[rest-practices] Namespace qualifying respresentation types

[Daniel Veillard]

On Thu, Apr 29, 2010 at 11:55:07AM +0100, Eoghan Glynn wrote:
> Folks,
> 
> I wanted to get the group's feeling for the use of namespaces in
> representation types.
> 
> The context for the question initially comes from a discussion about how
> action/operations were to be modelled in RHEVM-API, particularly how to
> capture the action-specific parameter types in the schema. In XML schema
> terms, my first cut was to use an xs:any within the action element, but to
> restrict the contained type to the local namespace. Which of course requires
> that a target namespace is defined in the first place.

  Why XML Schemas ? Why not Relax-NG instead.
As an implementor, I *strongly* suggest to avoid XSD unless for very
very simple things. And even very simple thing are described in very
painful terms with no formal semantic. No later than today I had to dig
in
http://www.w3.org/TR/xmlschema-1/#Simple_Type_Definition_details
for some other Red Hat employees requesting help. This thing is horrid
please avoid it.
In contrast Relax-NG specification is 1/ clear 2/ includes a formal
semantic description 3/ has more expressive power than XSD and is
even an ISO standard.
http://www.oasis-open.org/committees/relax-ng/spec-20011203.html

  Actually I would be very surprised if there wasn't a way to use
xs:any allowing only non-namespaced elements, but considering the
lack of clarity of the spec, no matter everybody gets confused. In
the end people just try with the set of implementation tehy target
and don't try to understand the real expected semantic of their schemas,
which IMHO completely undermine the usefulness of XSD in the first
place.

> So I'm sensing there may be some opposition to this, given that namespacing
> isn't widely used in the existing RESTful APIs. I guess I'm wondering
> whether this ommission was a concious decision, or just that nobody saw the
> need for using namespaces but wouldn't be averse to their use in principle.

  I think that XSD is a bigger mistake than namespaces, though
namespaces have their set of annoyances. If you go REST I would
suggest to avoid both, but use Relax-NG as your schemas language.

> Obviously there are other RESTful examples out there in the wild where the
> xmlns attribute makes an appearance, Atom springs to mind here ... <feed
> xmlns="http://www.w3.org/2005/Atom">. And I don't need to rehearse the basic
> advantages of using namespaces, partitioning the global namespace, aiding in
> validation, encoding version information etc. But I would be interested in
> hearing if there's determined opposition to their use, and if so what the
> reasoning is.

  In general they make implementations more complex，for example in
XPath you will have to register all namespaces before being able to
execute a query, it's less readable, etc ...
  My suggestion would be to use a non-namespaced space and use the
namespaces only for custom extensions if the need arise.

Daniel

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