Fw: [dfdl-wg] simple way to study hard DFDL example problem - IBM Format VS rec ords as XML
Steve Hanson
smh at uk.ibm.com
Fri Nov 19 10:17:27 CST 2004
I agree with Jim that two DFDL layers are required, one that describes the
original logical structure and one to describe the desired logical
structure. The key thing to recognise is that there are two logical
structures here, and that a transformation of some kind (XSL, Java program,
...) is required to get one from the other.
I don't think we should get DFDL to treat IBM Format VS records as a purely
physical representation of some ideal logical structure - that gets way too
complicated and imposes a big burden on all DFDL implementations.
This is a pretty subjective area - it poses the philisophical question
"when does the physical format become so cryptic that it can be viewed as
changing the logical structure itself".
A structure that asks the same question is an IMS segment. These impose
themselves on the data such that the data is carved into segments that are
preceded with an LLZZ field, the LL containing the segment length. Do you
view the logical structure as a sequence of segments, or do you view it as
the content of the segments where the owning segment # is a physical
property of each field? On a project I worked on in the past, we took the
latter view, which meant that this IMS specific concept found its way into
the physical model, and we had to write specific code to parse & write
segments. I am not convinced that was the right decision.
Mike you say you are aware of 19 such legacy formats, and I bet there are
more. Well IBM's broker has no specific support for any of these, nor have
we been asked to incorporate them into our message model. Maybe we should
play the percentages game - if we see enough different subsystems that use
the same cryptic format then it becomes worth building the support into
DFDL.
Regards, Steve
Steve Hanson
WebSphere Business Integration Brokers,
IBM Hursley, England
Internet: smh at uk.ibm.com
Phone (+44)/(0) 1962-815848
----- Forwarded by Steve Hanson/UK/IBM on 19/11/2004 16:13 -----
mike.beckerle at asc
entialsoftware.co
m To
Sent by: jim.myers at pnl.gov,
owner-dfdl-wg at ggf dfdl-wg at gridforum.org
.org cc
Subject
19/11/2004 15:43 RE: [dfdl-wg] simple way to study
hard DFDL example problem - IBM
Format VS rec ords as XML
You are thinking along the lines I was; however, the challenge is that I
cannot find a way to do this using multilayer so I'm uncomfortable
suggesting that it's possible at all anymore. Here's some reasoning why.
In particular, it's the intersection of the induction across the items with
the first, middle*, last thing, and the spanning that seems to defy my
efforts to cut it up into progressive transformation layer by layer. In
some conversations I've referred to this problem as the "non-conforming
trees" problem. The fundamental shapes of the trees are not compatible, and
expressing the transformation between them isn't easily done via induction
of any kind on one or the other of the trees.
To me the First, Middle*, Last thing is very problematic. It's effectively
a little regular language (in the formal sense) that has to be recognized.
Generally this requires a finite-state-machine, and what makes FSMs
interesting and complex is always the way you diagnose malformed data in
addition to recognizing correct data.
Now, a finite-state-machine is, to my mind, the ultimate procedural
abstraction, the quintessential opposite of "declarative" expression. To be
declarative about a FSM you end up saying "recognize this regular
language", and providing a description of the regular language, which is of
course, just begging the question of how it actually works.
(And for us, we're not really talking about a regular language of character
text, but a pattern of usage in the binary data layout that obeys the
pattern of a regular language. So it's not like having a little regular
expression thing for validating text strings helps with this problem.)
I guess I'm arguing that a black box approach to this is not only
acceptable, but is highly likely to be the only "good" way to do it. In
light of this I've suggested a rep property called "streamFormat" (perhaps
should be renamed "recordFormat"), which gets values from the set VS, V,
VBS, FB, FBS, etc. etc. all these well-defined legacy data formats (there
are 19 of them I think). In additon, one should be able to extend this by
introduction of a blackbox transformation.
And ... here's the rub...if that's true for this case, then other "hard"
examples like run-length encoding seem also in this category.
There's several "leaps of faith" just made in these arguments, so i'd still
like people to take this "XML challenge" and see if there's some magic I'm
overlooking.
...mikeb
From: Myers, James D [mailto:jim.myers at pnl.gov]
Sent: Friday, November 19, 2004 9:52 AM
To: dfdl-wg at gridforum.org
Subject: RE: [dfdl-wg] simple way to study hard DFDL example problem - IBM
Format VS rec ords as XML
Without digging too much into the details, I'd say this is an example
where multi-layer comes in. The DFDL would describe a hidden layer in
which the first, middle, last data elements would be identified and put
into a list, and then that hidden list would be used as the input to
create items in the output layer.
I think this is conceptually similar to one of our run-length encoding
examples (more complex of course). If you read a sequence if ints and then
a sequence of floats and need to output a sequence of floats with int[i]
repeats of float[i], it would be easiest to create a hidden layer
representing the int and float sequences and to then produce output from
that. If you don't think about a layer, even this example gets painful - I
need to read an int, skip forward somewhere to find a float, skip back to
get the next int, etc.
