So what is a web service platform? The basic platform is the XML plus HTTP. HTTP is the ubiquitous protocol, which is running practically everywhere on a Internet. The XML provides the metalanguage in which you can write the specialized languages to express the complex interactions between the clients and the services or between the components of a composite service. Behind facade of the web server, an XML message gets converted to the middleware request and results converted back to the XML.

A Web needs to be augmented with few other platform services, which can maintain the ubiquity and the simplicity of Web, to constitute more functional platform. A full-function web services platform can be thought of as the XML plus HTTP plus SOAP plus WSDL plus the UDDI. At the higher levels, one may also add the technologies such as the XAML, XLANG, XKMS, and the XFS -- services which are not universally been accepted as the mandatory.

Below is the brief description of platform elements. It should be noted that while the vendors try to present a emergent web services platform as coherent, it is really the series of an in-development technologies. Often at higher levels there are, and may also remain, multiple approaches to same problem.

SOAP is the protocol specification that defines the uniform way of passing the XML-encoded data. It also defines the way to perform the remote procedure calls (RPCs) using the HTTP as a underlying communication protocol.

SOAP arises from a realization that no matter how nifty a current middleware offerings are, they need the WAN wrapper. Architecturally, sending the messages as a plain XML has advantages in terms of ensuring the interoperability (and debugging, as I can do well attest). A middleware players seem willing to put up with costs of parsing and serializing the XML in order to scale their approach to wider the networks.

The UDDI do provides the mechanism for clients to dynamically find the other web services. Using the UDDI interface, a businesses can dynamically connect to the services provided by the external business partners. The UDDI registry is similar to the CORBA trader, or it can be thought of as the DNS service for the business applications. The UDDI registry has two kinds of the clients: businesses which want to publish the service (and the usage interfaces), and the clients who want to obtain the services of certain kind and bind programmatically to them. The table below is the overview of what an UDDI provides. The UDDI is layered over the SOAP and do assumes that the requests and the responses are UDDI objects sent around as the SOAP messages. A sample query is been included below.

WSDL defines the services as the collection of the network endpoints or the ports. In a WSDL the abstract definition of the endpoints and the messages is separated from their concrete network deployment or the data format bindings. This allows reuse of the abstract definitions of the messages, which are the abstract descriptions of a data being exchanged, and the port types, which are the abstract collections of the operations. The concrete protocol and the data format specifications for the particular port type constitute the reusable binding. The port is defined by associating the network address with the reusable binding; the collection of ports define the service. And, thus, the WSDL document do use the following elements in a definition of the network services:

A traditional notion of the database transaction is atomic, that is a, all-or-none action; either an entire action happens or it does not. Providing this kind of guarantee in the distributed infrastructure involves the expensive process called a two-phase commit. An alternative optimistic model has been proposed in a database research (originally called as sagas and proposed by the Hector Garcia-Molina), where the actions have the explicit compensatory actions which negate effect of the action. In a real world of the actions, the existence of the compensatory actions is quite common. For an instance if I debit the credit card $52, a compensatory action is to credit a credit card $52. If I sent out the e-mail saying you will get the product you hav ordered in seven days", compensatory action is to send the e-mail saying, "oops, it is going to take longer". XLang is the notation for expressing an compensatory actions for any request that do need to be undone. An web services infrastructure can leverage the XLang specifications to perform a complex undo operations.

An XKMS is the effort by Microsoft and the Verisign to integrate the PKI and the digital certificates (which are used for securing the Internet transactions) with an XML applications. The key idea is to delegate a signature processing to the trust server on Web, so that the thin or a mobile clients do not have to carry around smarts to do all this themselves. An XKMS relies on a XML Signature specification which is already being worked on by W3C and on the anticipated work at the W3C on the XML encryption specification.