Hi Mike
I agree. When I wrote the SWIFT example
expression that I realised this was becoming a bit too complicated. IBM
MRM has a quick way of dispatching a choice that uses a similar mechanism
to the one you propose.
It has an optional property called 'Message
Alias'. The parser first tries to match the tag against the element name,
and if that fails it tries the alias. I've modified my proposal accordingly.
Regarding wildcards, the approach you
show was what we had agreed upon a couple of years ago when we dropped
wildcards. I would prefer not to add them back in for 1.0, but I have had
some feedback from IBMers modeling envelope/payload formats that they want
a loose-coupling approach as that fits better with other tools and is scalable.
Let's treat this as a separate issue. I'll go back to the IBMers so that
I fully understand their concerns, and if necessary I will bring back to
the WG as a new agenda item. So all we need to ensure for now is
that whatever mechanism we come up with is extensible to wildcards, if
needed.
Revised details:
A new dfdl:element property is added
called dfdl:elementAlias of type xs:NCName. This provides an alternative
name for the element. Only allowed on local element and global element,
not on element reference or simple type.
A new dfdl:choice property is added called dfdl:choiceBranchRef of type
DFDL Expression. The expression must evaluate to xs:NCName. The string
must match either the element name or the dfdl:elementAlias property of
one of the element branches of the choice, and if so discriminates in favour
of that branch. The parser then goes straight to that branch, ignoring
schema order. Because the branch is 'known to exist' no backtracking takes
place if a processing error occurs.
Rules:
- Both properties behave 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.
- Both properties are only used when parsing.
- When dfdl:choiceBranchRef is preent, all choice branches must be local
elements or element references. It is a schema definition error otherwise.
- It is a processing error if the resolved xs:string value of dfdl:choiceBranchRef
does not match one of the branches.
- It is a schema definition error if a choice has both dfdl:choiceBranchRef
set and dfdl:initiatedContent="yes".
So we now have the ability to apply a simple lookup 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.
Questions:
- What happens if we encounter a discriminator
once we are processing the branch and its point of uncertainty is the choice
?
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
----- Forwarded by Steve
Hanson/UK/IBM on 27/03/2012 13:41 -----
From:
Mike Beckerle <mbeckerle.dfdl@gmail.com>
To:
Steve Hanson/UK/IBM@IBMGB
Cc:
dfdl-wg@ogf.org
Date:
21/03/2012 15:57
Subject:
Re: [DFDL-WG]
Action 145: 'dispatch' way of discriminating a choice for better performance
(updated)
Sent by:
dfdl-wg-bounces@ogf.org
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
| From:
| Steve Hanson <smh@uk.ibm.com>
|
| To:
| dfdl-wg@ogf.org,
|
| Date:
| 24.03.2012 02:27
|
| Subject:
| [DFDL-WG] Action 145: 'dispatch' way
of discriminating a choice for better performance |
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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Registered office: PO Box 41, North Harbour, Portsmouth, Hampshire PO6
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3AU