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:
The plan calls for subsequent versions of the specification, including the following items with status:
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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