[dfdl-wg] Opaque/BLOB/Uninterpreted/Raw - also hexBinary - (was Re: split into multiple topics - Re: [dfdl-wg] Issues: additional data types)

[Mike Beckerle]

re: Opaque or uninterpreted or raw fields. These are sometimes called 
Blobs, though database people reserve that term for the acronym "BLOB" 
which stands for Binary Large Object, which has to do with size being too 
large for the smaller binary SQL type objects. I.e., there's no such thing 
as a small BLOB in databases. I think in our mailing list we've used blob 
to mean "opaque bytes" of any size at all.

I believe use of the 'hexBinary' type is also probably this same topic. 
I.e., how to deal with data where you don't know its proper 
interpretation, though you can express how big it is so that we can at 
least copy it from place to place. 

I think there are two choices here. One is just use "occuring" bytes. 
E.g., here's uniterpreted data of length 1234 bytes:

<element name="ignoreMe" type="byte" minOccurs="1234" maxOccurs="1234" 
dfdl:repType="binary"/>

This is a basic binary byte array. I think this works fine as a 
blob/opaque type.  I believe we do not need any other kind of raw/opaque 
type. If we had one, we'd have to have a way to express its length, and be 
specific about the units of that length, and the above accomplishes that 
with pretty much minimum baggage.  You name it what you want, i.e, 
"unused" or "dummy" or "ignore" or whatever you want. 

We might want an annotation to indicate that this data should not be 
accessed, to distinguish this case from an actually array of bytes that 
you DO want to access, but I'm not sure that's worth it. Note that the OMG 
CAM model does have an access control attribute. Perhaps we can use that. 
However, I doubt it allows distinguishing copy from access. 

The alternative is to use the "hexBinary" type for this. In that case we 
need to express the size in the DFDL annotation:

<element name="ignoreMe" type="hexBinary" dfdl:repLength="1234" 
dfdl:repType="binary"/> 

I can think of one advantage of hexBinary over the occuring bytes 
approach, which is suppose you do want to use DFDL in the obvious way to 
convert data into XML format. Never mind that DFDL is supposed to enable 
avoiding this, suppose it's what you want to do. Then my above byte array 
for the "ignoreMe" element ends up as:

<ignoreMe>0</ignoreMe><ignoreMe>0</ignoreMe><ignoreMe>0</ignoreMe><ignoreMe>0</ignoreMe><ignoreMe>0</ignoreMe><ignoreMe>0</ignoreMe>....<ignoreMe>0</ignoreMe>

Which is big compared to: <ignoreMe>000000000000...00</ignoreMe> which is 
what we'd get if we allow hexBinary as a type. 

Note that if we add the hexBinary type, you'll still be able to do it the 
other way, so the hexBinary notion is not strictly speaking necessary or 
minimalist. 

...mikeb

Mike Beckerle
Architect, Scalable Computing
IBM Software Group
Information Integration Solutions
Westborough, MA



Mike Beckerle/Worcester/IBM at IBMUS 
Sent by: owner-dfdl-wg at ggf.org
09/02/2005 04:34 PM

To
"Robert E. McGrath" <mcgrath at ncsa.uiuc.edu>
cc
dfdl-wg at gridforum.org, owner-dfdl-wg at ggf.org
Subject
split into multiple topics - Re: [dfdl-wg] Issues: additional data types







I'd like to split this topic into several distinct ones: 

Arrays - I have a placeholder for this in the doc. 

Opaque and "code" types are separate. This is related also to the concept 
of "open content". 

Enums 

Bitfields 

Pointers 


Mike Beckerle
Architect, Scalable Computing
IBM Software Group
Information Integration Solutions
Westborough, MA 


"Robert E. McGrath" <mcgrath at ncsa.uiuc.edu> 
Sent by: owner-dfdl-wg at ggf.org 
09/02/2005 03:13 PM 


To
dfdl-wg at gridforum.org 
cc

Subject
[dfdl-wg] Issues: additional data types








Greetings,

Here is an "issue" for the DFDL: additional data types that should
be considered.

Please see attached.

---
Robert E. McGrath
National Center for Supercomputing Applications
University of Illinois, Urbana-Champaign
Champaign, Illinois 61820
(217)-333-6549

mcgrath at ncsa.uiuc.edu 
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