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Subject: Catalogs 1.1 WD diff
The presentation of this document has been augmented to identify changes from a previous version. Three kinds of changes are highlighted: new, added text, changed text, and deleted text. Working Draft 1.1, 02 Oct 2003
Copyright © 2000, 2001, 2002, 2003 OASIS Open, Inc. All Rights Reserved. Table of Contents
Appendixes
In order to make optimal use of the information about an XML external resource, there needs to be some interoperable way to map the information in an XML external identifier into a URI reference for the desired resource. This Working Draft defines an entity catalog that handles two simple cases:
Though it does not handle all issues that a combination of a complete entity manager and storage manager addresses, it simplifies both the use of multiple products in a great majority of cases and the task of processing documents on different systems. This entity catalog is designed to be compatible with [TR 9401] catalogs as mandated by the Technical Committee [Requirements]. This Working Draft provides several schema language descriptions of XML Catalogs in non-normative appendices. The semantics of XML Catalogs are defined normatively by the prose of this specfiication, not by any one of those schemas. The key words must, must not, required, shall, shall not, should, should not, recommended, may, and optional in this Working Draft are to be interpreted as described in [RFC 2119]. Note that for reasons of style, these words are not capitalized in this document. The terms URI and URI reference are to be interpreted as described in [RFC 2396]. The term external identifier is to be interpreted as defined in Production 75 of [XML]. External identifiers have two parts, an optional public identifier and a system identifier. The terms public identifier and system identifier in this Working Draft always refer to the respective part of an external identifier. NoteAll system identifiers are URI references, but not all URI references are system identifiers. A system identifer is always logically part of an external identifier, even when the public identifer is not provided. The logical input to a catalog processor is an external identifier (some combination of public and system identifiers) or a URI reference. The logical output of the catalog processor is a URI reference. (This Working Draft does not attempt to define an API for catalog processors so the logical interfaces and the practical interfaces may differ.) A catalog is a logical structure that contains "mapping" information. A catalog may be physically contained in one or more catalog entry files. A catalog entry file is a document that contains a set of catalog entries. This Working Draft defines an application-independent entity catalog that maps external identifiers and URI references to (other) URI references. It also defines a format for catalog entry files in terms of [XML] and [XML Namespaces]. The principal task of a catalog processor is to find entries in the catalog that match the input provided and return the associated URI reference as the output. The first such match is always used, and there is no requirement for the catalog processor to search for additional matches. This catalog is used by an application's entity manager. This Working Draft does not dictate when an entity manager should access this catalog; for example, an application may attempt other mapping algorithms before or after accessing this catalog. The catalog is effectively an ordered list of (one or more) catalog entry files. It is up to the application to determine the ordered list of catalog entry files to be used as the logical catalog. (This Working Draft uses the term "catalog entry file" to refer to one component of a logical catalog even though a catalog entry file can be any kind of storage object or entity including—but not limited to—a table in a database, some object identified by a URI reference, or some dynamically generated set of catalog entries.) Each entry in the catalog associates a URI reference with information about an external reference that appears in an XML document. For example, the following are possible catalog entries that associate a URI reference with a public identifier: <public publicId="ISO 8879:1986//ENTITIES Added Latin 1//EN" uri="iso-lat1.gml"/> <public publicId="-//USA/AAP//DTD BK-1//EN" uri="aapbook.dtd"/> <public publicId="-//Example, Inc.