Table of Content:
- General overview
- The definition
- Using catalogs
- Some examples
- How to tune catalog usage
- How to debug catalog processing
- How to create and maintain catalogs
- The implementor corner quick review of the
API
- Other resources
What is a catalog? Basically it's a lookup mechanism used when an entity
(a file or a remote resource) references another entity. The catalog lookup
is inserted between the moment the reference is recognized by the software
(XML parser, stylesheet processing, or even images referenced for inclusion
in a rendering) and the time where loading that resource is actually
started. It is basically used for 3 things:
- mapping from "logical" names, the public identifiers and a more
concrete name usable for download (and URI). For example it can associate
the logical name
"-//OASIS//DTD DocBook XML V4.1.2//EN"
of the DocBook 4.1.2 XML DTD with the actual URL where it can be
downloaded
http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd
- remapping from a given URL to another one, like an HTTP indirection
saying that
"http://www.oasis-open.org/committes/tr.xsl"
should really be looked at
"http://www.oasis-open.org/committes/entity/stylesheets/base/tr.xsl"
- providing a local cache mechanism allowing to load the entities
associated to public identifiers or remote resources, this is a really
important feature for any significant deployment of XML or SGML since it
allows to avoid the aleas and delays associated to fetching remote
resources.
Libxml, as of 2.4.3 implements 2 kind of catalogs:
- the older SGML catalogs, the official spec is SGML Open Technical
Resolution TR9401:1997, but is better understood by reading the SP Catalog page from
James Clark. This is relatively old and not the preferred mode of
operation of libxml.
- XML
Catalogs is far more flexible, more recent, uses an XML syntax and
should scale quite better. This is the default option of libxml.
In a normal environment libxml2 will by default check the presence of a
catalog in /etc/xml/catalog, and assuming it has been correctly populated,
the processing is completely transparent to the document user. To take a
concrete example, suppose you are authoring a DocBook document, this one
starts with the following DOCTYPE definition: <?xml version='1.0'?>
<!DOCTYPE book PUBLIC "-//Norman Walsh//DTD DocBk XML V3.1.4//EN"
"http://nwalsh.com/docbook/xml/3.1.4/db3xml.dtd">When validating the document with libxml, the catalog will be
automatically consulted to lookup the public identifier "-//Norman Walsh//DTD
DocBk XML V3.1.4//EN" and the system identifier
"http://nwalsh.com/docbook/xml/3.1.4/db3xml.dtd", and if these entities have
been installed on your system and the catalogs actually point to them, libxml
will fetch them from the local disk. Note: Really don't use this
DOCTYPE example it's a really old version, but is fine as an example. Libxml2 will check the catalog each time that it is requested to load an
entity, this includes DTD, external parsed entities, stylesheets, etc ... If
your system is correctly configured all the authoring phase and processing
should use only local files, even if your document stays portable because it
uses the canonical public and system ID, referencing the remote document. Here is a couple of fragments from XML Catalogs used in libxml2 early
regression tests in test/catalogs : <?xml version="1.0"?>
<!DOCTYPE catalog PUBLIC
"-//OASIS//DTD Entity Resolution XML Catalog V1.0//EN"
"http://www.oasis-open.org/committees/entity/release/1.0/catalog.dtd">
<catalog xmlns="urn:oasis:names:tc:entity:xmlns:xml:catalog">
<public publicId="-//OASIS//DTD DocBook XML V4.1.2//EN"
uri="http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd"/>
... This is the beginning of a catalog for DocBook 4.1.2, XML Catalogs are
written in XML, there is a specific namespace for catalog elements
"urn:oasis:names:tc:entity:xmlns:xml:catalog". The first entry in this
catalog is a public mapping it allows to associate a Public
Identifier with an URI. ...
<rewriteSystem systemIdStartString="http://www.oasis-open.org/docbook/"
rewritePrefix="file:///usr/share/xml/docbook/"/>
...A rewriteSystem is a very powerful instruction, it says that
any URI starting with a given prefix should be looked at another URI
constructed by replacing the prefix with an new one. In effect this acts like
a cache system for a full area of the Web. In practice it is extremely useful
with a file prefix if you have installed a copy of those resources on your
local system. ...
