The METS Board also has a generic URL that points to the current version 
of the METS schema.  However,  at least the last time we--the METS 
Board--discussed the matter, we agreed that the schema we maintain at at 
the generic METS URL would always be backwards compatible with previous 
versions of METS.  (There have, in fact and alas, been two exceptions to 
this rule--but these were to correct outright schema errors--in 
particular our incorrect xlink target namespace reference).  If we ever 
implement a non-backwards compatible version of METS, we expect both to 
assign it a new version number (version 2) and to make it available both 
at a "generic" version 2 location (something like and at a specific version 
2 location (something like for version 2.1).  
However, this is a matter that the METS Board should probably take up 
again to make sure we are all still in agreement.

A generic URL for the location of a metadata schema can be a great 
convenience--providing, as Karen notes, that the updates are backwards 

Rick Beaubien

Karen Coyle wrote:
> Andrew, it sounds like you are saying that there shouldn't be a 
> generic URL... ? I believe that people asked for it in the past, but 
> maybe it's not a good idea. It seems to work when updates are backward 
> compatible but not when they aren't.
> kc
> Houghton,Andrew wrote:
>>> From: Metadata Object Description Schema List [mailto:[log in to unmask]] On
>>> Behalf Of Rick Beaubien
>>> Sent: Thursday, March 12, 2009 5:02 PM
>>> To: [log in to unmask]
>>> Subject: [MODS] URL for the current version of mods
>>> The current version of MODS is available both through a version
>>> specific
>>> URL ( as well as a
>>> more generic URL (  Can
>>> users count on the more generic URL always pointing to the most recent
>>> version of the MODS schema that is backwards compatible through version
>>> 3.1?  (I believe that there is a break in compatibility between version
>>> 3.0 and 3.1).  Or, to phrase the question a little differently, if a
>>> new
>>> version of the MODS schema was produced that was not backwards
>>> compatible with versions 3.1-3.3, would it replace the 3.3 schema that
>>> is now available via the generic URL
>>>  I am asking this because
>>> of
>>> its implications for how the schemaLocation in mods instances should be
>>> set up.
>> What goes in xsi:schemaLocation is the namespace URI and a URI to the 
>> XML
>> schema for MODS.  The namespace URI must match the targetNamespace 
>> attribute
>> in the XML schema for MODS which currently is:
>>   targetNamespace=""
>> for the XML schema URI you should be using the version specific URI 
>> in the
>> xsi:schemaLocation attribute of your XML instance document.  
>> Otherwise, as
>> your question suggests when version 4 of the MODS specification comes 
>> out
>> and it is not backward compatible, LC will have to change the target
>> namespace URI to something like:
>>   targetNamespace=""
>> anybody still using version 3 of MODS and who mistakenly used the 
>> generic URI in their xsi:schemaLocation attribute of their XML 
>> instance documents will now have a problem since the targetNamespace 
>> in the XML schema returned from the generic URI will not match the 
>> namespace URI they used in the xsi:schemaLocation attribute specified 
>> in their XML instance documents.  If
>> the XML instance document is being read by an XML parser that is schema
>> aware, then the XML parser will throw an error, otherwise the 
>> application processing the MODS instance document will not know there 
>> is a problem.  The generic XML schema URI should never be used in 
>> xsi:schemaLocation.
>> Unfortunately, the LC web site doesn't make it clear which one you 
>> should be
>> using and when you dereference the generic URI you get back an HTTP 
>> 200 OK
>> status with the HTTP entity containing the latest version of the MODS 
>> schema.
>> What I believe the LC web site should be doing when dereferencing the 
>> generic URI is to respond back with an HTTP 302 Found status and an 
>> HTTP Location
>> header containing the URI to the latest version of the MODS schema.  
>> Then it
>> would be obvious that you should be using the version specific URI 
>> instead
>> of the generic URI.
>> Andy.

Rick Beaubien
Software Engineer, Research and Development
U.C. Berkeley Library

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