//DTD Report//EN" uri="http://www.example.com/dtds/report.dtd"/> This Working Draft defines the following catalog entry types: catalog, delegatePublic, delegateSystem, delegateURI, group, nextCatalog, public, rewriteSystem, rewriteURI, system, and uri. In order to be conformant with this Working Draft, an application must implement all of these entry types with the semantics described herein. The namespace name defined by this Working Draft is "urn:oasis:names:tc:entity:xmlns:xml:catalog". The public identifier for XML Catalogs is "-//OASIS//DTD XML Catalogs V1.0//EN". This Working Draft reserves all elements and attributes from its namespace for current and future use. In addition, unqualified attributes on elements in its namespace, other than the attributes explicitly described in this Working Draft, are reserved for future use. To provide for possible future extension and other applications of this catalog, its format allows for "other information" indicated by elements and attributes from namespaces other than the one defined by this Working Draft. A catalog can be used in two different, independent ways: (1) it can be used to locate the replacement text for an external entity, or (2) it can be used to locate an alternate URI reference for a resource. Although these functions are similar in nature, they are distinct and exercise two different sets of entries in the catalog. In either case, the following entries in the catalog are interpreted as follows:
The nextCatalog entry can be used to insert a new catalog entry file into the current list of catalog entry files. The catalog attribute on a nextCatalog entry is used to locate another catalog entry file that is inserted into the catalog entry file list after the current catalog entry file. Multiple nextCatalog entries are allowed, and the referenced catalog entry files are inserted into the existing working catalog entry file list in the order in which they occur in the current catalog entry file (document order). Catalog entry files identified by nextCatalog entries will only be examined after all other entries in the current catalog entry file have been considered and none of them provide a match for the current input. In the discussion that follows, note that catalog resolution semantics are not recursive. Once a matching catalog entry has been found, the value that results from that entry is returned without further examination of the catalog. External Identifiers, as defined in [Production 75] of [XML], identify the external subset, entities, and notations of an XML document. They are not used to identify other resources such as namespace names, stylesheets, and schema languages other than DTDs; URI entries are used for that purpose. For the purposes of resolving external identifiers, a catalog-based resolver considers the following entries:
Although system identifiers are assumed to be "URI reference[s]…meant to be dereferenced to obtain input for the XML processor to construct the entity's replacement text", in some circumstances (such as when the document was generated on another system, when the document was generated in another location on the same system, or when some files referenced by system identifiers have moved since the document was generated), the specified system identifiers are not always the best identifiers for the replacement text. For this or other reasons, it may be desirable to prefer the public identifier over the system identifier in determining the entity's replacement text. Therefore, this Working Draft defines two modes for searching the catalog: "prefer system identifier" mode and "prefer public identifier" mode.
The prefer attribute can be used on catalog and group entry types to indicate, for the enclosed set of catalog entries, if system or public entry matches are preferred. Each occurrence of a prefer attribute specifies the search strategy mode for entries contained within the catalog or group element on which it occurs. A public or delegatePublic entry encountered when prefer is "public" will be considered for possible matching whether or not the external identifier has an explicit system identifier. A public or delegatePublic entry encountered when prefer is "system" will be ignored during lookups for which the external identifier has an explicit system identifier. No other entry types are affected by the prefer attribute. The initial search strategy in force at the beginning of each catalog entry file depends on the preference as determined by the application. An application must provide some way (e.g., a runtime argument, environment variable, preference switch) that allows the user to specify which of these modes to use in the absence of any occurrence of the prefer attribute on the catalog entry. When doing a catalog lookup, an entity manager generally uses whatever is available from among the entity declaration's system identifier and public identifier to find catalog entries that match the given information. A match in one catalog entry file will take precedence over any match in a later catalog entry file (and, in fact, the entity manager need not process subsequent catalog entry files once a match has occurred). URI references that are not part of an external identifier, such as namespace names, stylesheets, included files, graphics, and hypertext references, simply identify other resources. They are resolved using URI entries as described below. The input to a resolver that locates resources is simply the original URI reference. For the purposes of resolving URI references, a catalog-based resolver considers the following entries:
