DataDDX

• Use a multipart MIME document to hold the DDX and one data blob per DDX. Adopt the same use of Multipart. MIME as WCS uses.

• consider the following design idea for embedding type information within the data stream:

In DAP2, the DDS is used as a descriptive header for data. For gridded data that works OK but for point data it doesn't work so well. When the data are read as from a stream, it's OK, but when the data are read and stored, then used, they need to be in a linked structure. Because the DDS contains the data type definition and not the value, it does not hold a structure suitable for holding values. Look at how the d_values field in Sequence works (see Sequence::print_val(), deserialize()). A better approach would be to encode the type of a variable in the data stream itself and have the reader (client most of the time) build instances as needed. For arrays, structures and simple types this is mostly a wash, but for Sequences it would be a major plus because the protocol could support sequences of complex objects more easily. It would also get rid of the odd situation where a DDS holds a type definition for a nested sequence while the top-most sequence holds the tree of objects which hold values. (I.E. the child sequences in the DDS don't hold data at all).

As an alternative, suppose we build a data response using the DDX in a multipart document and then encoded type information in the data stream as well? This would provide a way to bundle attributes with variables in the data response and locate type information with the data values (for sequences mostly).

Normative References

Multipart MIME

Xlink

Example DataDDX response

 HTTP/1.1 200 OK
 Server: Apache-Coyote/1.1
 Content-Type: multipart/related; type="text/xml"; start="<080B6DC4AC8AF0C43041C57CE8DE9646>"; boundary="--mimepart_7_9651610.1145395859678"
 Date: Tue, 18 Apr 2006 21:30:59 GMT
 XDAP: 3.2
 Connection: close
 
 --mimepart_7_9651610.1145395859678
 Content-Type: text/xml; charset=UTF-8
 Content-Transfer-Encoding: binary
 Content-Id: <080B6DC4AC8AF0C43041C57CE8DE9646>
 
 <?xml version="1.0" encoding="UTF-8"?>
 
     <Dataset name="fnoc1.nc" 
              xmlns="http://xml.opendap.org/ns/DAP/3.2#"
              xmlns:xml="http://www.w3.org/XML/1998/namespace"
              xmlns:xlink="http://www.w3.org/XML/1999/xlink"
              xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
              xsi:schemaLocation="http://xml.opendap.org/ns/DAP/3.2#  http://xml.opendap.org/dap/dap3.2.xsd"
              xml:base="http://test.opendap.org/dap/data/nc/fnoc1.nc.ddx"
              xlink:href="cid:6efa6ea4:98eda872192:-1ed1">

        <xlink href="cid:6efa6ea4:98eda872192:-1ed1" rel="ref"/>

        <Attribute name="NC_GLOBAL" type="Container">
	     <Attribute name="base_time" type="String">
	         <value>"88- 10-00:00:00"</value>
	     </Attribute>
	     <Attribute name="title" type="String">
	         <value>" FNOC UV wind components from 1988- 10 to 1988- 13."</value>
	     </Attribute>
	 </Attribute>
	 <Attribute name="DODS_EXTRA" type="Container">
	     <Attribute name="Unlimited_Dimension" type="String">
	         <value>"time_a"</value>
	     </Attribute>
	 </Attribute>
	 <Array name="v">
	     <Attribute name="units" type="String">
		 <value>"meter per second"</value>
	     </Attribute>
             <Attribute name="long_name" type="String">
		 <value>"Vector wind northward component"</value>
	     </Attribute>
	     <Attribute name="missing_value" type="String">
		 <value>"-32767"</value>
	     </Attribute>
	     <Attribute name="scale_factor" type="String">
		 <value>"0.005"</value>
	     </Attribute>
	     <Int16/>
	     <dimension name="time_a" size="16"/>
	     <dimension name="lat" size="17"/>
	     <dimension name="lon" size="21"/>
	 </Array>
     </Dataset>
 </xml>

 --mimepart_7_9651610.1145395859678
 Content-Type: application/octet-stream
 Content-Transfer-Encoding: binary
 Content-Id: 6efa6ea4:98eda872192:-1ed1
   
   Here be the XDR encoded binary stuff that is the data from the GetDATA request
   
 --mimepart_7_9651610.1145395859678--