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Re: Comments on Assigning Media Types to Binary Data in XML

From: David RR Webber (XML) <w3c@drrw.info>
Date: Wed, 08 Dec 2004 08:25:32 -0500
Message-ID: <41B700CC.7040006@drrw.info>
To: "Henry S. Thompson" <ht@inf.ed.ac.uk>
CC: public-ws-media-types@w3.org

Henry,

If the goal here is to transport binary payloads across a SOAP 
interface, then this
is all already implemented and available - with packaging extensions to the
base SOAP enveloping to handle all that.

There is also an open source toolset available here:

 http://sourceforge.net/projects/ebxmlms

I was recently able to send a 110Mb binary using this toolset so I know
that it works - and the specification is robust.

Thanks, DW

Henry S. Thompson wrote:

>I have produced these comments at the request of the XML Schema WG,
>but the WG has not yet seen them, much less endorsed them.
>None-the-less they are intended to be comments specifically from the
>perspective of XML Schema, and I hope they may be helpful.
>
>1) Architecture
>
>I'm sympathetic to what you're trying to achieve, i.e
>
> a) Enable e.g. a SOAP message to say "this bit of binary encodes an
>    application/png image";
> b) Enable e.g. an XML Schema involved in a WSDL doc. to say "the
>    binary allowed here should be marked (per (a) above) as encoding
>    one or more of the following media types. . ."
>
>But I'm not entirely happy with the way you've separated these two
>goals and introduced a new mechanism for cross-validation between
>them.
>
>There are already two relevant mechanisms available in XML Schema
>which it seems to me would serve your needs much more
>straightforwardly.
>
>The first mechanism is the type-derivation hierarchy; one way to use it
>for your purposes goes as follows:
>
> It makes sense to think about e.g. image/png encoded as base64 as a
> _subtype_ of xs:base64Binary.  It even makes sense to think about
> image/png encoded as base64 as a subtype of image/* encoded as
> base64.
>
> So why not define a family of types in a basic XML Schema for the
> xmlmime namespace, one rooted at xs:base64Binary and the other,
> parallel, one rooted at xs:hexBinary, as follows:
>
>                     xs:base64Binary
>                        |
>                   xmlmime:base64Binary
>                       /|\
>                      / | \
>                     /  |  \
>                    /   |   \
>                   /    |    \
>                  /     |     \
>                 /      |      \
>    xmlmime:image xmlmime:text xmlmime:application    . . . audio, video
>          .            /|\           .
>          .           / | \          .
>          .          /  |  \         .
>                    /   |   \
>                   /    |    \
>                  /     |     \
>                 /      |      \
>                /       |       \
>               /        |        \
>              /         |         \
>             /          |          \
>            /           |           \
>xmlmime:text_plain xmlmime:text_xml  xmlmime:text_html
>
>Then you can get rid of xmlmime:expectedMediaType altogether, so that
>e.g.
>
>    <xs:complexType name="JPEGPictureType" 
>            type="xs:base64Binary"
>            xmlmime:expectedMediaType="image/jpeg"/> 
>
>    <xs:element name="JPEGPicture" type="tns:JpegPictureType"/>
>
>becomes simply
>
>    <xs:element name="JPEGPicture" type="xmlmime:image_jpeg"/>
>
>and you don't need xmlmime:contentType if you're confident of schema
>processing, but _if_ you want to be explicit, you can use xsi:type
>instead, e.g.
>
><Picture xsi:type="xmlmime:image_png">/aWKKapGGyQ=</Picture>
>
>I realise this doesn't cover the full generality of your proposal, in
>so far as you appear to be allowing _any_ kind and number of
>'parameters' after the type/subtype.  I am not at all sure that's the
>right thing to do, and at the very least I think you need some
>argumentation to establish the need for that much generality.
>
>Note furthermore that the general rules for type substitution and
>unions can be used to establish arbitary sets of media types, so that
>the functionality your now achieve by allowing a list of media types in
>is not lost.
>
>----------------
>
>The other mechanism already present in schema reflects the fact that
>this problem has been around for a long time.  A solution was already
>present in SGML DTDs and carried forward into XML DTDs and XML Schema,
>namely NOTATION attributes (see [1] and [2]).  There is an example in
>[2] which directly addresses your concerns, and would allow you to
>eliminate xmlmime:expectedMediaType and connect xmlmime:contentType
>more clearly to the IETF media type definitions via the URL hierarchy
>rooted at http://www.iana.org/assignments/media-types/, e.g. by
>changing the example from [2] to read
>
><xs:notation name="jpeg"
>  public="image/jpeg"
>  system="http://www.iana.org/assignments/media-types/image/png" />
>
>Again, on this approach the XML Schema for the xmlmime namespace would
>contain declarations for many useful Notations and simple types
>derived from NOTATION, for use in WSDL schemas.
>
>2) Low-level points
>
>If the discussion above has persuaded you to change the current
>design, well and good.  If it has not, there are some minor ways in
>which the current design could be improved.
>
>2a) Using xs:string is almost certainly not what you want -- that
>makes whitespace variation significant, so that e.g. 
>  xmlmime:contentType="image/png "
>is not the same as
>  xmlmime:contentType="image/png"
>
>I would recommend xs:token instead.
>
>2b) Please provide a concrete reference for "IANA media type token".
>
>2c) In example 1, you probably mean
>
>  <xs:restriction base="xmlmime:base64Binary">
>
>2d) In example 4, you probably mean
>
>  <xs:complexType name="JPEGPictureType>
>   <xs:complexContent>
>    <xs:restriction base="xmlmime:base64Binary"
>                    xmlmime:expectedMediaType="image/jpeg"/>
>   </xs:complexContent>
> </xs:complexType>
>
>ht
>
>[1] http://www.w3.org/TR/2004/REC-xmlschema-2-20041028/datatypes.html#NOTATION
>[2] http://www.w3.org/TR/2004/REC-xmlschema-1-20041028/structures.html#declare-notation
>  
>
Received on Wednesday, 8 December 2004 13:55:01 GMT

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