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Re: Media types and versioning

From: <noah_mendelsohn@us.ibm.com>
Date: Tue, 22 May 2007 13:06:25 -0400
To: "Mark Baker" <distobj@acm.org>
Cc: "David Orchard" <dorchard@bea.com>, "Marc de Graauw" <marc@marcdegraauw.com>, mark@coactus.com, "Marc de Graauw" <mdegraau@xs4all.nl>, www-tag@w3.org
Message-ID: <OFB758FFBA.47690BF3-ON852572E3.004DDC34-852572E3.005E06E1@lotus.com>

Mark Baker writes:

> I'm ok with the second part of that, but if a new version is
> compatible, then a new media type shouldn't be required.

Which begs the question, what do you mean by compatible?  Almost surely, a 
new version of a language will introduce both new syntax and corresponding 
new information conveyed by that syntax.  So, in that sense, there's 
almost always some incompatibility when new constructs are used. 

Sometimes the new information is in some sense orthogonal to the old, 
allowing one to ask separately which pieces of the content are 
"understood".  Sometimes, the new rules change the implications of the old 
fields.   For example, if the old version says that case doesn't matter 
for a field, and the new version of the language says that case matters, 
then even documents that were legal in the old language may be taken to 
convey information that might not have been intentional when the document 
was authored.   Even in the simple case where the new content is 
completely orthogonal syntactically and semantically, there is the 
question of whether users consider it OK to to ignore if it's not 

For reaons like these, I don't think using the word "compatible" without 
explanation or qualification is appropriate.  I do agree that for some 
families of languages, one can define useful notion of compatibility that 
make it appropriate to consider using a single media type for all of the 
versions.  I don't agree that those styles of language evolution are 
always what people want.

Noah Mendelsohn 
IBM Corporation
One Rogers Street
Cambridge, MA 02142

"Mark Baker" <distobj@acm.org>
Sent by: mark@coactus.com
05/18/2007 08:04 AM
        To:     "David Orchard" <dorchard@bea.com>
        cc:     noah_mendelsohn@us.ibm.com, "Marc de Graauw" 
<marc@marcdegraauw.com>, "Marc de Graauw" <mdegraau@xs4all.nl>, 
        Subject:        Re: Media types and versioning

On 5/17/07, David Orchard <dorchard@bea.com> wrote:
> What about incorporating a version # in the media type?
> Say application/soap+1dot2+xml, etc.

That's fine.  On the ietf-types list, I think I've recommended
something similar for at least one format which made backwards
incompatible changes.

> Then say in the media type definition that by definition any minor
> version change is compatible, and any incompatible change will be
> accompanied by a major version change.

I'm ok with the second part of that, but if a new version is
compatible, then a new media type shouldn't be required.

Received on Tuesday, 22 May 2007 17:06:44 UTC

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