[DFDL-WG] lengthKind='prefixed' clarification needed
Steve Hanson
smh at uk.ibm.com
Thu Oct 27 05:29:43 CDT 2011
Hi Mike
I agree that forcing someone to put dfdl:alignment on the element when
they really want it on the simple type is not good - but the spec does not
imply that today. Unless I am missing something I can write:
<simpleType name="string" dfdl:lengthKind="prefixed"
dfdl:prefixLengthType="stdStrPrefixType" dfdl:alignment="2"
dfdl:alignmentUnits="bytes" >
<restriction base="xs:string" >
<maxLength value='65535'/>
</restriction>
</simpleType>
<simpleType name="stdStrPrefixType" dfdl:representation="binary">
<restriction base="xs:unsignedShort"/>
</simpleType>
The properties of simple type "string" and the element are combined. This
alignment is applied before the prefix length is consumed, as per the
grammar. This seems fine to me. Taking a (hypothetical) example, if I was
modelling a PL/1 var char then I know that all var chars are aligned on
half-word boundaries, so I put the alignment on the type for the var char.
Have I misunderstood the problem?
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:
Mike Beckerle <mbeckerle.dfdl at gmail.com>
To:
Steve Hanson/UK/IBM at IBMGB
Cc:
dfdl-wg at ogf.org, Tim Kimber/UK/IBM at IBMGB
Date:
26/10/2011 16:02
Subject:
Re: [DFDL-WG] lengthKind='prefixed' clarification needed
Oops, a nit: I missed dfdl:outputValueCalc which should also be in the
list of things that cause a schema def error.
On Wed, Oct 26, 2011 at 10:55 AM, Mike Beckerle <mbeckerle.dfdl at gmail.com>
wrote:
Ok, I have a smaller simpler proposal to fix this which is really a very
small spec change The total wording of this would be these additional
errata:
Errata N Section 12.3.4 dfdl:lengthKind 'prefixed'
Replace phrase: "It is a schema definition error if the xs:simpleType
specifies dfdl:lengthKind 'delimited' or 'endOfParent' or a
dfdl:outputValueCalc" with "It is a schema definition error if the
xs:simpleType specifies dfdl:lengthKind 'delimited', dfdl:lengthKind
'endOfParent', dfdl:initiator, dfdl:terminator, dfdl:leadingSkip, or
dfdl:trailingSkip.
Errata M Section 9.1 DFDL Data Syntax Grammar
Replace "PrefixLength = SimpleContent" with "PrefixLength =
LeadingAlignment SimpleContent"
Today's errata on nested prefix length types would remain as errata.
The first new errata is important clarification, and the schema def error
is the conservative thing to do for the future. We have followed this
principle many other places in the spec.
The second new errata fixes a below-described issue with alignment and
information hiding/composition properties. It is, in some sense a very
small change to the spec to fix a rather glaring composition problem.
Here's the rationale:
To me lengthKind='prefixed' ought to handle the variable-length string
case like this:
In the schema for the specific application's data:
...
<element name="lname" type="fmt:string"/> // note use of string, but from
different namespace
<element name="addr1" type="fmt:string"/>
<element name="addr2" type="fmt:string"/>
<element name="postal" type="fmt:string"/>
...
Then, over in a different schema file.... which defines the namespace
bound to fmt above as its target namespace....
<simpleType name="string" dfdl:lengthKind="prefixed"
dfdl:prefixLengthType="stdStrPrefixType">
<restriction base="xs:string" >
<maxLength value='65535'/>
</restriction>
</simpleType>
<simpleType name="stdStrPrefixType" dfdl:alignment="2"
dfdl:alignmentUnits="bytes" dfdl:representation="binary">
<restriction base="xs:unsignedShort"/>
</simpleType>
The highlight here is that the alignment restriction is most naturally
expressed on the simpleType, the details of which are not near the string
itself's element declaration. The spec specifically allows this placement
on the simpleType for the dfdl annotations right now. It's only the
grammar that implies it would be ignored, as would initiator, terminator,
leading and trailing skip all be ignored.
