Generating a WSDL from an XSD file

Question

I need to generate a WSDL file given an XSD file. How do I do this? Can I do this in VS2005? What is the simplest way to do this?

Answer 1

You cannot - a XSD describes the DATA aspects e.g. of a webservice - the WSDL describes the FUNCTIONS of the web services (method calls). You cannot typically figure out the method calls from your data alone.

These are really two separate, distinctive parts of the equation. For simplicity's sake you would often import your XSD definitions into the WSDL in the <wsdl:types> tag.

(thanks to Cheeso for pointing out my inaccurate usage of terms)

Answer 2

I'd like to differ with marc_s on this, who wrote:

a XSD describes the DATA aspects e.g. of a webservice - the WSDL describes the FUNCTIONS of the web services (method calls). You cannot typically figure out the method calls from your data alone.

WSDL does not describe functions. WSDL defines a network interface, which itself is comprised of endpoints that get messages and then sometimes reply with messages. WSDL describes the endpoints, and the request and reply messages. It is very much message oriented.

We often think of WSDL as a set of functions, but this is because the web services tools typically generate client-side proxies that expose the WSDL operations as methods or function calls. But the WSDL does not require this. This is a side effect of the tools.

EDIT: Also, in the general case, XSD does not define data aspects of a web service. XSD defines the elements that may be present in a compliant XML document. Such a document may be exchanged as a message over a web service endpoint, but it need not be.

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Getting back to the question I would answer the original question a little differently. I woudl say YES, it is possible to generate a WSDL file given a xsd file, in the same way it is possible to generate an omelette using eggs.

EDIT: My original response has been unclear. Let me try again. I do not suggest that XSD is equivalent to WSDL, nor that an XSD is sufficient to produce a WSDL. I do say that it is possible to generate a WSDL, given an XSD file, if by that phrase you mean "to generate a WSDL using an XSD file". Doing so, you will augment the information in the XSD file to generate the WSDL. You will need to define additional things - message parts, operations, port types - none of these are present in the XSD. But it is possible to "generate a WSDL, given an XSD", with some creative effort.

If the phrase "generate a WSDL given an XSD" is taken to imply "mechanically transform an XSD into a WSDL", then the answer is NO, you cannot do that. This much should be clear given my description of the WSDL above.

When generating a WSDL using an XSD file, you will typically do something like this (note the creative steps in this procedure):

1. import the XML schema into the WSDL (wsdl:types element)
2. add to the set of types or elements with additional ones, or wrappers (let's say arrays, or structures containing the basic types) as desired. The result of #1 and #2 comprise all the types the WSDL will use.
3. define a set of in and out messages (and maybe faults) in terms of those previously defined types.
4. Define a port-type, which is the collection of pairings of in.out messages. You might think of port-type as a WSDL analog to a Java interface.
5. Specify a binding, which implements the port-type and defines how messages will be serialized.
6. Specify a service, which implements the binding.

Most of the WSDL is more or less boilerplate. It can look daunting, but that is mostly because of those scary and plentiful angle brackets, I've found.

Some have suggested that this is a long-winded manual process. Maybe. But this is how you can build interoperable services. You can also use tools for defining WSDL. Dynamically generating WSDL from code will lead to interop pitfalls.