Dancing with Web services: W3C chair talks choreography
By Nitin Bharti, News Editor
09 Mar 2005 | SearchWebServices.com
As companies embrace service-oriented architecture, the Business Process Execution Language (BPEL) continues to gain traction as a means to weave Web services into meaningful business processes. While BPEL allows existing services to be orchestrated into composite services, the Web Services Choreography Description Language (WS-CDL) goes a step further and describes the relationships between services in a peer-to-peer scenario. In this interview, Steve Ross-Talbot, co-chair of the W3C Web Services Choreography Working Group, describes choreography and how it differs from orchestration in the context of Web services. He compares the WS-CDL and BPEL specifications, looks at how the two can work together and describes four tools that will be needed to work with WS-CDL.
Could you tell us a little bit about yourself and what you do at the World Wide Web Consortium (W3C)?
Steve Ross-Talbot: I'm both the co-chair of the Web Services Choreography working group and I'm also the chair of Web Services Coordination. As co-chair, it's my job to ensure that the group can reach consensus. As a member of the working group, I also do technical work. I'm one of the authors for the requirements specification for WS-CDL.
What is the Web Services Coordination specification?
Ross-Talbot: This specification coordinates all the Web services standards within the W3C. So you've got a description that is the WSDL [Web Services Description Language] protocol, messaging which is SOAP, you've got choreography, you've got addressing, and there's also a link to the semantic Web domain and a link to the XML Core domain. All of these are reported to the WS-Coordination group and, all the chairs of those groups attend.
How does the W3C compare to other organizations like OASIS and the WS-I?
Ross-Talbot: Well firstly, the WS-I isn't a standards body; it never created a standard. What it does is take a bunch of standards, or in some cases proprietary specifications, such as WSDL 1.1 and SOAP 1.1 (which are really de facto and not official standards); so WS-I's job is to take de facto and actual standards, put them in a bag and give that bag a name. We call that Basic Profile.
OASIS is a very different kind of organization. By contrast, it's much more vendor-led than the W3C. The W3C is responsible for all the core building blocks of the Web, for Web services and for the semantic Web. All the ancillary things you need to make Web services happen, such as management and orchestration, are in OASIS.
What is WS-CDL and what problems does it address?
Ross-Talbot: The principal motivation behind WS-CDL is to ensure interoperability between a set of peer services. What Choreography says is 'I have a bunch of services. How do I write down what those services should do to engage with each other?' So I don't want to take a buyer's perspective, or a seller's perspective or a shipper's perspective. What I want to do is write down, from a high level, what's the exchange of messages that occur to buy a widget.
How do WS-CDL and WS-BPEL differ?
Ross-Talbot: Any real business transaction isn't just one function call, it's a sequence of function calls and it may be lots of things in parallel between different services that occur. BPEL is about 'how do I construct Web services out of existing Web services.' In other words, BPEL is about the orchestration of existing services to yield another service.
