I've spent today catching up with the recent DFDL spec discussions around Simon's comments to v0.19. Some comments of my own on the content of these and previous call minutes.

- General principle: The eventual consumers of DFDL will be users the majority of whom will not be data modelling experts, that's certainly the experience at IBM. Most see data modelling as a black art and find it difficult. I think that an over-reliance on hidden elements is not going to go down well. I would err on the side of caution here, and only if we are convinced a property will be very rarely used should we remove it and replace by a hidden element.

- Leading/Trailing Skip Bytes is a property intended to handle the byte skipping added by compilers, over and above simple byte alignment rules. The formulae for setting the values is beyond the ken of users to set manually, it would invariably be done using an automated COBOL -> DFDL translator, etc. I would not be too troubled if that went 'hidden'.

'finalTerminatorCanBeMissing' property. The rules for interpreting what trailing markup actually means are complex and properties like this will almost certainly be needed. (Aside: For Mike's second example, though, where data of max length n is terminated by markup only if actual length < n, wouldn't that be better expressed using a regular expression? finalTerminatorCanBeMissing is too general, and could lead the parser to validly parse data where the terminator was accidentally omitted).

- Infix/prefix/postfix separators. I believe this should be retained. It's in IBM WTX (Mercator) and I frequently have to apologise for the absence of postfix in IBM MRM. When a user sees (eg) x,y,z it's easier for him to comprehend that the comma after z is a postfix separator rather than the terminator of the parent group.

- Simon had a comment on the removal of 'applies' which I haven't seen discussed ("I find this cumbersome. I suggest this alternative: drop ‘applies’ and ‘dfdl:format’, insist on ‘dfdl:sequence’ and friends instead, and add local variants like ‘dfdl:sequenceLocal’. For attribute shorthand, add boolean attributes with the same name: sequenceLocal=”true” (optional, default false)."). I don't follow, the use of 'applies' is orthogonal to whether you use dfdl:format or one of the specific elements such as dfdl:sequence.

Regards, Steve

Steve Hanson
WebSphere Message Brokers
Hursley, UK
Internet: smh@uk.ibm.com
Phone (+44)/(0) 1962-815848

Mike Beckerle <beckerle@us.ibm.com>
Sent by: dfdl-wg-bounces@ogf.org

14/08/2007 14:23

To	dfdl-wg@ogf.org, "Simon Parker" <simon.parker@polarlake.com>
cc
Subject	Re: [DFDL-WG] Minutes from 2007-08-08 Call

I forgot to clarify Simon's question on sp165.

This was the 'finalTerminatorCanBeMissing" property.

We considered the comment that this might be unnecessary.

Use case: file of text format. Each "record" in the file is terminated by a CRLF so sez the user. At the top level this file contains an array of these records.

The file might or might not have a CRLF at the end of the file because human beings might have edited the file with a text editor, and either inserted or neglected to insert this final CRLF.

We want the file format to be legal with or without the final CRLF; however, all prior CRLFs in the file must be present.

So how to express this:
1) CRLF is a terminator of the record
2) CRLF is an occursSeparator of the enclosing array, records have no terminator. We enclose the array in a sequence group where the array is followed by a hidden "optional" (minOccurs=0 max=1) element of fixed="CRLF" string value.

Choice (1) requires that we have finalTerminatorCanBeMissing

Choice (2) is just modeling the behavior that is required directly via hidden elements. This is tantamount to saying that this keyword is not worth having because there is a way to model it already. This is true of many keywords. If we deem this one too obscure, then we need to revisit many others. (Leading/Trailing Skip Bytes is a good example. Trivially represented by a hidden element). What are our criteria for inclusion? Up until now our criteria have been to include things that existing systems already have found a need for. However, existing systems don't have hidden field capability.

Note that this same missing final terminator issue can come up not only with End-of-data, but with any bounded size structure.

E.g., suppose we say that an array has occursUnits="bytes" and occursPath="874". Then it is 874 bytes long. The array elements can be terminated by a particular data. E.g., semicolon. For the same reasons as the CRLF example above, we want to be able to tolerate a missing final semicolon before the end of the 874 bytes. In effect the byte-length-limit creates an implicit "end-of-data" for a sub-stream consisting of just those bytes.