To me the above pattern/idiom, where the format annotation cruft is
isolated on type definitions, is generally to be encouraged. THere's a
composition property I'm trying to preserve here, which is that you can
put two things side by side without having to worry about whether you are
meeting one or the other's alignment requirements. The alignment goes in
the package with the definition of the thing.
I don't think you should have to write:
...
<element name="lname" type="fmt:string" dfdl:alignment='2' />
<element name="addr1" type="fmt:string" dfdl:alignment='2'/>
<element name="addr2" type="fmt:string" dfdl:alignment='2'/>
<element name="postal" type="fmt:string" dfdl:alignment='2'/>
...
To me that violates a valuable principle of information hiding. I can hide
the type, but not hide its alignment requirements? Anywhere you can
abstract and hide a SimpleContent item that can be binary numeric where
alignment is a common requirement, you need also to be able to hide its
alignment requirements along side it so that using it won't be in the
error prone situation where the user can misalign it. PrefixLength is the
one place (That i know of) where we're violating this principle. It's an
easy omission, and an easy fix.
...mikeb
On Wed, Oct 26, 2011 at 6:15 AM, Steve Hanson <smh at uk.ibm.com> wrote:
Mike
Length kind 'prefixed' was intended to handle the case where the length is
tightly bound to the data, ie, there is nothing between the length and the
data. For example a PL/1 var char or ASN.1 BER. If the length causes the
length/data to be aligned then that has to be taken into account on the
element itself. Length kind 'prefixed' was not intended to cover more
complex cases where the length itself has independent alignment or there
are delimiters involved. For those you use length kind 'explicit' and an
expression. Otherwise the combinations become too complicated. If we wish
to extend 'prefixed' to include the more complex cases, I think that is a
post 1.0 thought and is best handled using a different length kind enum.
You say that ignoring the alignment property on the simple type used for
the length is strange, but if you allow that there is no way to align the
element's actual data separately. I think that it is even stranger.
The ASN.1 BER description at
http://en.wikipedia.org/wiki/Basic_Encoding_Rules describes how the length
itself can have a prefix (see sub-section 'Length').
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:
Mike Beckerle <mbeckerle.dfdl at gmail.com>
To:
Tim Kimber/UK/IBM at IBMGB, dfdl-wg at ogf.org
Date:
24/10/2011 02:16
Subject:
Re: [DFDL-WG] lengthKind='prefixed' clarification needed
Sent by:
dfdl-wg-bounces at ogf.org
Definintely need an agenda slot to discuss this matter.
I think we should redefine PrefixLength to allow it to have framing.
There is a significant issue which is that some prefixLengthTypes will be
multi-byte binary integers (typically 2 or 4 bytes), and these commonly
require alignment to a 2 or 4 byte boundary, as that's how the data
structures they live in would have been laid out.
The spec currently doesn't allow prefixLengthTypes to be aligned
themselves, because the grammar has them as SimpleContent, without the
surrounding ElementLeftFraming and RightFraming. This is also why they
cannot have lengthKind='delimited'. Because there are no initiator nor
terminator regions surrounding them. So the only way they can be aligned
is if the elements that have these prefixLengths are themselves aligned
properly.
However, if you specify alignment on a simple integer type, use it as a
prefixLengthType, and then that alignment annotation is *ignored* that
would seem strange, and buggy/hard-to-diagnose.
However, scoping rules for properties don't provide any way for this
alignment to get "into the scope chain", and I'd hate to start messing
with the scoping rules because of the corner case of prefixLength. We'd
need to put another scoping rule in just to handle this. I'd rather not go
there. Lots of our examples in the spec would have to change as they use
alignment as the example property...
