Internet-Draft (Informational) J. Kunze draft-kunze-dchtml-00.txt 18 March 1999 Dublin Core Expires 18 September 1999 Workshop Series Encoding Dublin Core Metadata in HTML ( 1. Status of this Document This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as ``work in progress.'' To learn the current status of any Internet-Draft, please check the ``1id-abstracts.txt'' listing contained in the Internet-Drafts Shadow Directories on (Africa), (Europe), (Pacific Rim), (US East Coast), or (US West Coast). Distribution of this document is unlimited. Please send comments to or to the discussion list. 2. Abstract The Dublin Core [DC1] is a small set of metadata elements for describing information resources. This document explains how these elements are expressed using the META and LINK tags of HTML [HTML4.0]. A sequence of metadata elements embedded in an HTML file is taken to be a description of that file. Examples illustrate conventions allowing interoperation with current software that indexes, displays, and manipulates metadata, such as [SWISH-E], [freeWAIS-sf2.0], [GLIMPSE], [HARVEST], [ISEARCH], etc., and the Perl [PERL] scripts in the appendix. 3. HTML, Dublin Core, and Non-Dublin Core Metadata The Dublin Core (DC) metadata initiative [DCHOME] has produced a small set of resource description categories [DC1], or elements of metadata (literally, data about data). Metadata elements are typically small relative to the resource they describe and may, if the resource format permits, be embedded in it. Two such formats are the Hypertext Markup Language (HTML) and the Extensible Markup Language (XML); HTML is currently in wide use, but once standardized, XML [XML] in conjunction with the Resource Description Framework [RDF] promise a significantly more expressive means of encoding metadata. This document explains how to encode metadata using HTML 4.0 [HTML4.0]. It is not concerned with element semantics, which are defined elsewhere. For illustrative purposes, some element semantics are alluded to, but in no way should semantics appearing here be considered definitive. The HTML encoding allows elements of DC metadata to be interspersed with non-DC elements (provided such mixing is consistent with rules governing use of those non-DC elements). A DC element is indicated by the prefix "DC", and a non-DC element by another prefix; for example, the prefix "ADMIN" is used with elements from the Admin Core [ADMIN]. 4. The META Tag The META tag of HTML is designed to encode a named metadata element. Each element describes a given aspect of a document or other information resource. For example, this tagged metadata element, says that Homer Simpson is the Creator, where the element named Creator is defined in the DC element set. In the more general form, the capitalized words are meant to be replaced in actual descriptions; thus in the example, ELEMENT_NAME was: Creator ELEMENT_VALUE was: Simpson, Homer and PREFIX was: DC Within a META tag the first letter of a Dublin Core element name is capitalized. DC places no restriction on alphabetic case in an element value and any number of META tagged elements may appear together, in any order. More than one DC element with the same name may appear, and each DC element is optional. The next example is a book description with two authors, two titles, and no other metadata. The prefix "DC" precedes each Dublin Core element encoded with META, and it is separated by a period (.) from the element name following it. Each non-DC element should be encoded with a prefix that can be used to trace its origin and definition; the linkage between prefix and element definition is made with the LINK tag, as explained in the next section. Non-DC elements, such as CreatorEmail from the Admin Core [ADMIN], may appear together with DC elements, as in This example also shows how some special characters may be encoded. The author name in the first element contains a diacritic encoded as an HTML character entity reference -- in this case an accented letter E. Similarly, the last line contains two double-quote characters encoded so as to avoid being interpreted as element content delimiters. 5. The LINK Tag The LINK tag of HTML may be used to associate an element name prefix with the reference definition of the element set that it identifies. A sequence of META tags describing a resource is incomplete without one such LINK tag for each different prefix appearing in the sequence. The previous example could be considered complete with the addition of these two LINK tags: In general, the association takes the form where, in actual descriptions, PREFIX is to be replaced by the prefix and LOCATION_OF_DEFINITION by the URL or URN of the defining document. When embedded in the HEAD part of an HTML file, a sequence of LINK and META tags describes the information in the surrounding HTML file itself. Here is a complete HTML file with its own embedded description. A Dirge
            Rough wind, that moanest loud
              Grief too sad for song;
            Wild wind, when sullen cloud
              Knells all the night long;
            Sad storm, whose tears are vain,
            Bare woods, whose branches strain,
            Deep caves and dreary main, -
              Wail, for the world's wrong!
