TheArchitect.co.uk - Jorgen Thelin's weblog

Does REST need WSDL ?

Mark Baker made some very valid comments on my recent posting entitled Web Services are more than just SOAP

However, this still seems to me to point strongly to the need for something like a WSDL definition of the syntax of a REST application's requests which involve XML, even if those requests contain multiple facets (namespaces) to the data, such as a payload for the ultimate recipient and some "out-of-band" data blocks for other interested listeners. Ultimately this is all handled using XML's namespace capabilities, irrespective of whether this is in a SOAP message or just a XML document fragment.

This appears to be both the advantage and disadvantage of the text/xml content type compared to say image/jpeg - in the latter case (jpeg), we know exactly how to decode the returned data (there is only one "format" standard), but with XML data we have to know what to look for (eg. which namespaces and schema types) in order to do any non-generic processing of the returned data, so there are multiple "format standards" for the data.

It does seems to be the general direction the W3C Web Services Description Working Group is heading with their fairly wide definition for the phrase Web Services which is clearly not tied to SOAP, and can easily accomodate and leverage the REST approach too:

Web Service [Definition: A Web Service is a software application identified by a URI [IETF RFC 2396], whose interfaces and binding are capable of being defined, described and discovered by XML artifacts and supports direct interactions with other software applications using XML based messages via Internet-based protocols. ]

We will have to wait to see what specifications and standards come out of that activity to see how much this can solve the "ESP Interface definition" problem I referred to previously