[DFDL-WG] Fw: Action 261
Steve Hanson
smh at uk.ibm.com
Wed Jun 25 06:41:44 EDT 2014
261
Implied separatorSuppressionPolicy for occursCountKind 'expression' (All)
10/6: Spec says it is 'never' (positional sequence) but you have to parse
to identify the position, so isn't that non-positional?
17/6: Some other issues noted around 'expression' as per email thread. IBM
have discussed this internally and will submit a proposal.
As was noted in the email for Action 260, if it is decided that the
meaning of "Each occurrence in the sequence can be identified by its
position in the data" is more strictly that an observer of the raw data
can identify an occurrence of an element in the sequence solely by
counting separators then that would appear to make dfdl:occursCountKind
'expression' more like 'parsed', and not eligible to be in a Positional
sequence. But if the meaning is a parser does not have to speculate to
identify an occurrence of an element in the sequence then it can be in a
Positional sequence.
While discussing the nature of 'expression' it was noted that it is very
easy for a DFDL user to create a data stream from an infoset and for that
data stream to be un-parse-able. If dfdl:outputValueCalc is not used, then
the element(s) in the infoset must be correctly set manually to match the
number of occurrences. The same observation applies to dfdl:lengthKind
'explicit' where dfdl:length is an expression.
To address this, IBM proposes the following changes to the DFDL
specification for occursCountKind 'expression' when unparsing:
When all occurrences have been obtained from the infoset (and defaulting
applied if needed), the occursCount expression is evaluated
If any element that is referenced by the expression has
dfdl:outputValueCalc, then that expression is (re-)evaluated as part of
the above; given that the number of occurrences is now known, the
outputValueCalc expression should now evaluate successfully
If the result of the occursCount does not match the number of occurrences
it is a processing error
This ensures the integrity of the data stream (the non-outputValueCalc use
case), while still allowing the infoset to dictate the count (the
outputValueCalc use case).
Similarly, IBM proposes the following changes to the DFDL specification
for lengthKind 'explicit', where length is an expression, when unparsing:
When an element has been obtained from the infoset (and defaulting applied
if needed) and unparsed but before padding is applied, the expression is
evaluated to give a length.
If any element that is referenced by the expression has
dfdl:outputValueCalc, then that expression is (re-)evaluated; given that
the unpadded length is now known, the outputValueCalc expression should
now evaluate successfully
Now that we have a length, the unparser behaviour is the same as if the
length was obtained from a fixed dfdl:length value. (That is, truncation,
padding, filling or error depending on other property settings).
This ensures the integrity of the data stream (the non-outputValueCalc use
case), while still allowing the infoset to dictate the length (the
outputValueCalc use case). Note that it means the 'awkward' behaviour
whereby lengthKind 'explicit' (expression) is a specified length when
parsing but variable length when unparsing is avoided - it is now always
specified length.
IBM believes that none of the above should impose additional burden on
implementers, as no brand new behaviour is being added.
Regards
Steve Hanson
Architect, IBM DFDL
Co-Chair, OGF DFDL Working Group
IBM SWG, Hursley, UK
smh at uk.ibm.com
tel:+44-1962-815848
----- Forwarded by Steve Hanson/UK/IBM on 25/06/2014 10:24 -----
From: Tim Kimber/UK/IBM
To: Steve Hanson/UK/IBM at IBMGB, Alex Wood1/UK/IBM at IBMGB, Andrew
Edwards/UK/IBM at IBMGB, Mark Frost/UK/IBM at IBMGB,
Date: 16/06/2014 22:08
Subject: Re: [DFDL-WG] Action 261
I think this needs to be discussed before the meeting tomorrow. I wanted
to avoid turning this thread into a monster - but I don't think it's
possible to discuss without the context so I've continued the thread as
before with <tk> tags.
I'm clear in my own mind that I understand the issues now. We should give
priority to the separatorSuppressionPolicy question because the current
rules are close to being unimplementable. The occursCountKind and
lengthKind questions are important but are at least implementable as they
stand.
