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Re: Postel's law, exceptions

• To: "Simon St.Laurent" <simonstl@s...>
• Subject: Re: Postel's law, exceptions
• From: Julian Reschke <julian.reschke@g...>
• Date: Tue, 13 Jan 2004 21:50:27 +0100
• Cc: xml-dev@l...
• In-reply-to: <r02000200-1028-D1176A3A460611D8AD220003937A08C2@[192.168.124.11]>
• References: <r02000200-1028-D1176A3A460611D8AD220003937A08C2@[192.168.124.11]>
• User-agent: Mozilla/5.0 (Windows; U; Windows NT 5.0; en-US; rv:1.6b) Gecko/20031205 Thunderbird/0.4

Simon St.Laurent wrote:

> This has come up before, but the chant "Postel's law has no exceptions"
> seems to be coming again, in the RSS context.
> 
> http://tbray.org/ongoing/When/200x/2004/01/11/PostelPilgrim
> 
> has Tim Bray's argument and lots of links to others.
> 
> Norm Walsh also has a piece at:
> 
> http://norman.walsh.name/2004/01/12/postel
> 
> Also an interesting bit at:
> 
> http://www.intertwingly.net/blog/1685.html

I think part of the problem are different interpreations about what the 
law actually is about. Very good summary by Ed Davis on 
<http://norman.walsh.name/2004/01/12/postel>:

> There are two interpretations of Postel's Law: the conservative and the liberal. The conservative interpretation is that if there is an ambiguity in a specification then you should:
> 
> 1. Be careful not to produce data whose compliance is ambiguous.
> 
> 2. Be careful to accept any data which might reasonably be considered to comply with the specification.
> 
> Also, maybe you should fix the specification.

(I'll add: excepting broken messages doesn't necessarily mean that they 
should silently be accepted, it just means that the recipient should act 
in a predictable way).

> The liberal interpretation of Postel's Law is that even data streams which unambiguously do not comply with the specification should be accepted.
> 
> I'm fully in favour of the conservative interpretation but I think anybody who supports the liberal interpretation rather misses the point of having a specification in the first place.

Needless to say, I'm convinced that application of the "liberal" 
interpretation actually *causes* interoperability problems (a recently 
shown by the experiment done here: 
<http://www.intertwingly.net/blog/1685.html>).

I'll also note that both the W3C ([1]) and the IETF (as far as usage of 
XML in protocols is concerned, [2]) seem to favor a conservative approach.

Julian

[1] <http://www.w3.org/TR/webarch/#no-silent-recovery>
[2] <http://greenbytes.de/tech/webdav/rfc3470.html#rfc.section.4.1>

-- 
<green/>bytes GmbH -- http://www.greenbytes.de -- tel:+492512807760

• References:
  • Postel's law, exceptions
    • From: "Simon St.Laurent" <simonstl@s...>

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