As when resolving URI references, a match in one catalog entry file will take precedence over any match in a later catalog entry file (and, in fact, the entity manager need not process subsequent catalog entry files once a match has occurred). Rewrite entries are provided as a convenience for performing redirection of a whole set of entities with a single catalog entry. Typical uses are website mirroring and dealing with fragment identifiers. Note that in the case of fragment identifiers, rewriting can be applied to the base URI that precedes the fragment identifier. The resolver never sees the fragment identifier part of the URI reference (the # or the characters that follow it). If the entire website at http://example.com/ has been mirrored onto your local system in file:///share/mirrors/example/, it is likely that you want any system identifier reference to the website to be redirected to your local system. One way of doing this would be to create a system entry for every relevant identifier. If there are many entities on the website, this may be tedious. Instead, a single rewrite entry can be used: <rewriteSystem systemIdStartString="http://www.example.com/" rewritePrefix="file:///share/mirrors/example/"/> Similarly, if you have a large number of references to a single document using many different fragment identifiers, it may be tedious to construct uri entries for every URI reference if the base document moves. Again, a single rewrite can be used instead: <rewriteURI uriStartString="http://www.example.com/old-location/" rewritePrefix="http://www.example.com/new-location/"/> The catalog files in Example 1, “A DocBook XML Catalog File: docbook.xml.” and Example 2, “A Stylesheet XML Catalog File: stylesheet.xml.” are complete examples of XML Catalog files. Example 1. A DocBook XML Catalog File: docbook.xml. <!DOCTYPE catalog PUBLIC "-//OASIS//DTD XML Catalogs 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" prefer="public"> <group xml:base="http://www.oasis-open.org/docbook/xml/4.1.2/"> <public publicId="-//OASIS//DTD DocBook XML V4.1.2//EN" uri="docbookx.dtd"/> <public publicId="-//OASIS//ENTITIES DocBook XML Notations V4.1.2//EN" uri="dbnotnx.mod"/> <public publicId="-//OASIS//ENTITIES DocBook XML Character Entities V4.1.2//EN" uri="dbcentx.mod"/> <public publicId="-//OASIS//ELEMENTS DocBook XML Information Pool V4.1.2//EN" uri="dbpoolx.mod"/> <public publicId="-//OASIS//ELEMENTS DocBook XML Document Hierarchy V4.1.2//EN" uri="dbhierx.mod"/> <public publicId="-//OASIS//ENTITIES DocBook XML Additional General Entities V4.1.2//EN" uri="dbgenent.mod"/> <public publicId="-//OASIS//DTD DocBook XML CALS Table Model V4.1.2//EN" uri="calstblx.dtd"/> </group> <public publicId="-//OASIS//DTD DocBook MathML Module V1.0//EN" uri="http://www.oasis-open.org/docbook/xml/mathml/1.0/dbmathml.dtd"/> <nextCatalog catalog="stylesheets.xml"/> </catalog> Example 2. A Stylesheet XML Catalog File: stylesheet.xml. <!DOCTYPE catalog PUBLIC "-//OASIS//DTD XML Catalogs 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" prefer="public"> <!-- Circumvent relative URI in spec.xsl that doesn't work online --> <uri name="http://www.oasis-open.org/committes/tr.xsl" uri="http://www.oasis-open.org/committes/entity/stylesheets/base/tr.xsl"/> </catalog> Together, these two catalog files provide sufficient resolution information to parse and format the XML source for this Working Draft. Applications conforming to this Working Draft must provide some (implementation dependent) mechanism that allows the user to establish the initial list of catalog entry files. This may be a preferences dialog, an environment variable, an application properties file, or any other appropriate mechanism. All conforming processors must accept and process catalog entry files written in the format described by this specification. They may also accept and process other formats, but they are not required to do so. If an application encounters a catalog entry file in a format that it does not understand, it must treat it as a resource failure. If a document contains external identifiers or URI references, it may be useful for the document to identify a catalog that is likely to aid in the resolution of those references. For example, XML documents stored on the www.example.com server may wish to indicate that http://www.example.com/catalog is a useful public catalog to use when parsing them. This Working Draft defines the processing instruction "<?oasis-xml-catalog?>" for this purpose. The <?oasis-xml-catalog?> processing instruction has a single pseudo-attribute, catalog, that identifies a single catalog entry file. If a document contains one or more <?oasis-xml-catalog?> processing instruction(s), the catalog entry file(s) identified must be used during resolution of external identifiers and URI references within that document. Catalog entry files referenced by the processing instruction are added to the end of any system- or user-defined catalog entry file list. For example, in <?xml version="1.0"?> <?oasis-xml-catalog catalog="http://example.com/catalog.xml"?> <!DOCTYPE doc PUBLIC "-//Example//DTD Document V1.0//EN" "http://www.example.com/schema/doc.dtd"> ... The URI "http://example.com/catalog.xml" is added to the end of the of the list of catalog entry files used for resolution within this document. The following constraints apply:
