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Catalog support

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Table of Content:

  1. General overview
  2. The definition
  3. Using catalogs
  4. Some examples
  5. How to tune catalog usage
  6. How to debug catalog processing
  7. How to create and maintain catalogs
  8. The implementor corner quick review of theAPI
  9. Other resources

General overview

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 lookupis inserted between the moment the reference is recognized by the software(XML parser, stylesheet processing, or even images referenced for inclusionin a rendering) and the time where loading that resource is actuallystarted.

It is basically used for 3 things:

  • mapping from "logical" names, the public identifiers and a moreconcrete name usable for download (and URI). For example it can associatethe 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 bedownloaded

    http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd

  • remapping from a given URL to another one, like an HTTP indirectionsaying 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 entitiesassociated to public identifiers or remote resources, this is a reallyimportant feature for any significant deployment of XML or SGML since itallows to avoid the aleas and delays associated to fetching remoteresources.

The definitions

Libxml, as of 2.4.3 implements 2 kind of catalogs:

  • the older SGML catalogs, the official spec is SGML Open TechnicalResolution TR9401:1997, but is better understood by reading the SP Catalog pagefromJames Clark. This is relatively old and not the preferred mode ofoperation of libxml.
  • XMLCatalogsis far more flexible, more recent, uses an XML syntax andshould scale quite better. This is the default option of libxml.

Using catalog

In a normal environment libxml2 will by default check the presence of acatalog in /etc/xml/catalog, and assuming it has been correctly populated,the processing is completely transparent to the document user. To take aconcrete example, suppose you are authoring a DocBook document, this onestarts 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 beautomatically consulted to lookup the public identifier "-//Norman Walsh//DTDDocBk XML V3.1.4//EN" and the system identifier"http://nwalsh.com/docbook/xml/3.1.4/db3xml.dtd", and if these entities havebeen installed on your system and the catalogs actually point to them, libxmlwill fetch them from the local disk.

Note: Really don't use thisDOCTYPE 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 anentity, this includes DTD, external parsed entities, stylesheets, etc ... Ifyour system is correctly configured all the authoring phase and processingshould use only local files, even if your document stays portable because ituses the canonical public and system ID, referencing the remote document.

Some examples:

Here is a couple of fragments from XML Catalogs used in libxml2 earlyregression 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 arewritten in XML, there is a specific namespace for catalog elements"urn:oasis:names:tc:entity:xmlns:xml:catalog". The first entry in thiscatalog is a publicmapping it allows to associate a PublicIdentifier with an URI.

...
    <rewriteSystem systemIdStartString="http://www.oasis-open.org/docbook/"
                   rewritePrefix="file:///usr/share/xml/docbook/"/>
...

A rewriteSystemis a very powerful instruction, it says thatany URI starting with a given prefix should be looked at another URIconstructed by replacing the prefix with an new one. In effect this acts likea cache system for a full area of the Web. In practice it is extremely usefulwith a file prefix if you have installed a copy of those resources on yourlocal 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, SystemIdentifier or URI prefixes it instructs the catalog software to look upentries in another resource. This feature allow to build hierarchies ofcatalogs, the set of entries presented should be sufficient to redirect theresolution of all DocBook references to the specific catalog in/usr/share/xml/docbook.xmlthis one in turn could delegate allreferences for DocBook 4.2.1 to a specific catalog installed at the same timeas the DocBook resources on the local machine.

How to tune catalog usage:

The user can change the default catalog behaviour by redirecting queriesto its own set of catalogs, this can be done by setting theXML_CATALOG_FILESenvironment variable to a list of catalogs, anempty one should deactivate loading the default /etc/xml/catalogdefault catalog

How to debug catalog processing:

Setting up the XML_DEBUG_CATALOGenvironment variable willmake libxml2 output debugging informations for each catalog operations, forexample:

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 makesthe base URI unavailable and the the "title.xml" entity cannot be loaded.Setting up the debug environment variable allows to detect that an attempt ismade to load the /etc/xml/catalogbut since it's not present theresolution fails.

But the most advanced way to debug XML catalog processing is to use thexmlcatalogcommand shipped with libxml2, it allows to loadcatalogs and make resolution queries to see what is going on. This is alsoused 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 verbositylevel to indicate the processing done (adding a second flag also indicatewhat 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 actuallyused heavily to debug the XML Catalog implementation itself.