6. Encoding Recommendations HTML allows more flexibility in principle and in practice than is recommended here for encoding metadata. Limited flexibility encourages easy development of software for extracting and processing metadata. At this early evolutionary stage of internet metadata, easy prototyping and experimentation hastens the development of useful standards. Adherence is therefore recommended to the tagging style exemplified in this document as regards prefix and element name capitalization, double-quoting (") of attribute values, and not starting more than one META tag on a line. There is much room for flexibility, but choosing a style and sticking with it will likely make metadata manipulation and editing easier. The following META tags adhere to the recommendations and carry identical metadata in three different styles: Use of these recommendations is known to result in metadata that may be harvested, indexed, and manipulated by popular, freely available software packages such as [SWISH-E], [freeWAIS-sf2.0], [GLIMPSE], [HARVEST], [ISEARCH] among others. These conventions also work with the metadata processing scripts appearing in the appendix, as well as with most of the [DCPROJECTS] applications referenced from the [DCHOME] site. Software support for the LINK tag and qualifier conventions (see the next section) is not currently widespread. Ordering of metadata elements is not preserved in general. Writers of software for metadata indexing and display should try to preserve relative ordering among META tagged elements having the same name (e.g., among multiple authors), however, metadata providers and searchers have no guarantee that ordering will be preserved in metadata that passes through unknown systems. 7. Dublin Core in Real Descriptions In actual resource description it is often necessary to qualify Dublin Core elements to add nuances of meaning. While neither the general principles nor the specific semantics of DC qualifiers are within scope of this document, everyday uses of the qualifier syntax are illustrated to lend realism to later examples. Without further explanation, the three ways in which the optional qualifier syntax can supplement the META tag may be summarized as follows: Accordingly, a posthumous work in Spanish might be described with 8. Encoding Dublin Core Elements This section consists of very simple Dublin Core encoding examples, arranged by element. Title (name given to the resource) ----- Creator (entity that created the intellectual content) ------- Subject (topic or keyword) ------- Description (textual description or abstract) ----------- Publisher (entity that made the resource available) --------- Contributor (other entity that made an intellectual contribution) ----------- Date (of creation or availability of resource; [WTN8601] recommended) ---- Type (category or genre) ---- Format (data format, plus optional dimensions) ------ Identifier (of the resource) ---------- Source (information about the resource's origin) ------ Language (of the content of the resource; [RFC1766] recommended) -------- Relation (relationship to a second resource plus its identifier) -------- Coverage (spatial or temporal features of the intellectual content) -------- Rights (text or identifier of a rights management statement) ------ 9. Security Considerations Metadata element encoding poses no risk to computers and networks. It poses minimal risk to searchers who might obtain incorrect information due to deliberate misrepresentation of resource metadata (e.g., index spam). It also poses minimal risk to providers who might accidentally expose private metadata. Neither of these risks is greater for this encoding than for any of the mainstream methods for expressing metadata (e.g., [MARC]). No other security concerns are likely to be raised by the HTML encoding documented here. 10. Copyright Notice Copyright (C) The Internet Society (date). All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implmentation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process 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 the Internet Society or its successors or assigns. This document and the information contained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE 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. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights which may cover technology that may be required to practice this standard. Please address the information to the IETF Executive Director. 