Catalog-aware applications should support the <?oasis-xml-catalog?> processing instruction. If the processing instruction is supported, they must provide a facility which allows a user to request that all <?oasis-xml-catalog?> processing instructions be ignored. One common idiom for controlling parser features is the use of a feature URI. This Working Draft defines the following feature URI for this purpose: http://www.oasis-open.org/committees/entity/features/catalog-pi If this feature is disabled, <?oasis-xml-catalog?> processing instructions must be ignored. XML Catalog files are XML documents and as such may contain external identifiers and URI references. Conformant processors are not required to be able to perform resolution of those identifiers through the XML Catalog. Implementations are encouraged to provide some sort of bootstrapping functionality to resolve external identifiers and URIs that the implementation needs to load catalog entry files. For example, presented with the following catalog entry file: <!DOCTYPE catalog PUBLIC "-//OASIS//DTD XML Catalogs 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" prefer="public"> <public publicId="-//OASIS//DTD DocBook XML V4.1.2//EN" uri="docbookx.dtd"/> </catalog> an implementation should recognize the standard external identifier used on the catalog and provide the parser with access to that DTD in some implementation defined way if it's necessary. Users can avoid any problems that might arise by limiting the external identifiers and URIs used to those that do not need resolution. Note that this only applies to external identifiers and URIs that must be resolved in order to load the catalog entry file. For example, if a local copy of the XML Catalog DTD is available at /etc/xml/catalog.dtd, the problems of resolution associated with loading this file can be avoided by pointing directly to that local copy in the catalog entry file: <!DOCTYPE catalog PUBLIC "-//OASIS//DTD XML Catalogs V1.0//EN" "/etc/xml/catalog.dtd"> <catalog xmlns="urn:oasis:names:tc:entity:xmlns:xml:catalog" prefer="public"> <public publicId="-//OASIS//DTD DocBook XML V4.1.2//EN" uri="docbookx.dtd"/> </catalog> As noted, catalog resolution semantics are not recursive. Once a matching catalog entry has been found, the value that results from that entry is returned without further examination of the catalog. In other words, if the catalog contains the following entries and only these entries: <uri name="http://example.com/path/resource" uri="http://example.com/alternate/resource"/> <uri name="http://example.com/alternate/resource" uri="http://example.com/final/resource"/> An attempt to resolve http://example.com/path/resource will return "http://example.com/alternate/resource". The fact that the URI returned would be subject to a different interpretation if it was passed to the resolver has no effect on the resolver: it stops when a match is found. (This example uses URI entries, but the semantics hold true for all entry types.) This avoids an obvious opportunity for circular reference inside the resolver. However, applications are free to make multiple calls to the resolver if they wish, in which case it is the responsibility of the application to handle any circularities that arise. Even so, catalog circularities may arise. Implementations should detect circularity, but it may be impractical or impossible in some circumstances. If a circularity is detected, it must be treated as an error. Applications may recover from this error by indicating to the calling application that no match was found. Given the dynamic nature of resources on the internet, it may not always be possible for implementations to detect circular references. Failure to detect circularity of references is not a failure to conform to this specification. Each catalog entry file consists of some number of catalog entries. Catalog entries can be identified by the namespace name defined by this Working Draft. Elements and attributes from other namespaces are are allowed, but they must be ignored for the purposes of resolution defined by this Working Draft. If an element is ignored, all of its descendants must also be ignored, regardless of their namespace. There are two attributes common to most elements: id and xml:base. The id is provided so