|
|
|
|||||||||||||||||||||||||
 |
||||||||||||||||||||||||||
|
||||||||||||||||||||||||||
|
A Web Services Primer by
Venu Vasudevan PAGE 2 SOAP SOAP is a protocol specification that defines a uniform way of passing XML-encoded data. In also defines a way to perform remote procedure calls (RPCs) using HTTP as the underlying communication protocol. SOAP arises from the realization that no matter how nifty the current middleware offerings are, they need a WAN wrapper. Architecturally, sending messages as plain XML has advantages in terms of ensuring interoperability (and debugging, as I can well attest). The middleware players seem willing to put up with the costs of parsing and serializing XML in order to scale their approach to wider networks. Submitted in 2000 to the W3C as a Note by IBM, Microsoft, UserLand, and DevelopMentor, the further development of SOAP is now in the care of the W3C's XML Protocols Working Group. This effectively means that SOAP is frozen and stable until such time as the W3C Working Group delivers a specification. UDDI (Universal Description, Discovery and Integration Service) UDDI provides a mechanism for clients to dynamically find other web services. Using a UDDI interface, businesses can dynamically connect to services provided by external business partners. A UDDI registry is similar to a CORBA trader, or it can be thought of as a DNS service for business applications. A UDDI registry has two kinds of clients: businesses that want to publish a service (and its usage interfaces), and clients who want to obtain services of a certain kind and bind programmatically to them. The table below is an overview of what UDDI provides. UDDI is layered over SOAP and assumes that requests and responses are UDDI objects sent around as SOAP messages. A sample query is included below.
WSDL (Web Services Definition Language) WSDL provides a way for service providers to describe the basic format of web service requests over different protocols or encodings. WSDL is used to describe what a web service can do, where it resides, and how to invoke it. While the claim of SOAP/HTTP independence is made in various specifications, WSDL makes the most sense if it assumes SOAP/HTTP/MIME as the remote object invocation mechanism. UDDI registries describe numerous aspects of web services, including the binding details of the service. WSDL fits into the subset of a UDDI service description. WSDL defines services as collections of network endpoints or ports. In WSDL the abstract definition of endpoints and messages is separated from their concrete network deployment or data format bindings. This allows the reuse of abstract definitions of messages, which are abstract descriptions of the data being exchanged, and port types, which are abstract collections of operations. The concrete protocol and data format specifications for a particular port type constitute a reusable binding. A port is defined by associating a network address with a reusable binding; a collection of ports define a service. And, thus, a WSDL document uses the following elements in the definition of network services:
So,
in plain English, WSDL is a template for how services should
be described and bound by clients. XLANG The traditional notion of a database transaction is an atomic, that is, all-or-none action; either the entire action happens or it doesn't. Providing this kind of guarantee in a distributed infrastructure involves an expensive process called two-phase commit. An alternative optimistic model has been proposed in database research (originally called sagas and proposed by Hector Garcia-Molina), where actions have explicit compensatory actions which negate the effect of the action. In the real world of actions, the existence of compensatory actions is quite common. For instance if I debit a credit card $52, the compensatory action is to credit the credit card $52. If I sent out an e-mail saying "you'll get the product you ordered in seven days", the compensatory action is to send an e-mail saying, "oops, it's going to take longer". XLang is a notation for expressing the compensatory actions for any request that needs to be undone. The web services infrastructure can leverage XLang specifications to perform complex undo operations.
|