Type: text/xml; charset=utf-8
Content-Length: YYYY

<?xml version="1.0" encoding="utf-8"?>
<soap:Envelope xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/">
  <soap:Body>
    <ToFahrenheitResponse xmlns="http://www.dur.ac.uk/barry.cornelius/webservices/">
      <ToFahrenheitResult>32</ToFahrenheitResult>
    </ToFahrenheitResponse>
  </soap:Body>
</soap:Envelope>

4.4.2. Providing a Web Service

Visual Studio.NET has a wizard that generates the files necessary to provide a Web Service.

All you need do is to provide the code of each method flagging each method with a WebMethod attribute:

[WebService]
public class Service1 : System.Web.Services.WebService
{
    ...
    [WebMethod]
    public double ToFahrenheit(double pCentigrade)
    {
        return 32 + pCentigrade*9/5;
    }
}

An external site can now invoke the ToFahrenheit method by sending a request, e.g., using the SOAP given earlier.
Note that you do NOT write the code:
- to decode the incoming HTTP requests,
- to generate the XML of the reply.
All of this is done elsewhere.

4.4.3. Accessing a Web Service

Suppose we now want to access the ToFahrenheit method.
Visual Studio.NET has a wizard that generates a proxy class.
You just supply the wizard with the URL of the Web Service.
The wizard queries the Web Service to discover the methods that it provides together with their signatures.

The wizard automatically generates the code of a class that has similar methods:

public class Service1 : System.Web.Services.Protocols.SoapHttpClientProtocol
{
    ...
    public double ToFahrenheit(double pCentigrade)
    {
        object[] results = this.Invoke("ToFahrenheit", new object[]{pCentigrade});
        return (double) results[0];
    }
    ...
}

So the client of the Web Service just calls the method of the proxy class:

Proxy.Service1 tService1 = new Proxy.Service1();
double tFahrenheit = tService1.ToFahrenheit(tCentigrade);

When a method of the proxy class is called, the code of the method:
- sends the appropriate SOAP request to the Web Service;
- decodes the XML that is returned by the Web Service;
- and returns an appropriate value to the caller of the method.
Note that you do NOT write the code:
- to generate the HTTP request (and its XML),
- to parse the XML of the reply.
All of this is done by the code of the proxy class.
If you have not got Visual Studio.NET, you can instead generate a proxy class with a tool, called wsdl.exe, that is part of the .NET Framework (which is free).