regards,
Tim Kimber,
IBM Integration Bus Development (Industry Packs)
Hursley, UK
Internet: kimbert at uk.ibm.com
Tel. 01962-816742
Internal tel. 37246742
From: Steve Hanson/UK/IBM
To: Tim Kimber/UK/IBM at IBMGB,
Cc: dfdl-wg at ogf.org, dfdl-wg-bounces at ogf.org
Date: 11/06/2014 16:12
Subject: Re: [DFDL-WG] Action 261
Replies in <smh> tags
Regards
Steve Hanson
Architect, IBM DFDL
Co-Chair, OGF DFDL Working Group
IBM SWG, Hursley, UK
smh at uk.ibm.com
tel:+44-1962-815848
From: Tim Kimber/UK/IBM at IBMGB
To: dfdl-wg at ogf.org,
Date: 11/06/2014 13:58
Subject: Re: [DFDL-WG] Action 261
Sent by: dfdl-wg-bounces at ogf.org
comments in <tk>tags
regards,
Tim Kimber,
IBM Integration Bus Development (Industry Packs)
Hursley, UK
Internet: kimbert at uk.ibm.com
Tel. 01962-816742
Internal tel. 37246742
From: Steve Hanson/UK/IBM
To: Tim Kimber/UK/IBM at IBMGB,
Cc: dfdl-wg at ogf.org, dfdl-wg-bounces at ogf.org
Date: 11/06/2014 10:47
Subject: Re: [DFDL-WG] Action 261
Some thoughts on this...
I agree that the definition of positional sequence in the spec needs
tightening as it is ambiguous as it stands and could be interpreted as a)
or b). If we adopted b) then that would appear to allow 'expression' to
appear in a positional sequence, but wouldn't it also allow 'stopValue'?
<tk>Yes - according to definition b) stopValue would be allowable in a
positional sequence. We could still disallow it if we do not believe there
is any benefit in allowing it. I don't believe it introduces any
particular complexities for an implementer.</tk>
occursCountKind 'expression' is analogous to lengthKind 'explicit' with an
expression and to lengthKind 'prefixed'. Both these lengthKinds are
classified as 'specified length' when parsing but 'variable length' when
unparsing. We are observing that occursCountKind 'expression' is like
'fixed' when parsing but not quite so like 'fixed' when unparsing - which
is why section 16 groups 'expression' with 'parsed' for unparsing.
<tk>Yes - we took a decision that the unparser should ignore the
expression in lengthKind/occursCountKind, and just output whatever data
happens to be in the info set. I'm not sure that it saves a lot of effort
in the implementation and it certainly is not easy to justify as a
consistent behaviour. For me, the unparser should treat
lengthKind='explicit' the same way whether the value is static or
calculated. And the unparser should treat occursCountKind='expression' the
same way as occursCountKind='fixed'. </tk>
When unparsing occursCountKind 'expression' you don't always have the
calculated array length N. If the infoset was derived from XML, there is
likely no 'count' element, just a bunch of elements with the same name
that make up the 'array'. DFDL gives you the choice whether to a) manually
set the count element, or b) have the unparser set it automatically via
outputValueCalc. In the former case, you can create a document that can
not be parsed; the unparser could check the 'count' element matches the
infoset, but that would involve reverse engineering an arbitrarily complex
expression and is why the specification does not say that.
<tk>It would involve evaluating the expression. In most cases, that will
not require any lookahead because the Length/Count field will precede the
array or element. Not sure where the reverse engineering comes in?</tk>
<smh>I see what you are saying. Just evaluate the expression and see what
it gives for N. That handles case a) but not b) where I explicitly want
the unparser to set the count via outputValueCalc - which is presumably
referring to the number of elements in the array, which is not known. For
case b) N has to be the number in the infoset. Given that we have to
support case b) the unparser can not treat occursCountKind 'expression'
exactly the same as 'fixed' when unparsing.</smh>
<smh>Similarly with lengthKind 'explicit' with an expression. For the
equivalent to case a) the length is known which makes the length fixed,
but for the equivalent of case b) with outputValueCalc the length is not
known so it is variable. When this was discussed in the past, it was
decided not to bifurcate the expression scenario. Hence the spec is the
way it is. </smh>
<tk>
That helps. So your belief is that case a) is workable but case b) is not
because the number of elements in the array is not known. I don't think
that holds up under scrutiny.
In case b), the outputValueCalc expression cannot be evaluated until all
of the array has been received. So if the info set is received as an event
stream then the unparser must wait until the array completes before
evaluating the expression. Note that the 'count' ( or 'length' ) field
cannot be serialized until its value is known. But the array ( or
variable-length field ) comes *after* the count/length field. By the time
unparsing of the array/field begins, the value *is* known.
The implementation of case a) is actually less straightforward. If the
field is an array length and the value is less than the number of items in
the info set then I think the unparser must issue an error. If greater
then the unparser could output default values ( if available ) or
delimiters ( if the parent sequence is a positional sequence with a
delimiter ) or else an error if neither are possible. More simply, the
unparser could simply insist that the value must correctly describe the
data, and I think that's a reasonable rule.
