[DFDL-WG] New scoping rules

Wed Sep 30 06:06:03 CDT 2009

>From an IBM point of view, if DFDL is not at least as easy to use as WTX 
type trees or WMB MRM, then something has gone wrong.  It's the scoping 
rules that concern me most. 

The IBM WTX type tree model today does not have scoping rules. There is no 
equivalent of a defineFormat block. Every property you need for an object 
must be set on that object. An object can 'inherit' properties from 
another object, but that is a static inheritance performed at creation 
time, it is not dynamic. In other words the properties are copied once and 
once only. All properties have implicit model defaults.  There is no 
concept of local v global. Everything is global. When an object is used in 
a 'group', there is no overriding of properties at point of use.

The IBM MRM model today is annotated schema like DFDL, and has the 
equivalent of a defineFormat block, which must be referenced from all 
objects in the schema. The MRM Format block contains schema wide 
properties of two kinds: a) ones that are only at schema level and not on 
objects (eg, escape scheme, timezone, encoding, byte order), and b) ones 
that are at both schema and object level (eg, separator, terminator) and 
therefore act as defaults. The majority of MRM properties are not in the 
Format block, are only on objects, and therefore have implicit model 
defaults.  There are no MRM properties on simple types, so no issue about 
merging element/type properties. There is local v global, but only a 
handful of properties are able to be overridden. 

Both the above models have issues over their flexibility.  DFDL is 
intended to address these - but it should not do so at the expense of 
decreased usability.

I think we are all agreed that implicit defaults cause problems. We have a 
huge amount of code in the MRM model that sets defaults depending on the 
values of other properties, the simple type, and so on. It is a 
maintenance headache. It is much cleaner to push this back to the user, by 
saying that there are no defaults and you must supply a format block and 
ref it if you want defaults.  The only problem this causes is cited by 
Alan - that there are some properties where a default is obvious - eg, 
initiator default is that there is no initiator. Yet we force the user to 
set dfdl:initiator="".  The mitigation for this is that there will be a 
range of example defineFormat blocks available that people will invariably 
use. I am ok with that.

The requirements I get from MRM customers are that they want to set a 
default for a property for the whole schema, and for some properties to 
set a default that depends on logical type - for example pad character.  I 
have yet to see a requirement for setting defaults at the level of a 
complex type.

I think Suman is suggesting that dfdl:ref is made mandatory. I actually 
don't have a problem with that, as it's how MRM works today. But I think 
making it optional is better. At the end of the day, the user either 
specifies all properties needed by the object on the object, or he uses 
dfdl:ref plus one or more properties on the object.  If he wants to do the 
former, then let him. 

As a starter, I think we should consider dropping the use of dfdl:format 
on a complex type. If the user wants to pick up a set of defaults, he must 
use dfdl:ref. 

I think the proposed merging of element/simple type properties makes 
sense, and is the only sensible rule we can apply.

The sticking point is element/group references.

Regards

Steve Hanson
Programming Model Architect, WebSphere Message  Brokers,
OGF DFDL WG Co-Chair,
Hursley, UK,
Internet: smh at uk.ibm.com,
Phone (+44)/(0) 1962-815848

From:
Suman Kalia <kalia at ca.ibm.com>
To:
Mike Beckerle <mbeckerle.dfdl at gmail.com>
Cc:
dfdl-wg at ogf.org, dfdl-wg-bounces at ogf.org
Date:
29/09/2009 19:03
Subject:
Re: [DFDL-WG] New scoping rules
Sent by:
dfdl-wg-bounces at ogf.org

Mike -  We have to be pragmatic and ensure that vendors can provide 
efficient implementations for the specification in order for DFDL to take 
off.  All we required was to explicitly specify a ref property to global 
format on global complex and group definitions so we can validate the 
contents during the development phase rather than deferring to runtime 
phase where the problem determination becomes more complex and time 
consuming.  Even if we had the original proposal in place, we would have 
allowed the ref property to be specified on these global constructs in 
which case it would have overridden all properties in scope from the 
element or inherited from parent in case this global element was included 
through element reference.  You can view the current proposal as a 
restricted form of the original proposal. 

Also when we say referential transparency; your reference is to element 
declaration and group definition -> I think it is there to a large extent 
but what we have restricted is that the properties from element do not 
scope over its contents unless you explicitly model using variables which 
I think is creating lot more complexity based on the examples seen from 
last week. We may want to remove this from V1 specification. 

In my personal opinion, I think it is a reasonable restriction for V1 of 
the specification keeping in view the complexity of initial implementation 
(tooling and runtime).  We can relax these restrictions in the later 
version of the specification.   

We will have more discussion on this topic tomorrow.. 

Suman Kalia
IBM Toronto Lab
WMB Toolkit Architect and Development Lead
WebSphere Business Integration Application Connectivity Tools 

http://www.ibm.com/developerworks/websphere/zones/businessintegration/wmb.html

Tel : 905-413-3923  T/L  969-3923
Fax : 905-413-4850 T/L  969-4850
Internet ID : kalia at ca.ibm.com 

From: 
Mike Beckerle <mbeckerle.dfdl at gmail.com> 
To: 
Alan Powell <alan_powell at uk.ibm.com> 
Cc: 
dfdl-wg at ogf.org 
Date: 
09/29/2009 08:52 AM 
Subject: 
Re: [DFDL-WG] New scoping rules

Alan, 

I've done some thinking on the scoping, and I think we've talked ourselves 
into a bad position. 

