Re: [saml-dev] Metadata extensions

[Scott Cantor <cantor.2@osu.edu>]

Alistair Young wrote:
> I'm defining an new implementation of SAML2 Metadata RoleDescriptor and
> was wondering why the schema didn't allow substitution groups. It seems I
> need to use xsi:type to extend RoleDescriptor and use <RoleDescriptor> in
> the instance document rather than defining a new element.

Because substitution is complex, and because it significantly aids 
implementers if they can depend on the element name at a given extension 
point. Handling substitution is harder than xsi:type if you're 
processing without validation, as most people do. With validation, it's 
still a little easier to only have one method to worry about instead of 
two. SAML 2 disallows all use of substitution in the schemas.

-- Scott