Visual Basic.NET How to Program (PDF)

1036 ASP .NET and Web Services Chapter 21
Testing and Debugging Tip 21.1
Using the ASMX page of a Web service to test and debug methods makes that Web service
more reliable and robust; it also reduces the likelihood that others who use the Web service
will encounter errors. 21.1
21.3 Simple Object Access Protocol (SOAP) and Web Services
Simple Object Access Protocol (SOAP) is a platform-independent protocol that uses XML to
make remote procedure calls over HTTP. Each call and response is packaged in a SOAP
message—an XML message containing all the information necessary to process its contents.
SOAP messages are quite popular, because they are written in the easy-to-understand
and platform-independent XML. Similarly, HTTP was chosen to transmit SOAP messages
because HTTP is a standard protocol for sending information across the Internet. The use
of XML and HTTP enables different operating systems to send and receive SOAP messages.
Another benefit of HTTP is that it can be used with networks that contain firewalls—
security barriers that restrict communication among networks.
Another reason that programmers creating Web services use SOAP is its extensive
set of supported data types. Readers should note that the wire format used to transmit
requests and responses must support all data types passed between the applications. Web
services that use SOAP support a wider variety of data types than do Web services that
employ other wire formats. The data types supported by SOAP include the basic data
types, DataSet, DateTime, XmlNode and several others. SOAP also permits transmission
of arrays of all these types. In addition, user-defined types can be used; we demonstrate
how to do this in Section 21.8.
ASP .NET Web services send requests and responses to and from Web services via
SOAP. When a program invokes a Web-service method, the request and all relevant information
is packaged in a SOAP request message and sent to the appropriate destination.
When the Web service receives this SOAP message, it begins to process the contents called
the SOAP envelope, which specifies the method that the client wishes to execute and the
arguments the client is passing to that method. After the Web service receives this request
and parses it, the proper method is called with the specified arguments (if there are any),
and the response is sent back to the client in a SOAP response message. The client parses
the response to retrieve the result of the method call.
The SOAP response message portrayed in Fig. 21.5 was taken directly from the
Bigger method of the HugeInteger Web service (Fig. 21.3). This Web service provides
programmers with several methods that manipulate integers larger than those that can
be stored as a Long variable. Most programmers do not manipulate SOAP messages,
allowing the Web service to handle the details of transmission.
1 POST /HugeInteger/HugeInteger.asmx HTTP/1.1
2 Host: localhost
3 Content-Type: text/xml; charset=utf-8
4 Content-Length: length
5 SOAPAction: "http://www.deitel.com/Bigger"
6
Fig. 21.5 SOAP request message for the HugeInteger Web service (part 1 of 2).