>From the note on scoping: 
The proposal currently under consideration is: 
Schema objects inherit DFDL properties from a lexically enclosing 
xs:complexType or xs:group declaration 
DFDL properties on a referenced global schema object (except simpleTypes) 
cannot be overridden unless explicitly parameterized by the global object.
The above is problematic. This breaks referential transparency. 
DFDL properties explicity defined on an element and it's referenced 
simpeType are merged into a single set. It is an error is the same 
property is defined on both the element and simpleType. 
It must be possible to validate all global objects , except simpleTypes.
This last bullet is an unreasonable requirement, depending on how you 
define validity. This was put in to simplify a tooling requirement of some 
sort that I believe is likely not a good goal for us to accept. 
Validity can mean "is consistent", but should not require property 
specifications to be "complete". 
This is an area of some confusion in DFDL. We have stated that a schema 
must have "all required properties" specified, and that there is no 
defaulting of property values by implementations. The purpose of this is 
to avoid implementation-specific or platform specific assumptions from 
creeping in so that DFDL schemas are more likely to be portable. This 
statement has been misinterpreted in the following sense. Some have 
interpreted this as meaning that all properties that are defined in the 
DFDL spec must have values set in order for a schema to be "valid". But 
when stating the "all required properties" rule (largely at my 
insistance), this was definitely not my intention. Consider for example if 
a format is all text, and uses a single-byte character set encoding, then 
I claim that dfdl:byteOrder need not be specified as it will never be 
needed to interpret the data. The point of saying there are no defaults 
for property values is NOT to require dfdl:byteOrder to always be 
specified, it is to say that if the format requires dfdl:byteOrder - 
because it has binary multi-byte representations in it, or wide characters 
which have endianness, then dfdl:byteOrder must be specified by the 
schema, either directly, by an included schema referenced by the schema, 
or must be specified explicitly via some external mechanism - section 21 
of draft 035. The point is that the implementation cannot just say "there 
is an unstated default" in this implementation for dfdl:byteOrder based on 
the platform you are installed on. If an implementation were to do that, 
then the schemas usable with that implementation will not be portable for 
use with other implementations - something we are trying to avoid. 
The difference here is subtle but important. Section 22 of draft 035 is a 
place holder for some pre-defined include-files the inclusion of which 
will provide dfdl:defineFormat specifications for useful sets of 
properties. It is important for everyone to understand that including 
these in a DFDL schema is 100% optional, and is for convenience of 
obtaining consistent and meaningful sets of properties only. However, 
simple formats can be described without any inclusion of these at all. As 
another example: if a file contains only an array of binary floating point 
numbers, then no dfdl:encoding property is needed. Just a handful of 
properties are needed to parse/unparse such a file format, and those are 
the ones about binary floating point numbers, and in the case of an array, 
about multiple occurrences. 
Getting back to scoping and the validation of a global decl/def.... Upshot 
of all this: it means from the perspective of "validating" a global 
decl/def,  one can't have conflicting DFDL properties in a global type or 
element declaration, but properties can be unspecified/unstated also, to 
be provided by the way that global decl/def is used. 
If a top-level element declaration is incomplete in this style, then it is 
unsuitable for use as the document element of a data file/stream unless 
augmented by external information - something possible and which we 
discuss in chapter 21 (version 035) of the spec without giving specific 
mechanism. 
If a top-level element declaration is incomplete in this style, then it 
can be made complete by way of being used by reference from another point 
in the schema which surrounds it with a scope providing the needed 
properties, or which provides the needed properties directly at the point 
of reference. This preserves referential transparency, and makes the 
semantics of referential transparency be just plain textual substitution, 
which is the semantics in XML Schema in general. 
I believe total validity (Consistency AND completeness) for global 
decls/defs is not worth trying to achieve for the sake of a tooling goal. 
Tooling may have to be more sophisticated, but discarding referential 
transparency is not something we should do for the sake of simplifying 
some goal for tooling that isn't even clearly a requirement. 
A tooling "goal" might be to allow an interactive user to point at a 
schema anywhere and see a list of properties in effect at that point. 
Total validity (consistency and completeness) is required for a concrete 
answer to this. However, why do we think this tooling goal should be a 
requirement? The answer presented back to the user could be that some 
properties are "unspecified", while other properties have specific values. 
I don't see this as problematic. 
We carefully decided not to allow any lexical invocation of DFDL formats 
at top level in order to eliminate the issue of lexical closure for top 
level objects. This allows ordinary textual referential integrity to work. 
I.e., reference semantics is exactly that of textual substitution. This is 
very desirable, as it allows ordinary refactoring of DFDL schemas to share 
common decls/defs to work in the expected manner. 
To me this is very desirable, and is a primary composition principle which 
will allow creation of complex schemas from simpler parts. 

On Mon, Sep 7, 2009 at 11:55 AM, Alan Powell <alan_powell at uk.ibm.com> 
wrote: 

All 

Attached is the description of the new DFDL scoping rules. 

We did not discuss the rules for simpleType derivations so I have assumed 
that it uses the same rules as simpleType reference, namely that the 
properties are merged and there must not be any duplicate properties 
specified. 

I have removed most of the complicated examples as they no longer apply. 

Alan Powell

MP 211, IBM UK Labs, Hursley,  Winchester, SO21 2JN, England
Notes Id: Alan Powell/UK/IBM     email: alan_powell at uk.ibm.com 
Tel: +44 (0)1962 815073                  Fax: +44 (0)1962 816898

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