[DFDL-WG] DFDL: Minutes from OGF WG call, 13 Feb 2008

[Ian W Parkinson]

Open Grid Forum: Data Format Description Language Working Group

Weekly Working Group Conference Call
17:00 GMT, 13 Feb 2008


Attendees
Mike Beckerle (Oco)
Steve Hanson (IBM)
Suman Kalia (IBM)
Simon Parker (PolarLake)
Ian Parkinson (IBM)

Apologies
Alan Powell (IBM)


1. Specification Draft Status

Alan has distributed draft 31 of the DFDL specification. The meeting 
reviewed the plan for the next few versions of the drafts and updated it 
to reflect progress and revised target dates - a copy of the updated plan 
is attached to these minutes.
Simon's UML diagrams describing the schema components are now planned for 
inclusion in draft 32 but require further discussion. This discussion will 
be added to the agenda for the DFDL-WG call on 27th February, but Simon 
would appreciate comments via e-mail before then. These diagrams are 
intended to set a conceptual model for DFDL and to show where annotations 
may be attached, but will not be used more formally, e.g. to 
automatiically generate APIs.
The work on nulls/defaults/optionals is complete, except for some small 
details, which will be included in draft 32.
The 'valueCalc' work has been progressed but is not complete, and is also 
now targeted for draft 32.
Other items originally planned for draft 31 are complete and have been 
included in the draft.


2. Assertions, Discriminators and Choice

Suman has distributed an example showing the use of discriminators in 
choice constructs in DFDL schemas. 
The meeting discussed the distinction between discriminators and 
assertions - Mike described an assertion as simply a predicate which, if 
encountered within a choice, can cause backtracking. In contrast a 
succesful discriminator expression would lock the choice into a particular 
branch. If no discriminator matches, then the parse would fail - unless, 
as Simon pointed out, the choice itself was optional. Simon also suggested 
that the last branch of a choice could be left without a discriminator to 
act a a catch-all, but felt that the purpose of a discriminator should be 
more to help disambiguate between the possible branches rather than form 
such a "swtich" construct.

Steve asked about the timing attribute on assertions, and in particular 
whether we'd need a similar attribute for discriminators. Mike suggested 
that the timing attribute might have been included simply to make 
implementation easier; as without it an implementation would need to 
perform a significant amount of static analysis. The motivation for 
discriminators was to allow a choice to be resolved by data encountered 
before the choice. However as Steve and Suman thought there were use cases 
where a discriminator might need to refer to elements inside the choice, 
and so a timing option would be useful. Suman will prepare such an 
example, and Mike will schedule a further discussion of this topic.

Simon distributed an alternative example, showing the use of fixed fields 
instead of discriminators or assertions., which he felt might form a 
useful starting point for a full description of choice disambiguation.

(Steve left the meeting)


3. Presentation for next OGF conference

Simon suggested that Mike highlight the recent discussion topics of the 
working group, and items which have recently been added to the 
specification, and asked whether it would be useful to include the UML 
diagrams. Mike would like to display the diagrams and see whether they 
trigger a discussion amongst the delegates. He would also like to present 
work on variable markup and valueCalc.


Meeting closed, 18:00 GMT



Attachment: revised plan for specification drafts

Draft 31:
Improve (finish?) nulls/defaults/optionals (Mike, with input from Steve) - 
Done, apart from minor edit task
Expression language (Alan) - Done
Property precedence for parsing (Steve) - Done
Entities, including basic white space (Alan) - Done

Draft 32 ("vX+2"):
valueCalc (Mike) - Feb 27, 2008
Remaining aspects of null/default/optionals (Alan) - Mar 5, 2008
2-level description of schema components, including UML (Simon) - Feb 27, 
2008
Property precedence for writing (Steve) - Feb 15, 2008
Variable markup (Steve) - Feb 29, 2008
Regular expressions for lengths (Alan)
Bring supplements up-to-date (Steve) - Mar 7, 2008
Assertions, discriminators and choice, including discussion of timing 
option (Suman) - Feb 19, 2008
How speculative parsing works (combining choice and variable-occurence - 
currently these are separate) (TX person)
Reordering the properties discussion: move representation earlier, improve 
flow of topics (Alan)

Draft 33: ("vX+3"):
Escape schemes (Ian P) - Mar 21, 2008
String XML type (Ian P) - Mar 21, 2008
Variables (Mike)
Selectors (Suman) - Mar 3, 2008
Improvements on property descriptions (All - split TBD)
Envelopes and Payloads (Steve) - Mar 5, 2008

Extraneous to spec:
Develop Schema for DFDL xsd (Suman) - Mar 15, 2008
Develop Schema for Schema DFDL Subset xsd (Suman) - Mar 30, 2008 (might 
not be needed)



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





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