[DFDL-WG] DFDL: Minutes from OGF WG call, 23 Jan 2007 *CORRECTED*

[Ian W Parkinson]

A small correction. with thanks to Simon - it was Steve (rather than 
Simon) who had previously attracted a reasonable audience at the OGF 
conference.

Ian



Open Grid Forum: Data Format Description Language Working Group

Weekly Working Group Conference Call
17:00 GMT, 23 Jan 2008


Attendees
Mike Beckerle (Oco)
Simon Parker (PolarLake)
Ian Parkinson (IBM)
Alan Powell (IBM)

Apologies
Steve Hanson (IBM), Suman Kalia (IBM)

1. OGF22
The DFDL session at OGF22 is now booked for the Monday afternoon, and Mike 
has registered to attend. Mike will present our updated status, and Alan 
promised to upload the last set of presented slides to GridForge so that 
Mike can update them. Alan asked whether we should attempt to drum up 
interest in the DFDL session to encourage attendence; Simon thought that 
advertising may not make much difference and that Steve had a reasonable 
audience when he presented.

2. Specification drafts
Steve and Alan had previously assigned ownership of individual items from 
Mike's plan of contents for the next few drafts. Alan will assemble the 
next draft, due at the end of the month, and asked for input as soon as 
possible.

Looking at the plan for the next, "vX+1", draft, the group reported the 
following status:
Nulls/default/optionals - Mike reported no update.
Description of schema components - Simon is still working on this.
Regular expressions for lengths - Alan reported no progress.
Expression language - Alan will shortly distribute a new version of the 
proposal for review.
valueCalc - Mike is still to write this.
Property precedence - Following a discussion on the call last week, please 
provide review comments. Mike will add this to the agenda for next week.
Entities - Alan's recent proposal is to be discussed on the current call.
White space handling - Discussion is ongoing, and Steve is to make a 
proposal.

The plan calls for subsequent versions of the specification, including the 
following items with status:
Supplements - Steve is working to update the supplements
Speculative parsing - IBM has internally been discussing and reviewing WTX 
function, though no documentation presently exists covering this.

3. UML diagrams
Simon is revising the UML diagrams which describe the DFDL schema 
components. The previous meeting minutes included a number of comments on 
these diagrams, and the group took this opportunity to look at some of 
those comments:

"...I think it would be better to use the open source XML schema model as 
source model and show relationship of DFDL Annotations attached to the XSD 
schema model" - Mike noted that DFDL makes use of annotations on objects 
which are absent from the XSD schema model, and hence that it may be 
unnatural to base the DFDL schema model directly on the XSD model. Simon 
suggested that it would be cleanest to describe a modified version the XSD 
model including those XSD elements that we need to annotate, and use this 
as a basis for the DFDL model.

"The current diagram suggests that 'variable definition' can both be part 
of a format base or as a standalone annotation (outside of a format). Is 
this true?" - Mike suggested that variable definitions don't have to be 
part of a format block: so, yes, this is true.

Mike agreed to respond further to the set of comments by email.

4. Review of Entities proposal
Alan has distributed a proposal covering entities in DFDL, intended to 
allow characters which are disallowed by XML1.0 (or XML1.1) to be included 
in DFDL schemas. These follow a similar syntax to XML, using % instead of 
& as an escape, with an additional mechanism for specifying raw data. This 
latter is intended to supplant the escaping mechanism described in current 
versions of the specification (which also uses % as an escape).

The group felt that the description of the raw data entities should not be 
cast in terms of characters and character sets, but rather in terms of 
bytes. If treated as characters, schemas may need to be written when 
moving from single-byte to double-byte character sets; further, this 
incorrectly implies some codepage conversion is involved.

The proposal also introduces a list of predefined names for certain common 
control characters. Mike asked whether these are the existing XML names - 
Alan replied that XML does not define names for control characters. 

Ian asked how we should represent the literal % character in strings given 
this form of escaping. The present draft of the specification uses "%%" to 
handle this; Simon suggested a string like "%pc;". The meeting felt that 
%% might be marginally preferable.

Finally, the proposal defines some labels which aim to reduce the 
complexity of dealing with whitespace and newlines. The %NL; entity 
represents a newline on "the target platform" - Mike observed that DFDL 
presently does not have a concept of a target platform. Alan felt it 
important that a single DFDL schema be able to generate output documents 
targetted at different platforms. Mike proposed that we introduce a new 
property, "generatedNewLine", which describes the meaning of %NL; during 
unparse, and that %NL; should be tolerant of any common new line 
representation during parse. The group discussed whether this could 
instead be handled using a list of optional new line values, however this 
would not support schema portability. Simon suggested we introduce another 
new property to mean that %NL; should be the conventional new line 
representation on the platform on which an engine is running, however Mike 
pointed out that this simply requires appropriate configuration of the 
generatedNewLine property.

%WSP; and %OWSP; are introduced to mean any whitespace, and optional 
whitespace. This will be useful in describing some formats which allow 
arbitrary whitespace, such as MIME. Mike pointed out that we could model 
such whitespace using hidden fields, but that these entities may make a 
schema clearer. PolarLake have found that only one such label is 
necessary, which means, "one or more whitespace characters", and that this 
needs only to be made available as a delimiter - Mike agreed that this 
label may represent a special type of delimiter rather than a general 
purpose entity. Alan would like to work through the potential use cases to 
see if we can restrict it in this fashion, and will update the proposal to 
specify that these relate to just one character. Simon suggested we could 
introduce an extra label, perhaps %WPS*; to match multiple whitespace 
characters.

Meeting closed, 18:15


Ian Parkinson
WebSphere ESB Development
Mail Point 211, Hursley Park, Hursley, Winchester, SO21 2JN, UK





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