Mike

I'd like to make some progress on this if possible? The IBM z/TPF team are a heavy user of IBM DFDL. They are interested in adding offset support and also support for pointers (more on the latter on next WG call).

A problem with using DFDLVersion="2" is that it is pre-judging what is going to be in 2.0. I would prefer that new features appeared in DFDL 1.0 schemas as 'experimental' with their DFDL properties in a separate namespace. They would switch to the DFDL namespace only in a 2.0 schema. This gives us flexibility around property naming and behaviour - we might decide to change things when formalising 2.0.

For example this is what z/TPF have done for pointers:

<xs:element name="myString" type="xs:string" dfdl:lengthKind="explicit" dfdl:length="../len" tddt:indirectKind="pointer" tddt:indirectLength="4" />

It means that when a user migrates to DFDL 2.0 they would need to change their schemas but I think that is preferable to the current proposal where 2.0 bleeds into 1.0.

Regards

Steve Hanson

IBM Hybrid Integration, Hursley, UK
Architect, IBM DFDL
Co-Chair, OGF DFDL Working Group
smh@uk.ibm.com
tel:+44-1962-815848
mob:+44-7717-378890
Note: I work Tuesday to Friday

From: Mike Beckerle <mbeckerle.dfdl@gmail.com>
To: dfdl-wg@ogf.org
Date: 02/02/2018 22:07
Subject: Re: [DFDL-WG] Action Item: Versioning mechanism for DFDL v2.0 features/changes
Sent by: "dfdl-wg" <dfdl-wg-bounces@ogf.org>

So if a schema doesn't use dfdl:format at all, then it doesn't hurt to add <dfdl:format DFDLVersion="2"/> to it at top level in order to express version when DFDL v2.0 features are added to it.

I think the alternative would be a separate annotation element <dfdl:version value="2"/> that is explicitly for this purpose, is allowed only at top level, etc.
That would eliminate the complexity of getting a DFDLVersion by way of a dfdl:ref that includes a named format. It could be required to just be at lexical top level of every schema document that uses v2.0 features.

The reason not to change namespaces, even though that is what we thought we should do originally, is based on various guidance such as
http://www.xml.com/pub/a/2004/07/21/design.html

These comments mostly apply to versioning of individual DFDL schemas, not of the schema-defining language itself.

The quintessential example of a language version changing is XMLSchema itself which now has version 1.1.
This still uses the same namespace as XMLSchema 1.0.

Mike Beckerle | OGF DFDL Workgroup Co-Chair | Tresys Technology | www.tresys.com
Please note: Contributions to the DFDL Workgroup's email discussions are subject to the OGF Intellectual Property Policy

On Tue, Jan 23, 2018 at 11:36 AM, Steve Hanson <smh@uk.ibm.com> wrote:
Mike,

Your scheme as proposed has a couple of problems.
a) It is possible to create a schema that does not use dfdl:format at all. Unlikely but possible. With such a schema there is no way to indicate version.
b) What are the version implications for annotations that have nothing to do with dfdl:format? Asserts, discriminators, variables.

Most of the language you use refers to versions of schemas. That's not the same as version of a dfdl:format. Any reason you didn't just change the DFDL namespace URL to be http://www.ogf.org/dfdl/dfdl-2.0/, which must be declared before any DFDL feature can be used? I think that was always the intent of the DFDL namespace.

The key piece of work is validating any proposal against the scoping rules in section 8 of the spec.

Regards

Steve Hanson
IBM Hybrid Integration, Hursley, UK
Architect, IBM DFDL
Co-Chair, OGF DFDL Working Group
smh@uk.ibm.com
tel:+44-1962-815848
mob:+44-7717-378890
Note: I work Tuesday to Friday

From: Mike Beckerle <mbeckerle.dfdl@gmail.com>
To: dfdl-wg@ogf.org
Date: 23/01/2018 16:03
Subject: [DFDL-WG] Action Item: Versioning mechanism for DFDL v2.0 features/changes
Sent by: "dfdl-wg" <dfdl-wg-bounces@ogf.org>

There are some features now under discussion which will most likely not be part of the DFDL v1.0 standard, but a subsequent version 2.0 of the standard.

Implementations may have these features prior to the existence of that draft version of the DFDL standard; hence, a mechanism for indicating the support, and requirements that implies, on DFDL schemas is needed in order for these v2.0 features to be introduced.