But, the spec is not self-consistent, as the dfdl:alignment property can
be placed on a simple type definition, as well as on an element. So it
would seem a prefixLengthType could reference an aligned simple integer
type, but neither the grammar nor the scoping rules allow for using this
alignment property to control anything. Similarly, you can put an
initiator on a simple integer type, use it as a prefixLengthType, and have
the initiator be ignored.... because there is no initiator region for a
PrefixLength.
We need to fix this inconsistency.
I think prefixLengthType needs to be alignable, and one should be able to
specify alignment on a type definition, not just on an element.
I also think we're better off with a uniform general fix here, than a
handful of special case rules around prefix lengths. (E.g., the
prefixLengthType cannot have alignment, cannot have initiator/terminator
or lengthKind delimited warning or SDE if it does, etc. etc.)
So I think the grammar is wrong. I think
PrefixLength = SimpleElement
(where SimpleElement = ElementLeftFraming SimpleContent RightFraming )
is the right definition.
In working through examples, I'm convinced the current spec is
problematic. In the current spec one must model a 4-byte aligned binary
integer prefix length as a separate element (so that you can align it),
and use lengthKind='explicit' on the thing it controls. This is a lot of
hassle for a very common situation. The whole point of
dfdl:lengthKind='prefixed' is to provide an easier way to model the common
cases.
For the same reason there is no alignment, the definition of
dfdl:prefixLengthType says the named type cannot have
lengthKind='delimited'. That is because the DFDL grammar defines the
prefixLength region to be SimpleContent which is without any of the
surrounding framing regions where delimiters are found.
So, one cannot for example, put an initiator and terminator on the prefix
length type so as to have syntax separating it from the actual content.
Even if it is fixed length you can't do it - Like you cannot model this
data as 3 string elements using prefix length:
(11)9 Ocean Way(20)Southwest(SW) Harbor(02)ME
(Notice in the above the unescaped "(SW)", which is why this is not a
delimited format.)
You also cannot do:
11(9 Ocean Way)20(Southwest(SW) Harbor)02(ME)
because that puts the initiator of the string element itself after its
prefix length region, which is backwards from the way we have it in the
grammar currently. Both of the examples above require use of a separate
element and lengthKind="explicit" to pull off, even though they seem like
fairly natural ways to textualize a binary format.
Now consider
xx9 Ocean WayxxSouthwest(SW) HarborxxME
where the "xx" is a 16 bit (2 byte) binary integer holding the lengths 11,
20, and 2 respectively.
Except....That is, so long as the "xx" doesn't need to be on a 2-byte
alignment, because in my example the first element occupies 13 bytes
including the prefix itself, so the next "xx" starts on an odd boundary.
I could specify alignment on each of the 3 elements of my sequence here,
which is unmotivated/weird since they're string elements and their type
may be distant from where the elements are declared, so the motivation for
the alighment may not be clear....... the alignment constraint really
wants to be expressed on the prefixLengthType, and the dfdl annotation
syntax lets you specify alignment there, ... it just doesn't use it.
If we just redefine PrefixLength as SimpleElement, now all the example
formats above are easily modeled in the obvious way, and even the
combinations of text and binary lengths can be done naturally, as a binary
prefixLengthType integer type can have all the usual constraints binary
data likes to have, like alignment.
Even the 2-level ASN.1 wierd case "prefix-length of the prefix-length"
(see errata 2.13) works because ElementLeftFraming itself includes
PrefixLength. I believe we should put an explicit depth limit of 2 on this
however.
(Side note: I'd like to see an example of the ASN.1 format that supposedly
requires this nested prefix of the prefix situation.)
Changing the grammar in this way lets us drop the special case handling
around prefixLength where it can't have lengthKind="delimited" and ignores
initiators and terminators and alignment which is a bunch less special
cases to have to implement and test, and create special warnings for
(e.g., "Warning: prefixLengthType 'lenType' has alignment property which
will be ignored.")
If we want to be more minimal about the changes, just changing
PrefixLength = ElementLeftFraming SimpleContent RightFraming
is sufficient and achieves the fix of the real problem.