that individual entries can be uniquely identified; it has no impact on the semantics of the catalog as defined by this Working Draft. The xml:base attribute changes the base URI for the entry on which it occurs (and all entries contained within it, unless further modified by another xml:base attribute). The semantics of xml:base are normatively defined in [XML Base]. All of the attributes defined by this Working Draft are in the per-element-type partition. Use of qualified attributes, for example, <cat:group cat:id="groupId"> instead of <cat:group id="groupId"> is forbidden. In order to accurately and interoperably compare public identifiers, catalog processors must perform normalization on public identifiers in both the catalog and the input passed to them. All strings of white space in public identifiers must be normalized to single space characters (#x20), and leading and trailing white space must be removed. In order to accurately and interoperably compare system identifiers and URI references, catalog processors must perform normalization. The normalization described in this section must be performed on system identifiers and URI references passed as input to the resolver and on strings in the catalog that are compared to them. URI references require encoding and escaping of certain characters. The disallowed characters include all non-ASCII characters, plus the excluded characters listed in Section 2.4 of [RFC 2396], except for the number sign (#) and percent sign (%) characters and the square bracket characters re-allowed in [RFC 2732]. These characters are summarized in Table 1, “Excluded US-ASCII Characters”. Table 1. Excluded US-ASCII Characters
Catalog processors must escape disallowed characters as follows:
Note that this normalization process is idempotent: repeated normalization does not change a normalized URI reference. This Working Draft requires processors to implement special treatment of URNs in the publicid URN Namespace ([RFC 3151]). URNs of this form must, in some contexts, be "unwrapped" by the Catalog processor. This unwrapping translates the URN form of the public identifier back into the standard ISO 8879 form for the purposes of subsequent catalog processing. Unwrapping a urn:publicid: URN is accomplished by transcribing characters in the URN according to the following table after discarding the leading urn:publicid: string:
For example, the following URN in the publicid namespace: urn:publicid:-:OASIS:DTD+DocBook+XML+V4.1.2:EN Represents the public identifier: -//OASIS//DTD DocBook XML V4.1.2//EN URNs in the publicid namespace should always represent normalized public identifiers (Section 6.2, “Public Identifier Normalization”). In the event that an unwrapped public identifier is not normalized, the catalog processor must normalize it. URNs in the publicid namespace are intended for use in documents. Since the resolver is required to unwrap them before searching the catalog, they should never be used literally in the catalog. (Nothing will ever match them.) The root element of a catalog entry file is catalog. There are ten other element types: group, public, system, rewriteSystem, delegatePublic, delegateSystem, rewriteURI, delegateURI, uri, and nextCatalog. Each of these element types is described in one of the following sections. Each XML Catalog entry file consists of a single catalog element. This element may set the global prefer value and global base URI. It is otherwise just a container for the other elements.
The group element is a convenience wrapper for specifying a prefer setting or base URI for a set of catalog entries. It has no semantics other than scoping these settings.
NoteThe ability to scope the prefer and and base URI settings is required in order to reasonably translate existing [TR 9401] catalogs into XML Catalogs. The public element associates a URI reference with the public identifier portion of an external identifier.
A public entry matches a public identifier if the normalized value (Section 6.2, “Public Identifier Normalization”) of the public identifier is lexically identical to the normalized value of the publicId attribute of the entry. If the value of the uri attribute is relative, it must be made absolute with respect to the base URI currently in effect. The URI reference should not include a fragment identifier. The system element associates a URI reference with the system identifier portion of an external identifier.
A system entry matches a system identifier if the normalized value (Section 6.3, “System Identifier and URI Normalization”) of the system identifier is lexically identical to the normalized value of the systemId attribute of the entry. If the value of the uri attribute is relative, it must be made absolute with respect to the base URI currently in effect. The URI reference should not include a fragment identifier. The rewriteSystem element rewrites the beginning of a system identifier.
