[DFDL-WG] Action 145: 'dispatch' way of discriminating a choice for better performance
Steve Hanson
smh at uk.ibm.com
Wed Mar 21 08:32:20 EDT 2012
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 at uk.ibm.com
tel:+44-1962-815848
Unless stated otherwise above:
IBM United Kingdom Limited - Registered in England and Wales with number
741598.
Registered office: PO Box 41, North Harbour, Portsmouth, Hampshire PO6 3AU
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