<delegatePublic publicIdStartString="-//OASIS//DTD XML Catalog //"
catalog="file:///usr/share/xml/docbook.xml"/>
<delegatePublic publicIdStartString="-//OASIS//ENTITIES DocBook XML"
catalog="file:///usr/share/xml/docbook.xml"/>
<delegatePublic publicIdStartString="-//OASIS//DTD DocBook XML"
catalog="file:///usr/share/xml/docbook.xml"/>
<delegateSystem systemIdStartString="http://www.oasis-open.org/docbook/"
catalog="file:///usr/share/xml/docbook.xml"/>
<delegateURI uriStartString="http://www.oasis-open.org/docbook/"
catalog="file:///usr/share/xml/docbook.xml"/>
...Delegation is the core features which allows to build a tree of catalogs,
easier to maintain than a single catalog, based on Public Identifier, System
Identifier or URI prefixes it instructs the catalog software to look up
entries in another resource. This feature allow to build hierarchies of
catalogs, the set of entries presented should be sufficient to redirect the
resolution of all DocBook references to the specific catalog in
/usr/share/xml/docbook.xml this one in turn could delegate all
references for DocBook 4.2.1 to a specific catalog installed at the same time
as the DocBook resources on the local machine. The user can change the default catalog behaviour by redirecting queries
to its own set of catalogs, this can be done by setting the
XML_CATALOG_FILES environment variable to a list of catalogs, an
empty one should deactivate loading the default /etc/xml/catalog
default catalog Setting up the XML_DEBUG_CATALOG environment variable will
make libxml2 output debugging information for each catalog operations, for
example: orchis:~/XML -> xmllint --memory --noout test/ent2
warning: failed to load external entity "title.xml"
orchis:~/XML -> export XML_DEBUG_CATALOG=
orchis:~/XML -> xmllint --memory --noout test/ent2
Failed to parse catalog /etc/xml/catalog
Failed to parse catalog /etc/xml/catalog
warning: failed to load external entity "title.xml"
Catalogs cleanup
orchis:~/XML -> The test/ent2 references an entity, running the parser from memory makes
the base URI unavailable and the the "title.xml" entity cannot be loaded.
Setting up the debug environment variable allows to detect that an attempt is
made to load the /etc/xml/catalog but since it's not present the
resolution fails. But the most advanced way to debug XML catalog processing is to use the
xmlcatalog command shipped with libxml2, it allows to load
catalogs and make resolution queries to see what is going on. This is also
used for the regression tests: orchis:~/XML -> ./xmlcatalog test/catalogs/docbook.xml \
"-//OASIS//DTD DocBook XML V4.1.2//EN"
http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd
orchis:~/XML -> For debugging what is going on, adding one -v flags increase the verbosity
level to indicate the processing done (adding a second flag also indicate
what elements are recognized at parsing): orchis:~/XML -> ./xmlcatalog -v test/catalogs/docbook.xml \
"-//OASIS//DTD DocBook XML V4.1.2//EN"
Parsing catalog test/catalogs/docbook.xml's content
Found public match -//OASIS//DTD DocBook XML V4.1.2//EN
http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd
Catalogs cleanup
orchis:~/XML -> A shell interface is also available to debug and process multiple queries
(and for regression tests): orchis:~/XML -> ./xmlcatalog -shell test/catalogs/docbook.xml \
"-//OASIS//DTD DocBook XML V4.1.2//EN"
> help
Commands available:
public PublicID: make a PUBLIC identifier lookup
system SystemID: make a SYSTEM identifier lookup
resolve PublicID SystemID: do a full resolver lookup
add 'type' 'orig' 'replace' : add an entry
del 'values' : remove values
dump: print the current catalog state
debug: increase the verbosity level
quiet: decrease the verbosity level
exit: quit the shell
> public "-//OASIS//DTD DocBook XML V4.1.2//EN"
http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd
> quit
orchis:~/XML -> This should be sufficient for most debugging purpose, this was actually
used heavily to debug the XML Catalog implementation itself. Basically XML Catalogs are XML files, you can either use XML tools to
manage them or use xmlcatalog for this. The basic step is
to create a catalog the -create option provide this facility: orchis:~/XML -> ./xmlcatalog --create tst.xml
<?xml version="1.0"?>
<!DOCTYPE catalog PUBLIC "-//OASIS//DTD Entity Resolution XML Catalog V1.0//EN"
"http://www.oasis-open.org/committees/entity/release/1.0/catalog.dtd">
<catalog xmlns="urn:oasis:names:tc:entity:xmlns:xml:catalog"/>
orchis:~/XML -> By default xmlcatalog does not overwrite the original catalog and save the
result on the standard output, this can be overridden using the -noout
option. The -add command allows to add entries in the