A rewriteSystem entry matches a system identifier if the normalized value (Section 6.3, “System Identifier and URI Normalization”) of the system identifier begins precisely with the normalized value of the systemIdStartString attribute of the entry. If the value of the rewritePrefix attribute is relative, it must be made absolute with respect to the base URI currently in effect. Rewriting removes the matching prefix from the supplied system identifier and replaces it with the value of the rewritePrefix attribute, returning the entire URI with the new prefix. If more than one rewriteSystem entry matches, the matching entry with the longest normalized systemIdStartString value is used. Given the following catalog fragment: <rewriteSystem systemIdStartString="http://www.oasis-open.org/" rewritePrefix="file:///share/doctypes/oasis/"/> <rewriteSystem systemIdStartString="http://www.oasis-open.org/docbook/" rewritePrefix="file:///sourceforge/docbook/docbook/"/> <rewriteSystem systemIdStartString="http://www.oasis-open.org/committees/" rewritePrefix="file:///projects/oasis/"/> The first two entries match the system identifier "http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd", but the third does not. The rewritten system identifier in this case is: "file:///sourceforge/docbook/docbook/xml/4.1.2/docbookx.dtd". The delegatePublic element associates an alternate catalog with a partial public identifier.
A delegatePublic entry matches a public identifier if the normalized value (Section 6.2, “Public Identifier Normalization”) of the public identifier begins precisely with the normalized value of the publicIdStartString attribute of the entry. Given the following catalog fragment: <delegatePublic publicIdStartString="-//OASIS//" catalog="http://www.oasis-open.org/catalog"/> <delegatePublic publicIdStartString="-//OASIS//DTD DocBook " catalog="http://www.oasis-open.org/docbook/catalog"/> <delegatePublic publicIdStartString="-//OASIS//DTD XML Catalog //" catalog="http://www.oasis-open.org/committees/entity/catalog"/> The first two entries match the public identifier "-//OASIS//DTD DocBook V4.1.2//EN", but the third does not. If the value of the catalog attribute is relative, it must be made absolute with respect to the base URI currently in effect. The delegateSystem element associates an alternate catalog with a partial system identifier.
A delegateSystem entry matches a system identifier if the normalized value (Section 6.3, “System Identifier and URI Normalization”) of the system identifier begins precisely with the normalized value of the systemIdStartString attribute of the entry. Given the following catalog fragment: <delegateSystem systemIdStartString="http://www.oasis-open.org/" catalog="http://www.oasis-open.org/catalog"/> <delegateSystem systemIdStartString="http://www.oasis-open.org/docbook/" catalog="http://www.oasis-open.org/docbook/catalog"/> <delegatePublic publicIdStartString="http://www.oasis-open.org/committees/" catalog="http://www.oasis-open.org/committees/catalog"/> The first two entries match the system identifier "http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd", but the third does not. If the value of the catalog attribute is relative, it must be made absolute with respect to the base URI currently in effect. The uri element associates an alternate URI reference with a URI reference that is not part of an external identifier.
A uri entry matches a URI reference if the normalized value (Section 6.3, “System Identifier and URI Normalization”) of the URI reference is lexically identical to the normalized value of the name attribute of the entry. Given the following catalog fragment: <uri name="http://www.oasis-open.org/committees/docbook/#membership" uri="file:///projects/oasis/docbook/website/#membership"/> <uri name="http://www.oasis-open.org/committees/docbook/" uri="file:///projects/oasis/docbook/website/"/> The second entry matches the URI reference "http://www.oasis-open.org/committees/docbook/", but the first does not. If the value of the uri attribute is relative, it must be made absolute with respect to the base URI currently in effect. The rewriteURI element rewrites the beginning of a URI reference that is not part of an external identifier.
A rewriteURI entry matches a URI reference if the normalized value (Section 6.3, “System Identifier and URI Normalization”) of the URI reference begins precisely with the normalized value of the uriStartString attribute of the entry. If the value of the rewritePrefix attribute is relative, it must be made absolute with respect to the base URI currently in effect. Rewriting removes the matching prefix from the supplied URI reference and replaces it with the value of the value of the rewritePrefix attribute. If more than one rewriteURI entry matches, the matching entry with the longest normalized uriStartString value is used. The delegateURI element associates an alternate catalog with a partial URI reference.
