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:

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:

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@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@IBMGB, Alex Wood1/UK/IBM@IBMGB, Andrew Edwards/UK/IBM@IBMGB, Mark Frost/UK/IBM@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@uk.ibm.com
Tel. 01962-816742  
Internal tel. 37246742




From:        Steve Hanson/UK/IBM
To:        Tim Kimber/UK/IBM@IBMGB,
Cc:        dfdl-wg@ogf.org, dfdl-wg-bounces@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@uk.ibm.com
tel:+44-1962-815848




From:        Tim Kimber/UK/IBM@IBMGB
To:        dfdl-wg@ogf.org,
Date:        11/06/2014 13:58
Subject:        Re: [DFDL-WG] Action 261
Sent by:        dfdl-wg-bounces@ogf.org



comments in <tk>tags
regards,

Tim Kimber,
IBM Integration Bus Development (Industry Packs)
Hursley, UK
Internet:  kimbert@uk.ibm.com
Tel. 01962-816742  
Internal tel. 37246742




From:        Steve Hanson/UK/IBM
To:        Tim Kimber/UK/IBM@IBMGB,
Cc:        dfdl-wg@ogf.org, dfdl-wg-bounces@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@uk.ibm.com
tel:+44-1962-815848




From:        Tim Kimber/UK/IBM@IBMGB
To:        dfdl-wg@ogf.org,
Date:        10/06/2014 21:22
Subject:        [DFDL-WG] Action 261
Sent by:        dfdl-wg-bounces@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@IBMGB
To:        dfdl-wg@ogf.org,
Date:        10/06/2014 17:57
Subject:        [DFDL-WG] OGF DFDL WG Call Minutes 2014-06-10
Sent by:        dfdl-wg-bounces@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@uk.ibm.com
tel:+44-1962-815848 --
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Unless stated otherwise above:
IBM United Kingdom Limited - Registered in England and Wales with number 741598.
Registered office: PO Box 41, North Harbour, Portsmouth, Hampshire PO6 3AU--
 dfdl-wg mailing list
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Unless stated otherwise above:
IBM United Kingdom Limited - Registered in England and Wales with number 741598.
Registered office: PO Box 41, North Harbour, Portsmouth, Hampshire PO6 3AU