The XML C parser and toolkit of Gnome

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 the API
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 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.

The definitions

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.

Using catalog

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.

Some examples:

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.

How to tune catalog usage:

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

How to debug catalog processing:

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.

How to create and maintain catalogs:

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.

The implementor corner quick review of the API:

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.

Other resources

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

• You can find a good rant from Norm Walsh about the need for catalogs, it provides a lot of context information even if I don't agree with everything presented. Norm also wrote a more recent article XML entities and URI resolvers describing them.
• An old XML catalog proposal from John Cowan
• The Resource Directory Description Language (RDDL) another catalog system but more oriented toward providing metadata for XML namespaces.
• the page from the OASIS Technical Committee on Entity Resolution who maintains XML Catalog, you will find pointers to the specification update, some background and pointers to others tools providing XML Catalog support
• There is a shell script to generate XML 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 on the 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 requiring network accesses for the DTD or stylesheets

• I have uploaded a small tarball containing XML Catalogs for DocBook 4.1.2 which seems to work fine for me too
• The xmlcatalog manual page

If you have suggestions for corrections or additions, simply contact me:

Daniel Veillard