[DFDL-WG] How to choose the correct choice branch when serializing

[Steve Hanson]

Hi Tim

I've made some minor corrections to your summary of the problem.

If the user restructures his model to wrap the sequences in elements then 
the problem goes away.  So I think we should keep the solution to this as 
simple as we can while not being unnecessarily restrictive. 

Regards

Steve Hanson
Architect, Data Format Description Language (DFDL)
Co-Chair, OGF DFDL Working Group
IBM SWG, Hursley, UK
smh at uk.ibm.com
tel:+44-1962-815848



From:   Tim Kimber/UK/IBM at IBMGB
To:     dfdl-wg at ogf.org
Date:   13/04/2012 10:54
Subject:        [DFDL-WG] How to choose the correct choice branch when 
serializing
Sent by:        dfdl-wg-bounces at ogf.org



There is an interesting edge case which arises when the serializer 
encounters a choice group. 
A DFDL xsd is structured as follows: 

<root> 
    <choice> 
        <sequence> 
            <firstname/> 
            <lastname/> 
            <postcode/> 
        </sequence> 
        <sequence> 
            <lastname/> 
            <telephoneNumber/> 
        </sequence> 
    </choice> 
</root> 

Note that both branches of the choice are sequences, not elements. 

The infoset is 

<root> 
    <lastName/> 
    <telephoneNumber/> 
</root> 

The likely action of the serializer is: 
- pick the first branch of the choice ( because it contains lastname ) 
- output the default value of firstname ( assuming that firstname has 
minOccurs = 1 and has a default ) 
- output lastname 
- issue a processing error because telephoneNumber is found in the info 
set but is not in the first branch. 

...but from the infoset the user clearly intended: 
- select the second branch of the choice and successfully process the 
entire info set 

The DFDL specification does not state what the behaviour should be. I 
think the options are: 
a) state explicitly that the serializer will choose the first branch that 
contains a matching element, regardless of minOccurs 
b) invent a new rule that causes the parser to back out of a branch and 
try another branch if there is a minOccurs error while processing the 
branch 
c) disallow sequences and choices as immediate children of a choice group 

Currently I'm leaning toward a) by process of elimination, for the 
following reasons: 
b) would make this scenario work, but I think it would impose a lot of 
work on implementers because it would require the serializer to do 
backtracking. 
c) would simplify a lot of things, but I think it's too restrictive - I 
can imagine complex data formats where is might be useful to have a choice 
as the direct child of a choice because the discrimination rules might be 
easier to express in a two-level structure. 

regards,

Tim Kimber, Common Transformation Team,
Hursley, UK
Internet:  kimbert at uk.ibm.com
Tel. 01962-816742 
Internal tel. 246742





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