The enveope/payload style of data format is quite common, where the envelope provides control information and the payload contains the business data. Examples are SWIFT and SAP IDocs. Typically the envelope contains a tag that identifies the payload, which can be one of many types. For SWIFT there are 300 possible types. To model this today in DFDL requires an xs:choice with each type modeled as an xs:element branch of the choice. A discriminator on each xs:element refers back to the envelope tag element thus enabling the choice to be resolved.

There are two issues with this approach.

1) Performance. Even if the elements in the branches are ordered for expected frequency, there will still be cases when tens or hundreds of discriminators need to be evaluated before the choice is resolved.

2) Tight coupling. When a new type is added, a new element branch needs to be added to the choice.

Action 145 proposes a mechanism to solve issue #1 and which opens the door to a possible extension to DFDL to solve issue #2 - namely a faster way to resolve a choice.

Details:

A new dfdl:choice property is added called dfdl:choiceBranchRef of type DFDL Expression. The expression must evaluate to a QName which corresponds to one of the element branches of the choice, and asserts 'known to exist' for that branch.  Rules:

- The property behaves like dfdl:ref and dfdl:hiddenGroupRef in that it is not possible to set a value in scope by a dfdl:format annotation, and is only set at its point of use. This is because there is nothing sensible that could be set in scope. But it has the benefit that adding support for the property to existing DFDL implementations will not suddenly cause errors to appear in existing DFDL schemas.

- Empty string is not an allowed value.

- The property is only used when parsing.

- All branches must be local elements or element references. It is a schema definition error if any branch is a sequence, a choice or a group reference.  

- It is a processing error if the QName does not resolve to one of the branches when parsing..

- It is a schema definition error if a choice has the property set and also has dfdl:initiatedContent="yes" set locally.

- Because the expression must return a QName, the expression language must provide a constructor for creating a QName from a namespace string and a name string. If you take SWIFT MT103 payload as an example, the tag in the envelope says "103" but a DFDL schema would actually model the global MT103 element with name "Document" and namespace ="urn:swift:xsd:fin.103.2011".
So the dfdl:choiceBranchRef expression would have to look like:
{fn:QName(fn:concat(fn:concat('urn:swift:xsd:fin.', FinMessage/Block2/MessageType), ".2011"), 'Document')}

So we now have the ability to derive a QName and apply it before we start to process a choice. That makes the processing time for each branch of the choice independent of its order in the schema.

We still have issue #2 so when a new payload is added, a new branch must be added to the choice. A solution to this is to allows xs:any wildcard elements back into DFDL, then provide a property dfdl:wildcardRef which works in the same way as dfdl:choiceRef. So at the point of encountering the wildcard we know its resolution in the schema.  This obviously will require some further discussion, but you can see how this ability to evaluate an expression and return a QName can be used in multiple ways.

Regards

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



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