Similarly for the length. If the length of the *unparsed* value is greater
than the value in the info set then the unparser should issue an error. If
it is shorter and the field is simple then pad characters could be added.
But again, I think real-world usage of length counts suggests that padding
is unlikely to be wanted, and the unparser should simply insist on the
length field being correct.
</tk>
Here's a real example of such an expression (albeit with lengthKind
'explicit' but the principle is the same):
dfdl:length="{xs:nonNegativeInteger(fn:floor((../Length + 1) div
2))}"
Alex brought up the case where the expression evaluates to 0. In a
positional sequence, would you still expect a delimiter for this case?
<tk>Yes, unless it is in the trailing optional region of the group and
SSP='trailingEmpty'. In a positional sequence, every delimiter must be
present until suppression begins ( if allowed )</tk>
If 'yes' then the resultant zero length string must be treated as the
'absent representation' and ignored. If 'no' then is the sequence still
positional?
<tk>I don't understand the point. Why would it not be the 'empty
representation'? Why must it be 'ignored' if it does happen to be the
'absent representation'? What does 'ignored' mean?</tk>
<smh>The point is that the parser has been told there are 0 occurrences.
So it would be odd if the infoset ended up containing an occurrence, which
can happen if the normal nil/empty rules are followed. (Eg,
nilValue=%ES;).
<tk>
If the occursCount expression evaluates to zero then the parser will not
attempt to parse even one occurrence of the array. That's the natural
meaning of 'zero occurrences'. So nothing would go into the info set apart
from the 'count' field. This is entirely consistent with my definition b)
of 'positional' ( the identity of every delimited field is known before
parsing of the field begins ).
</tk>
Hence the 0 occurrence case must treat it as absent which means nothing is
added to the infoset. Take the ISO8583 bitmap use case - if the bit is 0
we must not try to parse anything at all for that element - it is totally
absent.</smh>
<tk>Yes - that's exactly my point. The fact that there is ( or could be )
a delimiter after the 'count' field is irrelevant.</tk>
Regards
Steve Hanson
Architect, IBM DFDL
Co-Chair, OGF DFDL Working Group
IBM SWG, Hursley, UK
smh at uk.ibm.com
tel:+44-1962-815848
From: Tim Kimber/UK/IBM at IBMGB
To: dfdl-wg at ogf.org,
Date: 10/06/2014 21:22
Subject: [DFDL-WG] Action 261
Sent by: dfdl-wg-bounces at ogf.org
Implied separatorSuppressionPolicy for occursCountKind 'expression '
(All)
10/6: Spec says it is 'never' (positional sequence) but you have to parse
to identify the position, so isn't that non-positional?
I think there are two alternative definitions of 'positional':
a) the identity of every delimited field is known before parsing of the
sequence group begins
b) the identity of every delimited field is known before parsing of the
field begins
As an implementer, b) is sufficient because it means that the parser never
needs to backtrack while parsing the group.
a) allows the field identities to be statically known, but that is less
important - it does not allow optimised extraction of a particular field
as would be the case for a fixed-length group ( the possibility of escaped
separators/terminators means that every character will need to be scanned
anyway ).
It may sound like a small point, but it affects two decisions
1. whether ock='expression' should be allowed within a positional sequence
group ( action 261 )
2. what the behaviour of the unparser should be w.r.t. ock='expression'.
My own feeling is that ock='expression' should be treated almost exactly
like ock='fixed', except that the calculated array length N is used
instead of maxOccurs.
- When parsing a positional sequence group it should cause N delimiters to
be expected for the array.
- When unparsing a positional sequence group it should cause N delimiters
to be written.
These rules are consistent and straightforward to describe and implement.
The current rule ( unparser outputs the occurrences that are in the info
set only ) allows the unparser to write a document that cannot be parsed
using the same schema.
regards,
Tim Kimber,
----- Forwarded by Tim Kimber/UK/IBM on 10/06/2014 20:34 -----
From: Steve Hanson/UK/IBM at IBMGB
To: dfdl-wg at ogf.org,
Date: 10/06/2014 17:57
Subject: [DFDL-WG] OGF DFDL WG Call Minutes 2014-06-10
Sent by: dfdl-wg-bounces at ogf.org
Please find minutes from the above call at
http://redmine.ogf.org/dmsf_files/13263?download=
Regards
Steve Hanson
Architect, IBM DFDL,
Co-Chair, OGF DFDL Working Group
IBM SWG, Hursley, UK
smh at uk.ibm.com
tel:+44-1962-815848 --
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