XML and the IETF: Making the case for RELAX NG
19:45, 4 Jun 2002 UTC | Michael Smith

In a message to the ietf-xml-use mailing list, James Clark makes a detailed case for giving RELAX NG at least "equal billing" with the W3C XML schema language as a recommended formalism in IETF protocols using XML.

Writing in response to a recently updated IETF Internet-Draft, Guidelines for the Use of XML within IETF Protocols (draft-hollenbeck-ietf-xml-guidelines-04.txt), Clark begins his message by writing:

I just had a look at draft-hollenbeck-ietf-xml-guidelines-04. Section 4.6 says "XML Schema should be used as the formalism in the absence of clearly stated reasons to choose another." I strongly disagree with this recommendation.

I believe RELAX NG is preferable in many situations to XML Schema and should receive at least equal billing. Concretely, I propose in the sentence above changing "XML Schema" to "XML Schema or RELAX NG".

[...] I don't think RELAX NG is just another mechanism. It is a solid, mature and stable specification. It has been developed in an open standards process (in OASIS). It has multiple, independent and interoperable implementations. It is based on a solid body of CS theory (tree automata). It is on track to become a fully-fledged International Standard: it recently went out as a Draft International Standard.

In the remainder of the message, Clark details a variety of reasons for preferring RELAX NG as a language to "communicate unambiguously and precisely to a human reader what XML documents are legal for [a particular XML application]". He notes that RELAX NG:

  • is designed to be simple and easy to understand; the specification for the W3C XML schema language, in contrast, is far from being simple or easy to understand

  • includes a normative, formal description of the semantics of a RELAX NG schema and has a solid basis in tree automata theory; the W3C XML schema language has no such basis

  • integrates attributes into content models; the W3C XML schema language's support for attributes is totally inadequate and provides no advance over DTDs

  • provides strong support for unordered content; the W3C XML schema language provides very weak support for unordered content

  • provides a modular approach to use of datatypes; the W3C XML schema language is totally lacking in modularity -- tied to the single collection of datatypes defined in its specification

  • has a clear, unambiguous notion of validity; the W3C XML schema language does not: for example, it provides no way to specify what is allowed as the root element of a document instance

  • treats validation as a process with two independent inputs: a schema and an instance to be validated with respect to the schema; the W3C XML schema language does not provide a similar clean separation between the two

  • never changes the information that an application receives -- it specifies purely what is valid and what is invalid; the W3C XML schema language, in contrast, supports infoset augmentation

In a followup to Clark's posting, Tim Bray concurs, saying that "to the extent that there is an IETF style of doing things, RNG is right there." That statement will probably ring true to many people familiar with the IETF and how its history of RFCs (Request For Comments) -- developed by small groups of technical experts and reviewed in an open, distributed fashion -- contrasts with the W3C's institutional process. A couple of excerpts from the Free On-Line Dictionary of Computing entry for RFC seem relevant:

The RFC tradition of pragmatic, experience-driven, after-the-fact standard writing done by individuals or small working groups has important advantages over the more formal, committee-driven process typical of ANSI or ISO.

The RFCs are most remarkable for how well they work -- they manage to have neither the ambiguities that are usually rife in informal specifications, nor the committee-perpetrated misfeatures that often haunt formal standards.

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Re: XML and the IETF: Making the case for RELAX NG (Terris Linenbach - 00:08, 6 Jun 2002)
Long live Relax NG, now if only Microsoft would support it...
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