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Why XTM 2.0 is different from 1.0

Posted in Technology on 2006-12-16 19:17

Many people have asked what the changes between versions 1.0 and 2.0 of XTM are, and what the rationales for the various changes are. The actual list of differences can be found in the standard itself, but the standard says nothing about why they were made, and so I thought I would give a quick overview of that here.

A bit of background may be useful. XTM 1.0 had a number of design goals specified up front, but there was no clear decision on what kind of format XTM 1.0 was supposed to be. For XTM 2.0 we took the position that the purpose of XTM 2.0 is to allow topic maps to be transferred from one place to another. Period. This meant that functionality to make it easier for people to work with topic maps stored in files (without using Topic Maps software) was not a priority at all, and some of the changes follow from this principle.

XLink and XML Base

XTM 1.0 used both XLink and XML Base, but XTM 2.0 uses neither. In both cases the reason is the same: using these technologies made the specification more complex, and using them achieved essentially nothing.

XML Base, for example, allowed one to specify a base URI for the XTM document that was different from the URI from which the document was retrieved. This allowed the use of relative URIs for URIs based on the base URI inside the XTM document, which makes the document shorter and simpler. However, it's rare for XTM documents to have so many similar URIs that this is helpful, and the savings are in any case small, given how verbose XTM is.

The downside of having XML Base is that the specification has to explain what it means to use this attribute, and one gets into interpretation issues regarding what URI IDs should resolve to, etc. Cutting this functionality really loses nothing of value, and at the same time reduces complexity.

The same applies to XLink, since XLink doesn't really provide any functionality that XTM has any use for. All XTM needs is the ability to put URIs into XTM documents, and a simple href attribute is sufficient for the purpose. And just as with XML Base XLink presents challenges such as what do with the required (but superfluous in XTM) xlink:type attribute, etc. Again, removing XLink makes the specification simpler, and in this case there is no loss of functionality at all.

The name changes

A lot of elements have simply changed names in the two versions, and for the most part the reasons are simple. The complete list is below.

The removals

We took out a few things in XTM 2.0:

The structural changes

A number of structural changes were made in XTM 2.0, mostly reflecting how the abstract model of Topic Maps was tightened by the introduction of TMDM. Some also reflect how we learned more about the role and use of XTM in the five years from the completion of XTM 1.0 in 2001.


A very few extensions have also been made in version 2.0:

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Trond - 2006-12-19 11:34:09

Thanks for the clarifying the differences between XTM 1.0 and XTM 2.0.

This meant that functionality to make it easier for people to work with topic maps stored in files (without using Topic Maps software) was not a priority at all.

Fair enough - and I agree, but didn't one of XTM's strengths over RDFS/OWL use to be that XTM is easier for people to read?

At least that's what Pepper claims in his Ten Theses on Topic Maps and RDF ... but now you're saying that this is no longer an important aspect of XTM? Or?

(I'm not saying that the (reading) complexity of XTM has reached that of RDF(S)/OWL (triples)).

In XTM 1.0 member elements could specify any number of role players (including zero). However, in TMDM, each role must have exactly one player, so in XTM 2.0 we changed the syntax accordingly.

This is an important change. The fact that XTM 1.0 imposed no such constraint did "compromise" the whole interchange aspect, IMHO, as different developers could come up with different ways of doing the same thing -- in a non-standard way. Implicit semantics versus explicit semantics ... the whole point of an international standard...

Lars Marius - 2006-12-19 14:33:20

It's certainly true that XTM is a lot simpler than RDF/XML (which is what you have to compare XTM with on the RDF side), but whether this amounts to anything much when you comare the two technology stacks is another question entirely. I'm not sure it does.

In any case, I think XTM 2.0 is quite a bit simpler than XTM 1.0, so we sacrificed none of that in the process. What we lost was a couple of very marginal conveniences for people who want to maintain their topic maps as sets of XTM files on disk. Personally I don't think this was a loss at all, on balance.

It's true as you say that XTM 1.0 left a bit much open to interpretation, but I don't think the member-with-multiple-players issue really stood out in any sense there. The main problem was the absence of a data model that told you how to interpret the syntax. Without that, things like the players issue become a problem, but so did many other issues throughout (hierarchical variants or flat? etc etc).

As someone who's argued for six years now that the absence of a data model in XTM 1.0 was a huge problem with the standard, I can hardly disagree with your last two sentences... :)

Trond - 2006-12-31 17:12:22

"XTM is a lot simpler than RDF/XML (which is what you have to compare XTM with on the RDF side)"

Lars, I am curious as to why the XTM community "always" (papers, mailing lists, etc.) compare XTM to RDF/XML and not to e.g. OWL Lite. RDF/XML lacks the ability to define new classes and relations / ontologies, which is the purpose of RDFS & OWL (as you would know). Is it really fair to compare XTM to a technology which was not designed to do this, when alternatives / more expressive competing technologies do exist? I do understand Ontopia's (and other companies') motivation behind this, but is the XTM community really better off by ignoring OWL as a competing language/tool?

If I was new to XTM and in need of a technology in which to define my ontologies, I'd most def. not compare XTM to RDF/XML, but to OWL (which, according to Anne Creagan, may also be used in order to express Topic Maps [1]).

(After all, XTM's ability to express ontologies using XML is a very important aspect of the technology)


Lars Marius - 2007-01-04 10:52:48

Good questions, Trond, and ones asked by many others as well. They really deserve a more substantial answer than a comment is suited for, so I'll write a separate blog posting on this.

Frederik - 2008-04-17 08:27:21

Hm, you said that the focus was on machine-readability, and not human-readability (or authoring, editing, etc.). So basically the XTM represents the TMDM in an XML file.

But I think there is one big exception from this principle: The instanceOf element. It is quite crucial for human readability, since it directly tells the class/type of a topic. But for machine readability, it's pointless, since it is resolved into a type-instance association, anyway.

So I am curious about round-tripping and preserving human readability in Topic Map processors. When I deserialize an XTM file, I store instanceOf-information in an association. When I serialize it, it would be natural for a machine to express it as an association in the XTM file, as well. But human readability would be lost.

So should XTM processors keep track of the XML representation of a Topic Map? Or should they even express all type-instance associations with instanceOf elements when serializing?

This is of course not the case for CXTM, which seems to be that hyper-standartized, human unreadable exchange format. But XTM might require a different emphasis.

Lars Marius - 2008-04-20 12:53:17

Frederik, it's true that the <instanceOf> element is strictly speaking not needed. It's really just included because it's convenient, and because it reduces the size of XTM files quite a lot. Human readability was not really a concern, although it does help with that.

I think if you store <instanceOf> as an explicit association you should output it that way as well. The two forms of expressing the associations are equivalent, and XTM is not very readable anyway.

As for CXTM, that's really for conformance testing. See my blog posting on CXTM.

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