A delegateURI entry matches a URI reference if the normalized value (Section 6.3, “System Identifier and URI Normalization”) of the URI reference begins precisely with the normalized value of the uriStartString attribute of the entry. If the value of the catalog attribute is relative, it must be made absolute with respect to the base URI currently in effect. The nextCatalog elements indicate additional catalog entry file(s) to be considered during the process of resolution.
If the value of the catalog attribute is relative, it must be made absolute with respect to the base URI currently in effect. Catalogs loaded due to a nextCatalog directive have an initial base URI that is dependent on the location of the loaded catalog entry file. No xml:base information is inherited from the originating catalog. This section describes how catalog resolution is performed. Resolution begins with a list of catalog entry files and either an external identifier or a URI reference. This section describes how catalog entries are used to resolve external identifiers. An external identifier will have at least one and perhaps both of the following:
Resolvers should respect the system identifiers provided by authors. If the system part of the external identifier is relative, resolution should use that relative URI as the system identifier. If resolution with the relative form fails, it's reasonable for resolvers to try again using the absolute form. If the public identifier is a URN in the publicid namespace ([RFC 3151]), it is converted into another public identifier by "unwrapping" the URN (Section 6.4, “URN "Unwrapping"”). This may be done, for example, so that a URN can be specified as the public identifier and a URL as the system identifier, in the absence of widely deployed URN-resolution facilities. If the system identifier is a URN in the publicid namespace, it is converted into a public identifier by "unwrapping" the URN. In this case, one of the following must apply:
Resolution follows the steps listed below, proceeding to each subsequent step if and only if no other action is indicated.
This section describes how catalog entries are used to resolve URI references. URI reference resolution always begins with a single URI reference. Resolvers should respect the URI references provided by authors. If the URI is relative, resolution should use that relative URI. If resolution with the relative form fails, it's reasonable for resolvers to try again using the absolute form. If the URI reference is a URN in the publicid namespace ([RFC 3151]), it is converted into a public identifier by "unwrapping" the URN (Section 6.4, “URN "Unwrapping"”). Resolution continues by following the semantics of external identifier resolution (Section 7.1, “External Identifier Resolution”) as if the public identifier constructed by unwrapping the URN had been provided and no system identifier had been provided. Otherwise, resolution of the URI reference proceeds according to the steps below. Resolution of a generic URI reference follows the steps listed below, proceeding to each subsequent step if and only if no other action is indicated.
The catalog processor is sometimes required to load a catalog entry file. This may occur at the beginning of processing, when dealing with the initial list of catalog entry files, or during subsequent processing of a nextCatalog entry or one of the delegate entries. If the processor attempts to load a resource and fails (because the resource does not exist or is not reachable, for example), it must recover by ignoring the catalog entry file that failed and proceeding. Similarly, if the resource retrieved is not an understandable catalog (because it is not in a format that the processor recognizes, or it purports to be XML but is not well-formed, or for any other reason), the processor must recover by responding as if the resource could not be loaded. In order for a resource to be considered an XML Catalog, the following conditions must hold:
It is not an error for catalog processors to accept other forms of catalog documents, but their identification and specification is outside the scope of this Working Draft. A. A W3C XML Schema for the XML Catalog (Non-Normative)This [W3C XML Schema] grammar defines the syntax for OASIS XML Catalog Working Draft entry files. This grammar has the following identifier:
B. A RELAX NG Grammar for the XML Catalog (Non-Normative)This [RELAX NG] grammar defines the syntax for OASIS XML Catalog Working Draft entry files. This grammar has the following identifier:
C. A DTD for the XML Catalog (Non-Normative)This [XML] DTD grammar partially[1] defines the syntax for OASIS XML Catalog Working Draft entry files. This DTD has the following identifiers:
D. Support for TR9401 Catalog Semantics (Non-Normative)This Working Draft defines a subset of the catalog entry types described in [TR 9401] that are applicable to XML. For implementors wishing to provide full TR9401 support, this appendix defines the elements that should be used for the remaining TR9401 catalog entry types. The elements described in this appendix provide full TR9401 semantics in the XML Catalog format. These are implemented as extension elements in the namespace: "urn:oasis:names:tc:entity:xmlns:tr9401:catalog". For a complete description of the semantics of these elements see [TR 9401]. The doctype element associates a DTD with a document element.
