An Introduction To XML SOAP Using ASP and VB6 - Step By Step (Page 2 of 5 )
As we know, SOAP is a call-response mechanism, which operates in a client-server paradigm. The client (your application) makes a call to the server (a web server, somewhere on the internet), passing in parameters; and the server provides a response. Both call and response are transported in the form of XML documents. Therefore, to build our own example SOAP system, we need both a client and a server – a caller and a responder.
The example we will follow is a very simple one: we will build a service that will calculate the tax due on a sales transaction. In traditional VB terms well will create a function with the following definition:
Public Function GetSalesTax(ByVal pSalesTotal As Double) as Double GetSalesTax = pSalesTotal * 0.04 End Function
Crude, but effective (if you live in a state where the sales tax rate is 4%!)
This function defines a method name (GetSalesTax), a parameter (pSalesTotal – representing the total value of the sale) and a return value (the result of the function). In more traditional object-oriented terms, we might think of pSalesTotal as an "IN" parameter, and the GetSalesTax result as an "OUT" parameter. Quite straightforwardly: sales-total in; sales-tax out. A SOAP service can be thought of in terms of "IN" and "OUT" parameters – the client passes "IN" parameters to the server, and then receives the "OUT" parameters in return. So, in order to create our SOAP service, we need a server that will listen for requests to GetSalesTax, accompanied by "IN" parameters, and which will respond with "OUT" parameters, indicating the correct tax amount. Let's start with the client – the caller. How do we call the SOAP service to make our request.
The Client In traditional VB terms, our request to the GetSalesTax function described above, would be something such as:
dblSalesTax = GetSalesTax(100)
Returning a value of $4.
If the GetSalesTax function was contained within an external object, such as an MTS server, then the call would need to reference the server DLL:
Dim objTax As New CTaxCalc
dblSalesTax = objTax.GetSalesTax(100
In a SOAP system the call is little different, only the request is formatted as an XML document, which is passed up to the server. The appropriate document contains the same details as the MTS call –a method to call, and the parameter name and value.
In order to ensure that the server can correctly identify and decrypt this method call, it is wrapped up in a larger document, called a SOAP envelope, which references the universal name-space of the SOAP envelope standard.
Finally, to complete our SOAP request document, we will add a name-space reference to our method call, which points to the object which contains the method – the equivalent of the object declaration (Dim objTax As New CTaxCalc) in our MTS method call.
Now that we have built our SOAP request document, we are ready to send it to the server. The request is a simple HTTP post - just like posting a web form in your internet browser. Of course, your internet browser masks all the complexity of sending a form to a server; and in the longer-term .NET will mask this process from your VB code too. But for now, we need to do the job ourselves; and I have been using Microsoft's XML HTTP Request object to give me a helping hand. (The XMLHTTPRequest is an object within the MSXML class library (MSXML.DLL), and it comes with IE5.) Assuming that strEnvelope contains the XML document described above, the request is formatted thus:
Dim objHTTP As New MSXML.XMLHTTPRequest Dim strEnvelope As String
'Set up to post to our localhost server objHTTP.open "post", "http://localhost/soap/soap.asp"
'Set a standard SOAP/ XML header for the content-type objHTTP.setRequestHeader "Content-Type", "text/xml"
'Set a header for the method to be called objHTTP.setRequestHeader "SOAPMethodName", _ "urn:myserver/soap:TaxCalc#GetSalesTax"
'Make the SOAP call objHTTP.send strEnvelope
'Get the return value strReturn = objHTTP.responseBody
Now that we have sent our request, we move to the server, to see how we set up a SOAP service to listen and respond to our calls.