The DDX is an attempt to combine the information in the both the DDS and DAS into one response to eliminate the need to cross-walk the two responses when applying DAS information to variables. It will also streamline access somewhat and will employ XML as the content-type, although these are secondary objectives.
1 DAP 3.1
DAP 3.1 contains an implementation of this feature, which is described, in spirit, in the original Draft DAP 4 specification document.
2 DAP 3.2
2.1.1 GRDDL Namespace
In version 3.2 the namespace for grddl is defined and the DDX to RDF XSL transform is the value of the grddl:transform attribute in the Dataset element.
2.1.2 BLOB element
Removed as of Version 3.2.
2.1.3 Version information and the structure of the Dataset element
The information in the DDX Dataset element is different starting with DAP 3.2. For this version onward, the Dataset element will contain a default namespace that is http://xml.opendap.org/ns/DAP/3.2# where the last pathname component will vary to reflect the version number. This will also be set as a prefix like dap3.2 so that attributes can be put in the namespace. The Dataset element will also have the schema location set to http://xml.opendap.org/dap/dap/3.2.xsd where, again, the last part of the pathname will reflect the version.
Here's the current stuff that appears in a 3.2 DDX's Dataset element:
<Dataset name="fnoc1.nc" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://xml.opendap.org/ns/DAP/3.2# http://xml.opendap.org/dap/dap/3.2.xsd" xmlns="http://xml.opendap.org/ns/DAP/3.2#" xmlns:dap="http://xml.opendap.org/ns/DAP/3.2#" dap_version="3.2">
Why have .../DAP/3.2# appear twice? Because this sets the default namespace to DAP 3.2 and sets the prefix dap to that as well. The default namespace is not used by XML attributes, only elements, so defining the symbol dap as a shorthand provides an easy way to place attributes is a given namespace.
The schemaLocation attribute is used by conforming validating parsers but is not used to assign namespaces for the elements and attributes in the document, so the information is repeated.
2.2 Added the Data element to ease defining the DataDDX response
See the design/discussion in DataDDX. The Data element is essentially the old BLOB element, but I'm calling it 'Data' this time ;-)
Adopt the use of MIME that is used by WCS 1.1.
Modify Attributes so that they appear 'anywhere' in the DDX and eliminate the Attribute element. At the same time, every Attribute must be in a namespace to bind that attribute to some semantic context. We will define an 'opendap' namespace for stuff that's not clearly part of CF, FGDC, et cetera.