Web Services 

• web services
• Web Services Description Language (WSDL)
• Simple Object Access Protocol (SOAP)
• network appliance languages/protocols
• see also:
  • Delphi Programming Main Index
  • WDSL 1.1 specification on Microsoft's website
  • WDSL-powered websites
  • XML & Delphi
  • Web Services and Delphi
  • Client Server middleware tools including LAN's, WAN's, pipes, etc

What are web services?

• web services are functions of a web server object that can be accessed by a web client
• for example, using http, a client application might access a web service function which then sends a response back to the client application

What are the various web service protocols?

• there are a variety of ways in which a web client can query a web server object:
  • CGI scripting
  • ISAPI/NSAPI dll functions
  • active server pages
• unfortunately, in the above there is no standard specification for accessing the functions and for the data structures that are to be sent or returned from the server, thus 2 specifications have been developed to attempt to resolve this:
  • Web Services Description Language (WSDL), and
  • Simple Object Access Protocol (SOAP)

Web Services Description Language WSDL:

• WSDL is an XML format for describing network services as a set of endpoints operating on messages, either document-oriented or procedure-oriented information.
• the operations and messages are described abstractly, then bound to a concrete network protocol and message format to define an endpoint.
• whilst it is extensible to allow communication through other network protocols, it was originally designed to work in conjunction with SOAP
• a WSDL document defines the following:
  • Types - a container for data type definitions using some type system (such as XSD)
  • Message - an abstract, typed definition of the data being communicated
  • Operation - an abstract definition of an action supported by the service
  • Port Type - an abstract set of operations supported by one or more endpoints
  • Binding - a concrete protocol and data type specification for a particular port type
    • in addition to the core services definition framework, v1.1 introduces specific binding extensions for:
      • SOAP 1.1
      • HTTP GET/POST
      • MIME
  • Port - a single endpoint defined as a combination of a binding and a network address
  • Service - a collection of related endpoints
• thus a web server application residing on a web server exposes its own interfaces that describe the operations available and their parameters so that a web client application can run those operations
• Delphi 6's compiler natively embeds WSDL, SOAP and XML functionality

Simple Object Access Protocol (SOAP):

• this is Microsoft's version of an XML interprocess communications and remote procedure call (RPC) mechanism
• it is particularly effective at passing through firewalls (which usually prevent many RPC and interprocess communications protocols from reaching their destinations) because it's piggybacked on top of HTTP, which is available through almost all firewalls. SOAP slips transparently through any firewall that allows web browser access.
• it is an integral part of Microsoft's XML object broker initiative "Universal Plug and Play (UPnP)" which was announced in Jan 1999.
• SOAP promises to level the playing field between COM/DCOM and CORBA by making it easier to translate between the two of them
• SOAP can also use FTP or SMTP as a transport medium
• thus, given a few lines of code, MS IIS or an Apache web server becomes a SOAP ORB, because SOAP relies on 2 existing protocols - HTTP and XML without adding new technology, but just codifies the use of XML as an HTTP payload.
  • the HTTP payload of a SOAP request is simply an XML document that contains the values of the [in] and [in,out] parameters of the method. These values are encoded as child elements of a distinguished call element that shares the method name and namespace URI of the SOAPMethodName HTTP header.
  • thus the HTTP request is as for usual EXCEPT:
    • content type = text/xml
    • additional header: SOAPMethodName: URN
    • the content must have:
      •  the SOAP tags <Envelope> and at end </Envelope>
      • the 1st child element under <Body> must match the SOAPMethodName
  •  
• it competes with XMLTP
• Microsoft provide a SOAP Toolkit for Visual Studio
• Delphi 6's compiler natively embeds WSDL, SOAP and XML functionality

Network appliance languages/protocols:

• this refers to a protocol whereby one or more control processes collect and retain information about all devices available on a network based on a series of messages multicast to the network as a whole by each device (ie. advertising and discovery mechanism)
• it seems that this would only be of practical use within a private LAN rather than a WAN
• several competing protocols are being developed:
  • Microsoft's Universal Plug and Play (UPnP)
  • HP's Appliance Plug and Play (APnP) which integrates UPnP with HP's Chai Java appliance technology
  • Sun's JVM or Jini
  •