On the applicability of catalog resolutionhttp://norman.walsh.name/2005/06/10/resolution8912005-06-10T05:11:04-04:00$Date: 2005-09-11 10:27:02 -0400 (Sun, 11 Sep 2005) $NormanWalsh2005Norman WalshAs a strong proponent of XML Catalogs, I'm sometimes asked, “should
catalog resolution be used for …?” The answer is “yes”.LaBruyèreThe things we have most longed for do not happen; or if they do,
it is never at the time nor under the circumstances when they could
have made us happiest.I'm a strong proponent of
XML Catalogs
.
Not everyone believes in them,
but I do.
As a consequence, I'm sometimes asked, “should catalog
resolution be used for” followed by some context in which a URI can
appear that an application might attempt to retrieve: the system and
public identifiers of documents and entities, the URI components of
schema
location hints in XML Schema,
the href pseudo-attribute of the
XML stylesheet
processing instruction, the href attributes
of include and import elements in XSLT, the URI in an
XSL
or CSS
property value, the href attribute of
HTML, etc.The answer is “yes”. Catalog resolution applies to any URI that a
web cache
applies to: all of them.If you're writing an application, an entity resolver should be
used for all external entities and a URI resolver
should be used for all other URIs. All
of them. Yes, that one. And that one. And that other
one, too.
The XML specification
gives
explicit license to parsers to perform catalog resolution:
Attempts to retrieve the resource identified by a URI MAY be
redirected at the parser level (for example, in an entity resolver) or
below (at the protocol level, for example, via an HTTP Location:
header).
To the extent that lack of such explicit license
in other specifications leads users and developers to imagine that such
resolution is not allowed, I think it was a mistake to call this out
in the XML specification.Your application doesn't need explicit license to use a resolver
any more than it has to give me explicit license to run
WWWOffle.
GETing a URI? Resolve it first!