11. References [AAT] Art and Architecture Thesaurus, Getty Information Institute, [ADMIN] Admin Core - Administrative Container Metadata, (in progress) [DC1] RFC 2413, Dublin Core Metadata for Resource Discovery, September 1998, [DCHOME] Dublin Core Initiative Home Page, [DCPROJECTS] Projects Using Dublin Core Metadata, [DCT1] Dublin Core Type List 1, DC Type Working Group, March 1999, [freeWAIS-sf2.0] The enhanced freeWAIS distribution, February 1999, [GLIMPSE] Glimpse Home Page, [HARVEST] Harvest Web Indexing, [HTML4.0] Hypertext Markup Language 4.0 Specification, April 1998, [ISEARCH] Isearch Resources Page, [ISO639-2] Code for the representation of names of languages, 1996, [ISO8601] ISO 8601:1988(E), Data elements and interchange formats -- Information interchange -- Representation of dates and times, International Organization for Standardization, June 1988. [MARC] USMARC Format for Bibliographic Data, US Library of Congress, [PERL] L. Wall, T. Christiansen, R. Schwartz, Programming Perl, Second Edition, O'Reilly, 1996. [RDF] Resource Description Framework Model and Syntax Specification, February 1999, [RFC1766] RFC 1766, Tags for the Identification of Languages, [SWISH-E] Simple Web Indexing System for Humans - Enhanced, http://sunsite.Berkeley.EDU/SWISH-E/ [TGN] Thesaurus of Geographic Names, Getty Information Institute, [WTN8601] W3C Technical Note - Profile of ISO 8601 Date and Time Formats [XML] Extensible Markup Language (XML), 12. Author's Address John A. Kunze Center for Knowledge Management University of California, San Francisco 530 Parnassus Ave, Box 0840 San Francisco, CA 94143-0840, USA Voice: +1 510-525-8575 Email: Fax: +1 415-476-4653 13. Appendix -- Perl Scripts that Manipulate HTML Encoded Metadata This section contains two simple programs that work with versions 4 and 5 of the Perl [PERL] scripting language interpreter. They may be taken and freely adapted for local organizational needs, research proposals, venture capital bids, etc. A variety of applications are within easy reach of implementors that choose to build on these scripts. Script 1: Metadata Format Conversion ------------------------------------- Here is a simple Perl script that correctly recognizes every example of metadata encoding in this document. It shows how a modest scripting effort can produce a utility that converts metadata from one format to another. Minor changes are sufficient to support a number of output formats. #!/depot/bin/perl # # This simple perl script extracts metadata embedded in an HTML file # and outputs it in an alternate format. Issues warning about missing # element name or value. # # Handles mixed case tags and attribute values, one per line or spanning # several lines. Also handles a quoted string spanning multiple lines. # No error checking. Does not tolerate more than one ") { next if (! //i) { while (<>) { $meta .= $_; last if (/>/); } } $name = $meta =~ /name\s*=\s*"([^"]*)"/i ? $1 : "MISSING ELEMENT NAME"; $content = $meta =~ /content\s*=\s*"([^"]*)"/i ? $1 : "MISSING ELEMENT VALUE"; ($scheme) = $meta =~ /scheme\s*=\s*"([^"]*)"/i; ($lang) = $meta =~ /lang\s*=\s*"([^"]*)"/i; if ($lang || $scheme) { $mod = " ($lang"; if (! $scheme) { $mod .= ")"; } elsif (! $lang) { $mod .= "$scheme)" } else { $mod .= ", $scheme)"; } } else { $mod = ""; } print " @|$name$mod; $content\n"; } print "@)urc;\n"; # ---- end of Perl script ---- When the conversion script is run on the metadata file example from the LINK tag section (section 5), it produces the following output. @(urc; @|DC.Title; A Dirge @|DC.Creator; Shelley, Percy Bysshe @|DC.Type; poem @|DC.Date; 1820 @|DC.Format; text/html @|DC.Language; en @)urc; Script 2: Automated Metadata Creation -------------------------------------- The creation and maintenance of high-quality metadata can be extremely expensive without automation to assist in processes such as supplying pre-set or computed defaults, validating syntax, verifying value ranges, spell checking, etc. Considerable relief could be had from a script that reduced an individual provider's metadata burden to just the title of each document. Below is such a script. It lets the provider of an HTML document abbreviate an entire embedded resource description using a single HTML comment statement that looks like Our script processes this statement specially as a kind of "metadata block" declaration with attached title. The general form is This statement works much like a "Web server-side include" in that the script replaces it with a fully-specified block of metadata and triggers other replacements. Once installed, the script can output HTML files suitable for integration into one's production Web server procedures. The individual provider keeps a separate "template" file of infrequently changing pre-set values for metadata elements. If the provider's needs are simple enough, the only element values besides the title that differ from one document to the next may be generated automatically. Using the script, values may be referenced as variables from within the template or the document. Our variable references look like "(--mbVARNAME)", and here is what they look like inside a template: (--mbtitle) The above template represents the metadata block that will describe the document once the variable references are replaced with real values. By the conventions of our script, the following variables will be replaced in both the template and in the document: (--mbfilesize) size of the final output file (--mbtitle) title of the document (--mblanguage) language of the document (--mbbaseURL) beginning part of document identifier (--mbfilename) last part (minus .html) of identifier (--mbfilemodtime) last modification date of the document Here's an example HTML file to run the script on.

