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   XML Catalogs and Public Ids

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[apologizes if this appears twice, this is a resend after a mail problem]


Over the weekend I was reading through the XML Catalog specification [1] 
and toying with the implementation of a catalog resolver available 
through the Apache xml-commons project [2]. I was surprised about the way 
that system and public identifiers are handled, and thought I'd check in 
here to see whether I'm misinterpreting the spec (likely!) or could find 
some rationale for the behaviour.

Specifically, it would seem that System Identifiers are the preferred way 
of accessing a catalog, and Public Identifiers play second fiddle. Granted 
the spec does include a "preferred" mode for accessing the catalog but 
according to Section 7.1.2 of the spec if a system identifier can be matched 
in the catalog then that will always be used; an accompanying public 
identifier is only used as a fallback if the system id doesn't match 
(or is unavailable) and the prefer setting is "public". So public ids are 
only useful if the system identifier isn't available. Given that you 
have to provide both in XML, public ids seem pretty redundant.

Toying with the command-line resolver application shows behaviour consistent 
with this reading.

Everything I've read up to now set me up to expect the following: that 
Public Ids would be preferred and that System Ids would be the fallback.

What am I missing?

[2]. http://xml.apache.org/commons/


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