[DFDL-WG] Action 145: 'dispatch' way of discriminating a choice for better performance (updated)

[Mike Beckerle]

The whole point of this thing is to be faster, not more general, so my
reaction is too much XPath expression complexity here.

Consider this. If the tag is some 2-character code that does NOT want
to be the same as the element names (for example because they're
digits, so they can't be the element names exactly since element names
have to begin with an alpha char. Digits also aren't useful from
readability perspective as names), then you'll need a big lookup table
in the choiceBranchRef expression that translates from the codes to
the QNames. We don't have a case statement in the expression language,
so you've just moved the big linear evaluation chain out of evaluating
choice discriminators one after another and into a big if-then-else
nest in the choiceBranchRef expression. I don't see a performance gain
here.

I suggest dropping the QName stuff, and requiring a dfdl:choiceID
property on the elements that is an NCName.  (Well, we might want
those QName functions anyway in the expression language. But I
wouldn't use them for this rapid choice dispatch feature. You could
certainly use them in discriminators. )

The expression would then have to evaluate to a value that is matched
against this choiceID. I suggest exact match, not respecting
ignoreCase for example.

That eliminates all the QName complexity and is amenable to high-speed
compact lookup table implementation.

I tend to think the element names want to be a little bit more
descriptive than these tag values would want to be so using the
element names as the tags feels undesirable to me.
Particularly because we want the tags to be conveniently computed, for
example by just grabbing a fixed-length string out of a data field.

You end up with something like this:

    <element name="tag" type="string" dfdl:length="{ 2 }" ..../>
    <choice dfdl:choiceBranchRef="{ ../tag }">
        <element name="someName"     dfdl:choiceID="02" .../>
        <element name="anotherName" dfdl:choiceID="73" .../>
        ....
   </choice>

As for the wild-card issue. I think we can finesse this. Consider this model:

<element name="tag" type="string" dfdl:length="{ 2 }" ..../>
<choice>
    <!-- fast dispatch for known record types -->
    <choice dfdl:choiceBranchRef="{ ../tag }">
        <element name="someName"     dfdl:choiceID="02" .../>
        <element name="anotherName" dfdl:choiceID="73" .../>
        ....
   </choice>

   <!-- wildcard -->
   <element name="extensionRecord">
      <complexType>
        <sequence>
           <!-- keep tag copy in the extension -->
           <element name="extensionType" type="string"
dfdl:inputValueCalc="../../tag" .../>
           ....
        </sequence>
     </complexType>
  </element>
</choice>

The inner choice uses the fast dispatch. The outer choice lets me also
have an alternative that absorbs a more general syntax to provide some
way for a user to model their extensions to the choice set. The
extensionType field captures the tag and stores it inside the
extension record where it won't get disassociated.

The user's "extensionRecord" would not be a special DFDL wildcard
construct, just an element format they create which is general enough
to parse their extensions. Or this extension could be predefined as
part of a standard, to accept any standard-defined acceptable
extension record a user might need so long as there is some set of
rules all extension records must obey.

Given this, do we really need special wildcard constructs?

...mikeb

On Wed, Mar 21, 2012 at 10:10 AM, Steve Hanson <smh at uk.ibm.com> wrote:
> 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 string. XPath 2.0 provides
> such a function, xs:QName(), it's just not in the DFDL subset today. The
> string must be a lexical QName, ie, <prefix>:<name> and the prefix must be
> bound in what XPath calls the 'static context'.
>
> - DFDL should also include the XPath 2.0 function fn:QName() in its subset.
> This creates 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
>
>
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-- 
Mike Beckerle | OGF DFDL WG Co-Chair
Tel:  781-330-0412