From: Acting Shift Supervisor To: Plant Control Personnel RE: (--mbtitle) Date: (--mbfilemodtime)

Pursuant to directive DOH:10.2001/405aec of article B-2022, subsection regarding staff morale and employee productivity standards, the current allocation of doughnut acquisition funds shall be increased effective immediately. Note that because replacement occurs throughout the document, the provider need only enter the title once instead of twice (normally the title must be entered once in the HTML head and again in the HTML body). After running the script, the above file is transformed into this: Nutritional Allocation Increase

From: Acting Shift Supervisor To: Plant Control Personnel RE: Nutritional Allocation Increase Date: 1999-03-08

Pursuant to directive DOH:10.2001/405aec of article B-2022, subsection regarding staff morale and employee productivity standards, the current allocation of doughnut acquisition funds shall be increased effective immediately. Here is the script that accomplishes this transformation. #!/depot/bin/perl # # This Perl script processes metadata block declarations of the form # and variable references of the # form (--mbVARNAME), replacing them with full metadata blocks and # variable values, respectively. Requires a "template" file. # Outputs an HTML file. # # Invoke this script with a single filename argument, "foo". It creates # an output file "foo.html" using a temporary working file "". # The size of is measured after variable replacement, and is # later inserted into the file in such a way that the file's size does # not change in the process. Has little or no error checking. $infile = shift; open(IN, "< $infile") or die("Could not open input file \"$infile\""); $workfile = "$"; unlink($workfile); open(WORK, "+> $workfile") or die("Could not open work file \"$workfile\""); @offsets = (); # records locations for late size replacement $title = ""; # gets the title during metablock processing $language = "en"; # pre-set language here (not in the template) $baseURL = ""; # pre-set base URL here also $filename = "$infile.html"; # final output filename $filesize = "(--mbfilesize)"; # replaced late (separate pass) ($year, $month, $day) = (localtime( (stat IN) [9] ))[5, 4, 3]; $filemodtime = sprintf "%s-%02s-%02s", 1900 + $year, 1 + $month, $day; sub putout { # outputs current line with variable replacement if (! /\(--mb/) { print WORK; return; } if (/\(--mbfilesize\)/) # remember where it was { push @offsets, tell WORK; } # but don't replace yet s/\(--mbtitle\)/$title/g; s/\(--mblanguage\)/$language/g; s/\(--mbbaseURL\)/$baseURL/g; s/\(--mbfilename\)/$filename/g; s/\(--mbfilemodtime\)/$filemodtime/g; print WORK; } while () { # main loop for input file if (! /(.*)(.*)//) { $remainder = $1; } else { while () { $title .= $_; last if (/(.*)\s*-->(.*)/); } $title .= $1; $remainder = $2; } open(TPLATE, "< template") or die("Could not open template file"); while () # subloop for template file { &putout; } close(TPLATE); $_ = $remainder; &putout; } close(IN); # Now replace filesize variables without altering total byte count. select( (select(WORK), $| = 1) [0] ); # first flush output so we if (($size = -s WORK) < 100000) # can get final file size { $scale = 0; } # and set scale factor or else { # compute it, keeping width of size field low for ($scale = 0; $size >= 1000; $scale++) { $size /= 1024; } } $filesize = sprintf "%7.7s %sbytes", $size, (" ", "K", "M", "G", "T", "P") [$scale]; foreach $pos (@offsets) { # loop through saved size locations seek WORK, $pos, 0; # read the line found there $_ = ; # $filesize must be exactly as wide as "(--mbfilesize)" s/\(--mbfilesize\)/$filesize/g; seek WORK, $pos, 0; # rewrite it with replacement print WORK; } close(WORK); rename($workfile, "$filename") or die("Could not rename \"$workfile\" to \"$filename\""); # ---- end of Perl script ---- Internet-Draft (Informational) Expires 18 September 1999