The document element identifies a default document.
The dtddecl element associates an SGML declaration with a public identifier.
The entity element associates a document with an entity name.
The linktype element associates an external subset with a linktype declaration name.
E. OASIS Entity Resolution Committee (Non-Normative)The following individuals are members of the committee that developed this Working Draft:
The following additional individuals were members of the committee during the development of previous versions:
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However, this document itself may not be modified in any way, such as by removing the copyright notice or references to OASIS, except as needed for the purpose of developing OASIS specifications, in which case the procedures for copyrights defined in the OASIS Intellectual Property Rights document must be followed, or as required to translate it into languages other than English. The limited permissions granted above are perpetual and will not be revoked by OASIS or its successors or assigns. This document and the information contained herein is provided on an "AS IS" basis and OASIS DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. OASIS has been notified of intellectual property rights claimed in regard to some or all of the contents of this specification. For more information consult the online list of claimed rights. G. Intellectual Property RightsFor information on whether any patents have been disclosed that may be essential to implementing this specification, and any offers of patent licensing terms, please refer to the Intellectual Property Rights section of the Entity Resolution web page (http://www.oasis-open.org/committees/entity/) Normative[XML] Tim Bray, Jean Paoli, and C. M. Sperberg-McQueen, Eve Maler, editors. Extensible Markup Language (XML) 1.0 Second Edition. World Wide Web Consortium, 2000. [XML Namespaces] Tim Bray, Dave Hollander, and Andrew Layman, editors. Namespaces in XML. World Wide Web Consortium, 1999. [RFC 2119] IETF (Internet Engineering Task Force). RFC 2119: Key words for use in RFCs to Indicate Requirement Levels. S. Bradner. 1997. [RFC 3151] IETF (Internet Engineering Task Force). RFC 3151: A URN Namespace for Public Identifiers. N. Walsh, J. Cowan, P. Grosso, 2001. [XML Base] Jonathan Marsh, editor. XML Base. World Wide Web Consortium, 2000. Non-Normative[XML Schema Datatypes] Paul V. Biron and Ashok Malhotra, editors. XML Schema Part 2: Datatypes. World Wide Web Consortium, 2000. [RELAX NG] James Clark and MURATA Makoto, editors. RELAX NG Specification. OASIS. 2001. [XML Stylesheets] James Clark, editor. Associating Style Sheets with XML Documents Version 1.0. World Wide Web Consortium. 1999. [TR 9401] Paul Grosso, editor. OASIS Technical Resolution 9401:1997 (Amendment 2 to TR 9401). OASIS. 1997. [RFC 2279] IETF (Internet Engineering Task Force). RFC 2279: UTF-8, a transformation format of ISO 10646. F. Yergeau. 1998. [RFC 2396] IETF (Internet Engineering Task Force). RFC 2396: Uniform Resource Identifiers (URI): Generic Syntax. T. Berners-Lee, R. Fielding, L. Masinter. 1998. [RFC 2732] IETF (Internet Engineering Task Force). RFC 2732: Format for Literal IPv6 Addresses in URL's. R. Hinden, B. Carpenter, L. Masinter. 1999. [W3C XML Schema] Henry S. Thompson, David Beech, Murray Maloney, et al. editors. XML Schema Part 1: Structures. World Wide Web Consortium, 2000. [Requirements] Norman Walsh, editor. OASIS Entity Resolution Technical Committee Requirements Document. OASIS. 2000. [1] Any catalog file which is valid according to this DTD is valid according to this Working Draft. However, catalog files which make use of extension elements or attributes may be valid according to this Working Draft but invalid according to this DTD, due to the limits of DTD validation with respect to namespaces. |
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