Before talking about what the schema-seeking public really needs, who's calling the shots, and where all this is leading, let's see what the BizTalk and XML.org repository efforts offer and how they compare.

The BizTalk Repository

XML.org

Are Repositories Useful?

Here are the use cases projected for the repositories (explicitly for XML.org and implicitly for BizTalk.org) and summaries of why, on closer examination, I think the area of application may be significantly narrower in the near term:

1. Obtain schema (and other required supporting files, such as stylesheet) automatically on receipt of a document referencing an unknown schema.

   Counter: If you don't know the information model of the schema, or if it has changed, retrieving the schema won't automate interoperability. If you do know the information model underlying the schema, and you are using it on a real-time, transactional basis, you will likely download it once and maintain it locally, rather than hitting the repository server every time you need to parse an instance. If you aren't convinced by this argument, search for "purchase order" on BizTalk.org. The 19 hits (as of 1/20/00) include general and specialized documents, complementary and contradictory approaches, and pieces of larger schema frameworks. If you know ahead of time which one you want, finding it here might be convenient. If you don't know, this selection would represent the beginning of your research, not its conclusion.

2. Upload schema and supporting files, thus taking burden of being a schema server off of the creator. Files may be available for archival access (slow retrieval) or utility access, where server would require high speed and possibly high bandwidth. Posting material can also solicit useful feedback.

   Counter: The arguments against utility usage seem the same as above: if you use it frequently, you will fetch your own copy once. If an update is made, you will fetch the new schema once. This process can be automated as long as the revision does not affect the relationship to your local information model and how instances are processed locally. But how will the updated processing system know that a change in datatype means a commensurate change in local processing? It seems difficult to automate this level of discretion without a set of ground rules on the range of changes possible within an "update."

3. Register without deposit to gain visibility, but maintain local control from original site or alternate repository.

   Counter: None. This is the library or catalog function of the repository, consonant with the archival search and browse functions. This seems quite reasonable and immediately useful.

4. Browse or search for schema for new editing application. End user may not even be aware of use of XML or invocation of remote schema. (Example, I'm listing my house with a real estate broker, but don't have the right schema. My editing application hits a repository, finds and downloads the correct schema, and customizes itself for my data input.)

   Counter: At XML '99, all three vendors showing XML document editing tools promoted easily customizable, schema-specific applications (Arbortext's Adept Lite, SoftQuad's XMetaL, Excosoft's Documentor). Adapting any of these to a schema today is neither an automated nor an end-user process. The vendors have done much to lower the bar, but the task still requires integration and programming. Fully automating the process will require a major chunk of work in the implementation and execution of editing tools and interfaces. I'd really love to see this, and I applaud the writers of the OASIS Use Scenarios for looking ahead—it's a refreshing change from looking backward at the word processing paradigm—but I don't think this is a use case applicable to repositories in 2000 or 2001.

In summary then, the repository use cases that are compelling, at least for the near-term, are the discover-what's-out-there, look-at-it, and evaluate-it yellow pages scenarios.

What We Really Need in a Repository

There are essentially three levels of utility a repository could provide:

1. A yellow-pages-like listing of anyone willing to pay the price of admission and conform to minimal constraints
2. A reference-librarian or encyclopedia-like resource that informs and guides users to the information they really need
3. A dynamic, real-time source for schema location during transactional processing

Both XML.org and BizTalk could grow into a yellow pages for schemas. However, to guide users to the right schema—that is, to be a reference library rather than a phone book—the sites will need to put some more muscle and moxie into the project and produce more than just a flat list of everything that comes their way.

Currently, BizTalk touts its validation service, but if this is anything above a validating XML parser, it is not obvious. BizTalk also ranks schemas by what they call "use counts." This number is the number of individuals registered with BizTalk who ask to be notified if a change is made to a schema. Perhaps it indicates something, but moving from "tell me if Foo.xdr gets tweaked" to "Foo.xdr is mission-critical to my business" is an unwarranted leap of faith. XML.org proposes a similar metric: tracking the number of downloads. But this won't work either. Here's why:

Let's say I post Lioras.RadiologyExam.DTD on the XML.org site, and a dozen integrators—from the Mayo Clinic to King Faisal Hospital—download the thing to see what the heck I've done. The indicators would be "high usage." Meanwhile, the American College of Radiology has ACR.RadiologyExam.DTD, which is listed for reference on both sites. But everyone in the field has been tracking the development of the document, is a member of ACR, and gets their copy directly from the ACR site. Result: "low usage."

In short, measuring status or usage hasn't gotten more than a lick and a promise from either site. Users need to know what is really standards conformant. They need to find out what is used by whom; what experience others have had working with the schema; and its relationship to other schemas. If not a critical edition, at least we need an Amazon.com-like source of user feedback and a NY Times best seller list version of popularity.

Dan Rogers of BizTalk indicated that Microsoft had no plans to qualify or make judgements on schemas, and that usage indicators would become more representative as traffic to the site and use of schemas rose. Laura Walker, Executive Director of OASIS, on the other hand, suggested that "long term, OASIS will play more of a role in arbitrating the standards and offering opinions on the validity and viability of the standards." She believes that it is too early to add this layer of valuation, that the repository should get started on a "democratic" basis while experience is compiled on the various schemas. According to Walker, "More needs to be done in the process of downloading and testing, then using and applying the schemas. 12-18 months from now, this will change."

For one-stop schema shopping, a repository will not only need to guide a user to the right model, it will need to provide an unambiguous information model documenting its semantics. If I'm going to map my local database to information via that schema, I need to know its information model and its relationship to other models and schemas.

The OASIS Registry TC design principles call for "providing DTDs and schemas, and an interface to their metadata, before proceeding to other matters." BizTalk-hosted schemas have rudimentary documentation on site. Neither BizTalk nor OASIS will necessarily set the context required for "semantic interoperability"—the sine qua non of the exchange world.

Semantic interoperability means that when I send you my XML instance, you not only can parse it against a known schema, but you know what the components mean and can relate them to your local information model. To pull a schema off the shelf or down from a repository site and put it to work, the schema has to be a known quantity, part of a known framework of interoperable schemas or one with an unambiguous derivation from a known information model.

While the current sites are clearly intended to rise above the level of a yellow pages, neither has addressed the requirements for qualification or documentation of their wares.

So, what are they aiming at?