Wednesday, August 26, 2009


Paul kindly pointed to a blog entry about WADL use in REST, (WADL definition is found here). The Blog entry was interesting to read.

I want to quickly explain that WADL is meant to describe the possible interactions with a Web Application. Thus, it has methods, but those are HTTP commads, and it also describes the query variables (like the fields in a form) that the method needs to send to the web application. Finally, it describes the expected response.

Now, in the blog I see some misconceptions. First, it relates SOAP and WSDL to XML-RPC. That may seem true due to actual use of those two, but SOAP is just a wrapper and WSDL does not enforce XML-RPC, using document/style it reassembles a simply document sending paradigm. By the way, XML-RPC and WS used to do RPC are two completely different things.

Second, the REST based Service concept is defined to encompass even static web pages. May REST style architecture handle resources in the web, and REST “style” Services are services implemented using the REST underlying paradigm. But Static pages are not Services.

Third, WADL is at implementation level of the HTTP call, thus it does not define the service, just what to call and what to expect in response. That is not bad, it is actually good, but then WADL describes an HTTP interaction, while WSDL describes a messaging interaction. Although not actually standardized, WSDL may allow you to send messages using a message queue, SMTP, or any other transport.

Now, I want to explain my vote. I’m not a WSDL lover, I think it may be improved. Still, I’m carefull to compare, it is not as easy. We cannot say WADL is a description document for REST based Services and WSDL for RPC based ones. Right?

I may want to go even further. I may say REST Style Services may actually make use of both! Yes, while WSDL describes your Service at message level, defining the document and port, and a WADL that makes clearer the HTTP interaction (since WSDL inly indicates you are using HTTP as a transport).

Again, what do you think?

William Martinez Pomares.

Reference and other Comments: WADL, REST and WSDL

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