catalog: orchis:~/XML -> ./xmlcatalog --noout --create --add "public" \
"-//OASIS//DTD DocBook XML V4.1.2//EN" \
http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd tst.xml
orchis:~/XML -> cat tst.xml
<?xml version="1.0"?>
<!DOCTYPE catalog PUBLIC "-//OASIS//DTD Entity Resolution XML Catalog V1.0//EN" \
"http://www.oasis-open.org/committees/entity/release/1.0/catalog.dtd">
<catalog xmlns="urn:oasis:names:tc:entity:xmlns:xml:catalog">
<public publicId="-//OASIS//DTD DocBook XML V4.1.2//EN"
uri="http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd"/>
</catalog>
orchis:~/XML -> The -add option will always take 3 parameters even if some of
the XML Catalog constructs (like nextCatalog) will have only a single
argument, just pass a third empty string, it will be ignored. Similarly the -del option remove matching entries from the
catalog: orchis:~/XML -> ./xmlcatalog --del \
"http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd" tst.xml
<?xml version="1.0"?>
<!DOCTYPE catalog PUBLIC "-//OASIS//DTD Entity Resolution XML Catalog V1.0//EN"
"http://www.oasis-open.org/committees/entity/release/1.0/catalog.dtd">
<catalog xmlns="urn:oasis:names:tc:entity:xmlns:xml:catalog"/>
orchis:~/XML -> The catalog is now empty. Note that the matching of -del is
exact and would have worked in a similar fashion with the Public ID
string. This is rudimentary but should be sufficient to manage a not too complex
catalog tree of resources. First, and like for every other module of libxml, there is an
automatically generated API page for
catalog support. The header for the catalog interfaces should be included as: #include <libxml/catalog.h> The API is voluntarily kept very simple. First it is not obvious that
applications really need access to it since it is the default behaviour of
libxml2 (Note: it is possible to completely override libxml2 default catalog
by using xmlSetExternalEntityLoader to
plug an application specific resolver). Basically libxml2 support 2 catalog lists:
- the default one, global shared by all the application
- a per-document catalog, this one is built if the document uses the
oasis-xml-catalog PIs to specify its own catalog list, it is
associated to the parser context and destroyed when the parsing context
is destroyed.
the document one will be used first if it exists. Initialization routines:xmlInitializeCatalog(), xmlLoadCatalog() and xmlLoadCatalogs() should be
used at startup to initialize the catalog, if the catalog should be
initialized with specific values xmlLoadCatalog() or xmlLoadCatalogs()
should be called before xmlInitializeCatalog() which would otherwise do a
default initialization first. The xmlCatalogAddLocal() call is used by the parser to grow the document
own catalog list if needed. Preferences setup:The XML Catalog spec requires the possibility to select default
preferences between public and system delegation,
xmlCatalogSetDefaultPrefer() allows this, xmlCatalogSetDefaults() and
xmlCatalogGetDefaults() allow to control if XML Catalogs resolution should
be forbidden, allowed for global catalog, for document catalog or both, the
default is to allow both. And of course xmlCatalogSetDebug() allows to generate debug messages
(through the xmlGenericError() mechanism). Querying routines:xmlCatalogResolve(), xmlCatalogResolveSystem(), xmlCatalogResolvePublic()
and xmlCatalogResolveURI() are relatively explicit if you read the XML
Catalog specification they correspond to section 7 algorithms, they should
also work if you have loaded an SGML catalog with a simplified semantic. xmlCatalogLocalResolve() and xmlCatalogLocalResolveURI() are the same but
operate on the document catalog list Cleanup and Miscellaneous:xmlCatalogCleanup() free-up the global catalog, xmlCatalogFreeLocal() is
the per-document equivalent. xmlCatalogAdd() and xmlCatalogRemove() are used to dynamically modify the
first catalog in the global list, and xmlCatalogDump() allows to dump a
catalog state, those routines are primarily designed for xmlcatalog, I'm not
sure that exposing more complex interfaces (like navigation ones) would be
really useful. The xmlParseCatalogFile() is a function used to load XML Catalog files,
it's similar as xmlParseFile() except it bypass all catalog lookups, it's
provided because this functionality may be useful for client tools. threaded environments:Since the catalog tree is built progressively, some care has been taken to
try to avoid troubles in multithreaded environments. The code is now thread
safe assuming that the libxml2 library has been compiled with threads
support. The XML Catalog specification is relatively recent so there isn't much
literature to point at: If you have suggestions for corrections or additions, simply contact
me: Daniel Veillard | 
