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Turtles All the Way Up

April 2002: Turtles All the Way Up "XML is one of those dangerous technologies," Clay Shirky told a packed hall of SD West attendees during his Thursday afternoon keynote. "It's one that managers know about; you can almost see them sprinkling the magic pixie dust, saying 'Well, you should be able to do that, it's XML, right?' Meanwhile you're saying, 'You shouldn't even know that word!'" The audience members' rueful laughter revealed that many had been there. "XML is going to make the standards wars worse," Shirky continued. "That's the bad news. If it's easy to write up a standard, everybody's going to write one."

Shirky, a consultant and author specializing in peer-to-peer and other distributed technologies, attacked widely held assumptions about XML and Web services. Because XML by itself is so easy to customize, he said, "XML is not a lingua franca—it's not even a lingua—it's an alphabet." He pointed out that SOAP protocols don't come close to guaranteeing interoperability; five registered versions of a stock-ticker service that Shirky looked up couldn't even agree on the type of the content. "It's a five-digit number!" he exclaimed in disbelief.

No Cheap Shots
Shirky didn't waste time with cheap shots, however. Instead, he clearly delineated why we can't expect to take the borderless, stateless World Wide Web, map software onto it, and expect it all to hook up—and scale up. He showed how various "state horizons" appear as you attempt to connect software to the world, and provided some directions for coordinating across them. "The border is no longer tied to the physical bounds of the box," he claimed. For example, "trust horizons" define the limits of who can use resources on your computer. There are good security models for en route encryption, but that's not the point, he said. "The issue with the trust horizon is not packet sniffing, but do you honor the request when it gets there? There are no good solutions to this problem; three of the commonest are restriction, registration and identity."

Probably the most pressing question concerns the "coordination horizon": "Will Web services work at Internet scale—an uncoordinated market where millions are talking to millions," Shirky asked, and noted that this big unknown has its origins in the design patterns of the Web. "The Web got where it is by the simple definitions of HTTP and HTML, and by having a human being at the other. The 64-million-dollar question is: Does the Web work because it's so simple, and will Web services fail because it's so complex? Shirky then revealed his favorite four-letter acronym , in the UDDI spec: the Universal Service Interoperability Protocol, and then note dryly that this does not exist yet. "It's turtles all the way up," he said wryly. "You can't get to interoperability just by adding another layer at the top of the stack."

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