Mike's full example, not starting with the XML-ized version, might be
something that requires more than one layer - read the original into
something with with XML schema Mike defines, then a layer making a
sequence of data elements, and then something that has the desired logical
output.
I guess I would claim that this would not be too bad a way to describe a
fairly complex format in terms of a fairly different logical structure.
Whether one *should* do this in DFDL, or whether it would make more sense
to a) write a black box parser to get to items, or b) use DFDL to get to
the initial schema Mike wrote and use XSLT afterwards to convert to the
desired logical structure. I think there are enough cases where we need
the multilayer functionality in DFDL that are relatively simple that we
have to have it, which means it will then be possible to deal with complex
transformations in DFDL even if not simple/practical.
Jim
-----Original Message-----
From: owner-dfdl-wg at ggf.org [mailto:owner-dfdl-wg at ggf.org] On Behalf Of
mike.beckerle at ascentialsoftware.com
Sent: Thursday, November 18, 2004 9:53 PM
To: dfdl-wg at gridforum.org
Subject: [dfdl-wg] simple way to study hard DFDL example problem - IBM
Format VS rec ords as XML
I've come up with a way to articulate the difficulties I'm having with
DFDL for complex file formats.
This problem may not be that hard for someone with more XML, XPath or
XQuery experience, so I'd apprecate it if you could look it over and if
necessary even run it by your resident XML experts.
In case the emailer mangles all the line lengths, I've also attached the
below as a file.
<!-- Example motivated by DFDL for IBM Format-VS -->
<!-- see http://tinyurl.com/3s2bq for details on IBM Format-VS -->
<!-- Logically, our data is this: -->
<ITEM>The first item</ITEM>
<ITEM>This is the second item</ITEM>
<ITEM>The third</ITEM>
<!-- That is, data having this "logical" schema -->
<sequence>
<element name="ITEM" type="string" minOccurs="0" maxOccurs="unbounded"/>
</sequence>
<!-- But the below is the input data were starting from. What you see
below simulates
the structural issues of IBM Format-VS, but converting the problem
into an XML to XML
transformation problem -->
<BLOCK>
<SEGMENT>
<WHOLE/> <!-- a WHOLE segment holds a whole item (Duh!). This element
is really a type tag. -->
<DATA>The first item</DATA>
</SEGMENT>
</BLOCK>
<BLOCK>
<SEGMENT>
<FIRST/> <!-- a FIRST segment holds the first part of an item. -->
<DATA>Thi</DATA>
</SEGMENT>
</BLOCK>
<BLOCK>
<SEGMENT>
<MIDDLE/> <!-- a MIDDLE segment holds data from the center of an item
-->
<DATA>s is t</DATA>
</SEGMENT>
</BLOCK>
<BLOCK>
<SEGMENT>
<MIDDLE/>
<DATA>he sec</DATA>
</SEGMENT>
</BLOCK>
<BLOCK>
<SEGMENT>
<LAST/> <!-- a LAST segment holds data from the end of the item. -->
<DATA>ond item</DATA>
</SEGMENT>
<SEGMENT>
<WHOLE/><!-- This second segment in this block is a WHOLE segment.
However
in general the 2nd segment of a block could be a WHOLE or
the
FIRST segment of another multi-segment multi-block
spanning item -->
<DATA>Third item</DATA>
</SEGMENT>
</BLOCK>
<!-- Some observations: -->
<!-- Data is organized into BLOCKs -->
<!-- Each block contains 1 or 2 SEGMENTs -->
<!-- Each SEGMENT is either a WHOLE item, or the item spans 2 or more
SEGMENTs -->
<!-- Spanning data is broken on arbitrary boundaries across segments it
spans -->
<!-- Spanning involves a FIRST, MIDDLE*, LAST segment structure. -->
<!-- MIDDLE* means zero or more MIDDLE segments. -->
<!-- The question: how can we express the transformation into the desired
logical form?
Or is this beyond the call of duty for DFDL?
Goals include to be as declarative as possible, and ideally, do it as
a set of
XML Schema annotations in the GGF DFDL style. -->
<!-- here's an XSD (untested) for the input data structure -->
<complexType name="Format_VS_t">
<sequence>
<element name="BLOCK" type="Block_t" minOccurs="0"
maxOccurs="unbounded"/>
</sequence>
</complexType>
<complexType name="Block_t">
<sequence>
<element name="SEGMENT" type="Segment_t" minOccurs="1"
maxOccurs="2"/>
</sequence>
</complexType>
<complexType name="Segment_t">
<sequence>
<choice>
<element name="WHOLE">
</element>
<element name="FIRST">
</element>
<element name="LAST">
</element>
<element name="MIDDLE">
</element>
</choice>
<element name="DATA" type="string"/>
</sequence>
</complexType>
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