I still remember the day when I wrote my first DCOM (Distributed COM) sample application, few years ago.
I was really thrilled to see how simple it was to write an distributed application.
The code was straightforward, but configuration needed some work (using DCOMCNFG.exe). Also, this DCOM
application was not firewall friendly. Not just DCOM, but other Distributed Computing specifications (like CORBA, RMI, etc.)
also have issues and complexities when firewalls are involved. What we need is really simple, solid protocol to
facilitate distributed computing. It should be easy for the client to make calls to objects running on the server (a different machine,
possibly on the internet and across the firewall boundaries) and get the results. Enter SOAP! A W3C specification
based on two solid proven technologies (XML and HTTP).
SOAP, or Simple Object Access Protocol is an XML-based object invocation protocol. SOAP was originally developed for distributed applications to communicate over HTTP and through corporate firewalls. SOAP defines the use of XML and HTTP to access services, objects and servers in a platform-independent manner.
In this article, we'll learn how easy it is to write SOAP Web service and a client, that can communicate
over internet without any firewall issues. Our server or Web service is an ASP Page and client is an Visual Basic 6.0 application.
- About Sample Application
Our client is an Visual Basic 6.0 Application, with a single form having two edit boxes to accept username
and password. Here is how Visual Basic client looks like:
When clicked on Connect button, a method on the server (which is an ASP Page) is called which in turn returns
result indicating successful or incorrect login, depending on input parameters with the method call. Visual Basic application gets the results and just pops up a message box
indicating login success or failure.
To call a method on the server, we build a XML packet (document/fragment) and post it to the server. The
server (ASP Page) processes the input request, generates the result, again as an XML document and sends back
to client as response. Please note that the username and password are hard-coded in the ASP page server
code as PerfectXML and XML respectively (case-sensitive).
- The Client
The Visual Basic client accepts the username and password and calls DoLogin method on the server,
passing username & password as method parameters. With SOAP, method call and input parameters needs to be
XML-encoded, as below:
According to SOAP specification, the above method call information needs to be enclosed in what is called as SOAP envelop, which is finally
sent/posted to the server. The final SOAP request envelop is shown below:
The Visual Basic client application simply builds the above method call SOAP envelop (as string)
and now in order to post this to the server, we use XMLHTTPRequest object from MSXML.
Here is the part of code which builds SOAP request envelop and sends it to the server using XMLHTTPRequest:
As mentioned earlier, the server for our sample application is an ASP Page. This ASP page
is named simplesoap.asp and should be present in a virtual directory named soap. (You can keep the ASP
page wherever you wish, just change the URL in the above client code.).
Dim objXMLHTTP As New MSXML.XMLHTTPRequest
Dim objOutputXMLDoc As New MSXML.DOMDocument
Dim strMethodPkg As String
Dim strMethodResultXML As String
Dim iLoginResult As Integer
strMethodPkg = "<SOAP:Envelope xmlns:SOAP=""http://schemas.xmlsoap.org/soap/envelope/"">"
strMethodPkg = strMethodPkg & "<SOAP:Header></SOAP:Header>"
strMethodPkg = strMethodPkg & "<SOAP:Body><m:DoLogin xmlns:m=""urn:soapserver/soap:AuthorizationModule"">"
strMethodPkg = strMethodPkg & "<UserName>" & txtUserName.Text & "</UserName>"
strMethodPkg = strMethodPkg & "<Password>" & txtPassword.Text & "</Password>"
strMethodPkg = strMethodPkg & "</m:DoLogin></SOAP:Body></SOAP:Envelope>"
objXMLHTTP.open "post", "http://127.0.0.1/soap/simplesoap.asp", False
objXMLHTTP.setRequestHeader "Content-Type", "text/xml"
objXMLHTTP.setRequestHeader "SOAPAction", "soapserver/soap:AuthorizationModule#DoLogin"
Now let's have a look at our server application, simplesoap.asp.
- The Server
The server ASP page is very simple to understand - it uses MSXML DOM to get the username and password
values from the input request XML document, maches those values, builds the result XML document/envelop
and sends the response back to the client.
Set objXMLDOM = Server.CreateObject("Microsoft.XMLDOM")
varUserName = objXMLDOM.selectSingleNode("SOAP:Envelope/SOAP:Body/m:DoLogin/UserName").Text
varPassword = objXMLDOM.selectSingleNode("SOAP:Envelope/SOAP:Body/m:DoLogin/Password").Text
if varUserName = "PerfectXML" and varPassword = "XML" then
strResult = 1
strResult = -1
strResultXML = "<SOAP:Envelope xmlns:SOAP=""http://schemas.xmlsoap.org/soap/envelope/""><SOAP:Header></SOAP:Header>"
strResultXML = strResultXML & "<SOAP:Body><m:DoLoginResponse xmlns:m=""urn:soapserver/soap:AuthorizationModule"">"
strResultXML = strResultXML & "<LoginResult>" & strResult & "</LoginResult></m:DoLoginResponse>"
strResultXML = strResultXML & "</SOAP:Body></SOAP:Envelope>"
The client receives the response. And since response from ASP page (method call) is again an XML document, the VB client app, loads the document
and queries for result node's value. Depending on the result value, it then pops up either "login successful" or "login
failure" message box. Here is rest of VB client code:
strMethodResultXML = objXMLHTTP.responseText
iLoginResult = objOutputXMLDoc.selectSingleNode("SOAP:Envelope/SOAP:Body/m:DoLoginResponse/LoginResult").Text
If iLoginResult > 0 Then
MsgBox "Login Successful"
MsgBox "Login Failed!"
SOAP really is a way to build distributed application which are simple, extensible, and platform-independent.
In this small article, we learned how SOAP works using a Visual Basic client application connecting to ASP page,
acting as a server.
If you would like to learn more about SOAP or have any question about SOAP, check out
perfectxml.com's exclusive section dedicated to SOAP. We'll continue to
grow the SOAP resource center on perfectxml.com. If you have any ideas/suggestions/comments, please direct them to
DCOM or Distributed COM is Microsoft's distributed computing protocol and is built on top of DCE RPC, an RPC protocol developed by the Distributed Computing Environment consortium and adopted and extended slightly by Microsoft to support communication between COM objects across machines. One of the main design goals of DCOM is location transparency - a client program invoking a remote procedure should use the exact same syntax used for local procedure calls.
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