(This also eliminates the need for current errata 2.13 and 2.14, or rather
replaces those errata with this new stuff.)
...mikeb
--
dfdl-wg mailing list
dfdl-wg at ogf.org
http://www.ogf.org/mailman/listinfo/dfdl-wg
From:
Tim Kimber/UK/IBM at IBMGB
To:
Mike Beckerle <mbeckerle.dfdl at gmail.com>
Cc:
dfdl-wg at ogf.org, dfdl-wg-bounces at ogf.org
Date:
23/10/2011 21:12
Subject:
Re: [DFDL-WG] lengthKind='prefixed' clarification needed
Sent by:
dfdl-wg-bounces at ogf.org
Hi Mike,
I have always assumed that it works like this:
The Prefix region includes leading alignment, leading skip and initiator
The Content region contains the data, and the lengthKind property
describes how to determine the content length
The Suffix region includes Terminator and trailing alignment.
The lengthKind property describes the content region, and is not examined
until the Content region is reached. So the element's iniitator, if
defined, is not included in the length described by the prefix length.
If you view the prefixed length as describing the length of the *element*
(i..e its entire representation ) then this definition is not intuitive.
But I have always viewed lengthKind='prefixed' as being like the other
lengthKinds - it describes the length of the element's *content*.
So it's a consistent definition, but is it useful? I think so. In my
experience, prefixed lengths tend to be applied to complex elements (
structures ) rather than simple values. In such cases, the content of the
complex element will always be either a sequence group or a choice group,
and any initiator/terminator can be located on that group..
regards,
Tim Kimber, Common Transformation Team,
Hursley, UK
Internet: kimbert at uk.ibm.com
Tel. 01962-816742
Internal tel. 246742
From: Mike Beckerle <mbeckerle.dfdl at gmail.com>
To: dfdl-wg at ogf.org
Date: 22/10/2011 19:12
Subject: [DFDL-WG] lengthKind='prefixed' clarification needed
Sent by: dfdl-wg-bounces at ogf.org
For agenda/issues list
With respect to lengthKind='prefixed'. I'm concerned that there's a
complex interaction with initiator/terminator.
Can we have a prefix length and an initiator and terminator as well? If so
which comes first, and if it's the prefix does the prefix length include
the length of the initiator and terminator?
The grammar as written in current draft of spec has the initiator first,
then the prefix, then the content, and then the terminator. I think this
is wrong. I mean we can make it work, but it's not a useful, nor intuitive
behavior.
If we're going to fix this, I think we should make prefixed an alternative
to initiator and terminator, so that you can't have both on the element.
The alternative is to change the order around. Because initiator and
terminator can each be lists of alternative choices, the only sensible
composition of prefixed with these has prefix length providing the length
of a syntax which includes static initiator and terminator fields, which
are sort of like static padding to be trimmed off the string before
extracting the value.
E.g., prefix length of 10 preceeding these characters: [[123456]]
<element name='x' type='int' dfdl:initiator="[[" dfdl:terminator="]]"
dfdl:lengthKind='prefixed' .../>
But,....this is obscure enough that I'd rather make prefix length
exclusive of initiator/terminator. I.e. Schema Def Error if both are
specified.
Rationale: Even if such formats are possible, and even if they do exist
somewhere, it's possible to model this format differently with hidden
fields, lengthKind='explicit' etc., so it's not like removing this complex
interaction of prefix with initiator/terminator reduces DFDL's expressive
power in any way.
Summary: To reduce complexity, suggest that lengthKind='prefixed' is
exclusive of both initiator and terminator properties directly on the same
element. Schema Definition Error if both are specified.
--
Mike Beckerle | OGF DFDL WG Co-Chair
Tel: 781-330-0412
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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
--
Mike Beckerle | OGF DFDL WG Co-Chair
Tel: 781-330-0412
--
Mike Beckerle | OGF DFDL WG Co-Chair
Tel: 781-330-0412
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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