Conclusion: finalTerminatorCanBeMissing seems to be useful enough and comes up often enough that I think the keyword is worthwhile.

Implication: we should create a list of keywords or enumerated values for properties that we think are in the grey area where perhaps we want to drop them. Here's some candidates: byteOrderMarkPolicy, leading/trailingSkipBytes. Both these can be modeled readily as hidden elements. There are probably others.

Mike Beckerle
STSM, Architect, Scalable Computing
IBM Software Group
Information Platform and Solutions
Westborough, MA 01581
direct: voice and FAX 508-599-7148
assistant: Pam Riordan
priordan@us.ibm.com
508-599-7046

Mike Beckerle/Worcester/IBM

08/14/2007 08:40 AM

To	"Simon Parker" <simon.parker@polarlake.com>
cc	dfdl-wg@ogf.org
Subject	Re: [DFDL-WG] Minutes from 2007-08-08 CallLink

In conjunction with the annotated document these notes are clear, except for 'sp165'. Perhaps someone will recapitulate the discussion briefly at Wednesday's conference. I think only three annotations remain:

sp167 Absent and missing (expanded discussion on the wiki already)

This will be a major topic on a call.

sp172 separatorType="infix"

I'm happy to drop this strange stuff about separatorType=prefix or postfix and just say separator means infix. However, I would note that at least two major integration products (IBM WebSphere Transformation Extender - formerly Mercator, and Microsoft Biztalk, have this concept, so we may end up putting it back in. Presumably MS copied the earlier Mercator style, or both got it from common requirements in some EDI standard.

sp173 defaultWhenMissing (expanded discussion on the wiki already)

Same topic as sp167 above. Will have a call topic to discuss.

I've added another contribution to the wiki discussion on 'require'.

This seems to be at resolution I think, which is that we can express this using assertions. The general style of using DFDL to describe what fixed-data syntactic constructs look like is a good one.

However, I've amended the Wiki thread on this with a further issue for group consideration. See bottom of page:
https://forge.gridforum.org/sf/wiki/do/viewPage/projects.dfdl-wg/wiki/Require?_message=1187096164776

The 'length and occurs' proposal is an improvement, though I still have reservations to discuss; likewise the 'opaque data' proposal.

For a call, this week or soon. I will send out an agenda.

Mike Beckerle
STSM, Architect, Scalable Computing
IBM Software Group
Information Platform and Solutions
Westborough, MA 01581
direct: voice and FAX 508-599-7148
assistant: Pam Riordan
priordan@us.ibm.com
508-599-7046

"Simon Parker" <simon.parker@polarlake.com>
Sent by: dfdl-wg-bounces@ogf.org

08/13/2007 10:56 AM

To	<dfdl-wg@ogf.org>
cc
Subject	Re: [DFDL-WG] Minutes from 2007-08-08 Call

From: dfdl-wg-bounces@ogf.org [mailto:dfdl-wg-bounces@ogf.org] On Behalf Of Mike Beckerle
Sent: 08 August 2007 18:00
To: dfdl-wg@ogf.org
Subject: [DFDL-WG] Minutes from 2007-08-08 Call

MikeB, Geoff Judd, Alan Powell attended.

Continued through SP's comments.

sp37 - got it.

sp45 - agree. This whole part to be rewritten.

sp115 - ok. strict and "lax" as enums. No built-in default - we never use defaults in the processor itself. Only in the predefined formats.

sp118 - ok

sp123 - Proposal to simplify length, lengthKind, lengthUnits, and also occursKind, occursPath, occursPathUnits needed. (along the lines of byteCount, itemCount, length='delimited' enum, etc.)

sp154 - Need specific proposal to eliminate hexBinary and use what for opaque (consider also string with encoding='bytes'. ) Or introduce a dfdl:byteString type or dfdl:opaque type. (derived type - just a standard name).

sp158 - see sp123

sp165 - needed to have composition property for enclosing groups and or end-of-data. Regexp doesn't fix this.

Mike Beckerle
STSM, Architect, Scalable Computing
IBM Software Group
Information Platform and Solutions
Westborough, MA 01581
direct: voice and FAX 508-599-7148
assistant: Pam Riordan
priordan@us.ibm.com
508-599-7046-- dfdl-wg mailing list dfdl-wg@ogf.org http://www.ogf.org/mailman/listinfo/dfdl-wg
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