How to create and maintaincatalogs:

Basically XML Catalogs are XML files, you can either use XML tools tomanage them or use xmlcatalogfor this. The basic step isto 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 theresult on the standard output, this can be overridden using the -nooutoption. The -addcommand allows to add entries in thecatalog:

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 -addoption will always take 3 parameters even if some ofthe XML Catalog constructs (like nextCatalog) will have only a singleargument, just pass a third empty string, it will be ignored.

Similarly the -deloption remove matching entries from thecatalog:

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 -delisexact and would have worked in a similar fashion with the Public IDstring.

This is rudimentary but should be sufficient to manage a not too complexcatalog tree of resources.

The implementor corner quick review of theAPI:

First, and like for every other module of libxml, there is anautomatically generated API page forcatalog 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 thatapplications really need access to it since it is the default behaviour oflibxml2 (Note: it is possible to completely override libxml2 default catalogby using xmlSetExternalEntityLoadertoplug 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 theoasis-xml-catalogPIs to specify its own catalog list, it isassociated to the parser context and destroyed when the parsing contextis destroyed.

the document one will be used first if it exists.

Initialization routines:

xmlInitializeCatalog(), xmlLoadCatalog() and xmlLoadCatalogs() should beused at startup to initialize the catalog, if the catalog should beinitialized with specific values xmlLoadCatalog() or xmlLoadCatalogs()should be called before xmlInitializeCatalog() which would otherwise do adefault initialization first.

The xmlCatalogAddLocal() call is used by the parser to grow the documentown catalog list if needed.

Preferences setup:

The XML Catalog spec requires the possibility to select defaultpreferences between public and system delegation,xmlCatalogSetDefaultPrefer() allows this, xmlCatalogSetDefaults() andxmlCatalogGetDefaults() allow to control if XML Catalogs resolution shouldbe forbidden, allowed for global catalog, for document catalog or both, thedefault 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 XMLCatalog specification they correspond to section 7 algorithms, they shouldalso work if you have loaded an SGML catalog with a simplified semantic.

xmlCatalogLocalResolve() and xmlCatalogLocalResolveURI() are the same butoperate on the document catalog list

Cleanup and Miscellaneous:

xmlCatalogCleanup() free-up the global catalog, xmlCatalogFreeLocal() isthe per-document equivalent.

xmlCatalogAdd() and xmlCatalogRemove() are used to dynamically modify thefirst catalog in the global list, and xmlCatalogDump() allows to dump acatalog state, those routines are primarily designed for xmlcatalog, I'm notsure that exposing more complex interfaces (like navigation ones) would bereally useful.

The xmlParseCatalogFile() is a function used to load XML Catalog files,it's similar as xmlParseFile() except it bypass all catalog lookups, it'sprovided because this functionality may be useful for client tools.

threaded environments:

Since the catalog tree is built progressively, some care has been taken totry to avoid troubles in multithreaded environments. The code is now threadsafe assuming that the libxml2 library has been compiled with threadssupport.

Other resources

The XML Catalog specification is relatively recent so there isn't muchliterature to point at:

  • You can find a good rant from Norm Walsh about theneed for catalogs, it provides a lot of context informations even ifI don't agree with everything presented. Norm also wrote a more recentarticle XMLentities and URI resolversdescribing them.
  • An old XMLcatalog proposalfrom John Cowan
  • The Resource Directory DescriptionLanguage(RDDL) another catalog system but more oriented towardproviding metadata for XML namespaces.
  • the page from the OASIS Technical Committee on EntityResolutionwho maintains XML Catalog, you will find pointers to thespecification update, some background and pointers to others toolsproviding XML Catalog support
  • There is a shell scriptto generateXML Catalogs for DocBook 4.1.2 . If it can write to the /etc/xml/directory, it will set-up /etc/xml/catalog and /etc/xml/docbook based onthe resources found on the system. Otherwise it will just create~/xmlcatalog and ~/dbkxmlcatalog and doing:

    export XML_CATALOG_FILES=$HOME/xmlcatalog

    should allow to process DocBook documentations without requiringnetwork accesses for the DTD or stylesheets

  • I have uploaded asmall tarballcontaining XML Catalogs for DocBook 4.1.2 which seemsto work fine for me too
  • The xmlcatalogmanual page

If you have suggestions for corrections or additions, simply contactme:

Daniel Veillard