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Postel's Law has two parts

Sun, Aug 24, 2003; by Dave Winer.

The XML world has been debating Postel's Law the last few days.

"Be liberal in what you accept, and conservative in what you send."

Personally I disagree with the first half of the law when applied to XML -- the idea that aggregators should bend over backwards to accept poorly formed XML. I always understood that XML was trying to do something different, as a response to the awful mess that HTML became because browser vendors adopted the first half of Postel's philosophy.

When I adopted XML, in 1997, as I understood it -- I signed onto the idea of rejecting invalid XML. It was considered a bug if you accepted invalid XML, not a bug if you didn't.

Brent Simmons, an early player in this market, says users are better served if he reads bad feeds, but when he does that, he's raising the barrier to entry, in undocumented ways that are hard to reproduce.

His interests are served by high barriers to entry, but the users do better if they have more choice.

Now, the users are happy as long as Brent is around to keep updating his aggregator to work around feed bugs, but he might move on, it happens for all kinds of reasons. It's better to insist on tight standards, so users can switch if they want to, for any reason; so that next year's feed will likely work with this year's aggregator, even if it doesn't dominate the market.

I yearn for just one market with low barriers to entry, so that products are differentiated by features, performance and price; not compatibility.

Compatibility should be expressed in terms of formats, not products. We're dangerously heading down the latter path. The next thing is feed developers work around bugs in a particular aggregator, and then all aggregators have to emulate the bugs, if they can figure out how. That's a big if. Just ask developers who tried to compete with Excel, dBASE or Word.

There's nothing new about this game. What would be new is if compatiblity came to mean something in RSS.

Conservative in what you generate 

A picture named postel.jpgAnyway, the other half of Postel's Law is just as interesting, but so far no one is commenting.

Think about it, if everyone followed the second half, the first half would be a no-op. You could be fully liberal in an afternoon or less. No format-based barriers to entry.

That, btw, was the purpose of my funky campaign a few weeks ago. Maybe I should have called it Dave's Campaign for the Second Half of Postel's Law. ;->

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