The purpose of the versioning mechanism is to provide access to DFDL v2.0 features, while maintaining also backward compatibility with DFDL v1.0 schemas.

For example, new DFDL format properties may be added to the DFDL v2.0 version; however, those properties are not required to be specified unless the version of the schema is specifically set to indicate it is using version 2.0 features. Also, deprecated features should only generate deprecation warnings if the user is requesting version 2.0 of DFDL, and should be silent otherwise.

Proposal:

New attribute on the dfdl:format annotation: DFDLVersion with valid values "1", "2". All values are reserved for future use.

The DFDLVersion, if unspecified, defaults to "1" which means the implementation is of the DFDL v1.0 specification.

If the DFDLVersion is "2", then new requirements for version 2.0 of DFDL may be enforced on the schema, and schema definition errors will result if version 2.0 requirements are not met. DFDL Version 1.0 features that are deprecated will generate warnings.

Example:

<dfdl:format DFDLVersion="2" .../>

Example:

<dfdl:defineFormat name="myV2Format">
<dfdl:format DFDLVersion="2" .../>
</dfdl:defineFormat>

The DFDLVersion is not a format property and is not scoped. It can be added to a named format definition which is then referenced in the usual manner for named format definitions.

Backward Compatibility Requirement

DFDL Implementations that provide version 2.0 functionality must also provide DFDL v1.0 compatibility. That is, version 2.0 subsumes and extends version 1.0.

Mixed Version Schemas

If a schema consists of multiple schema documents, and their respective dfdl:format annotations do not all contain the same value for the DFDLVersion attribute, then the schema is said to be "mixed version". This is allowed.

A named dfdl:format carrying DFDLVersion "2" may not be referenced from a DFDL Schema document having DFDLVersion "1".

The opposite is allowed: A named dfdl:format carrying DFDLVersion="1" or lacking the DFDLVersion attribute may be referenced from a DFDL Schema having DFDLVersion="2".

There are really 3 ways that DFDL version 2.0 can differ from version 1.0, and these are that new features are added, old features are deprecated, or old features are removed. This latter is quite undesirable, but will be considered a possibility for purpose of discussion this versioning system.

These will commonly be in combination such as if an old feature is deprecated with the replacement feature being added.

These will now be discussed separately, with the emphasis on how mixed version schemas must work.

New Features Added

Each new required DFDL property introduced for version 2.0 will be defined along with what is called its "version 1.0 default value". When a mixed version schema exists, the DFDL v1.0 parts of it behave as if any required DFDLVersion 2 properties have their version 1.0 default value defined, at top level of the schema (as if defined with that default value in each DFDLVersion 1 schema document's dfdl:format annotation).

Features Removed

Incompatibilities are to be avoided wherever possible, and may not be needed at all. However, there is still the possibility that some feature will want to be entirely removed and replaced by something better, and in that case the versioning scheme needs to be able to support this.

In that situation, the schema definition error for version 2 is only generated when a schema has DFDLVersion 2 specified (in dfdl:format explicitly, or by reference to a named format carrying or referencing DFDLVersion 2). Mixed schemas that contain schema documents entirely using version 1 of DFDL do not generate this schema definition error.

Features deprecated

Deprecation warnings are generated only when a DFDLVersion="2" attribute is present or is incorporated by reference (to a named format). A version 1 schema that is incorporated with a version 2 DFDL schema but is not modified in any way, does not generate deprecation warnings about version 1.0 features that have been deprecated in version 2.0.

(Most likely deprecations would be revising property names or value enums for clarity.)

Implementations may provide mechanisms for suppressing warnings. These are implementation dependent.

Version 1.0 Existing Deprecations and Compatibility

During the process of standardization for DFDL v1.0 a number of properties names were changed. DFDL version 1.0 implementations accept both the old and new property names.
(Example: dfdl:separatorSuppressionPolicy, which replaced an older property name.) DFDL schemas that specify DFDLVersion="2" will not accept the older property names, and it is a schema definition error if the old (pre DFDL v1.0) property names are used.

Properly constructed mixed version schemas can still depend on the older property names in the version 1.0 parts of the schema.

------------------------------

Mike Beckerle | OGF DFDL Workgroup Co-Chair | Tresys Technology | www.tresys.com
Please note: Contributions to the DFDL Workgroup's email discussions are subject to the OGF Intellectual Property Policy -- dfdl-wg mailing list dfdl-wg@ogf.org https://www.ogf.org/mailman/listinfo/dfdl-wg

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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