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Network Working Group J. Kunze
Request for Comments: 2731 Dublin Core
Category: Informational Metadata Initiative
December 1999
Encoding Dublin Core Metadata in HTML
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (1999). All Rights Reserved.
1. 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.
2. 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. The [RDF] specification actually describes a way to use
RDF within an HTML document by adhering to an abbreviated syntax.
Kunze Informational [Page 1]
RFC 2731 Encoding Dublin Core Metadata in HTML December 1999
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 "AC" is used with elements from the A-Core [AC].
3. 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.
Kunze Informational [Page 2] RFC 2731 Encoding Dublin Core Metadata in HTML December 1999
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 Email from the A-Core [AC],
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.
4. 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
Kunze Informational [Page 3] RFC 2731 Encoding Dublin Core Metadata in HTML December 1999 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.
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!
5. 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. Kunze
Informational [Page 4] RFC 2731 Encoding Dublin Core Metadata in
HTML December 1999 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], and [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. 6.
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 is currently (subject
to change) used to supplement the META tag may be summarized as
follows: Kunze Informational [Page 5] RFC 2731 Encoding Dublin Core
Metadata in HTML December 1999 Accordingly, a posthumous work in
Spanish might be described with Note that the qualifier syntax and
label suffixes (which follow an element name and a period) used in
examples in this document merely reflect current trends in the HTML
encoding of qualifiers. Use of this syntax and these suffixes is
neither a standard nor a recommendation. 7. Encoding Dublin Core
Elements This section consists of very simple Dublin Core encoding
examples, arranged by element. Title (name given to the resource)
----- Kunze Informational [Page 6] RFC 2731 Encoding Dublin Core
Metadata in HTML December 1999 Creator (entity that created the
content) ------- Subject (topic or keyword) ------- Kunze
Informational [Page 7] RFC 2731 Encoding Dublin Core Metadata in
HTML December 1999 Description (account, summary, or abstract of
the content) ----------- Publisher (entity that made the resource
available) --------- Contributor (other entity that made a
contribution) ----------- Kunze Informational [Page 8] RFC 2731
Encoding Dublin Core Metadata in HTML December 1999 Date (of an
event in the life of the resource; [WTN8601] recommended) ---- Type
(nature, genre, or category; [DCT1] recommended) ---- Kunze
Informational [Page 9] RFC 2731 Encoding Dublin Core Metadata in
HTML December 1999 Format (physical or digital data format, plus
optional dimensions) ------ Kunze Informational [Page 10] RFC 2731
Encoding Dublin Core Metadata in HTML December 1999 Identifier (of
the resource) ---------- Source (reference to the resource's
origin) ------ Language (of the content of the resource; [RFC1766]
recommended) -------- Kunze Informational [Page 11] RFC 2731
Encoding Dublin Core Metadata in HTML December 1999 Relation
(reference to a related resource) -------- Coverage (extent or
scope of the content) -------- Kunze Informational [Page 12] RFC 2731 Encoding Dublin Core Metadata in HTML December 1999 Rights
(text or identifier of a rights management statement) ------ 8.
Security Considerations The syntax rules for encoding Dublin Core
metadata in HTML that are documented here pose no direct risk to
computers and networks. People can use these rules to encode
metadata that is inaccurate or even deliberately misleading
(creating mischief in the form of "index spam"), however, this
reflects a general pattern of HTML META tag abuse that is not
limited to the encoding of metadata from the Dublin Core set. Even
traditional metadata encoding schemes (e.g., [MARC]) are not immune
to inaccuracy, although they are generally followed in environments
where production quality greatly exceeds that of the average Web
site. Systems that process metadata encoded with META tags need to
consider issues related to its accuracy and validity as part of
their design and implementation, and users of such systems need to
consider the design and implementation assumptions. Various
approaches may be relevant for certain applications, such as adding
statements of metadata provenance, signing of metadata with digital
signatures, and automating certain aspects of metadata creation;
but these are far outside the scope of this document and the
underlying META tag syntax that it describes. Kunze Informational
[Page 13] RFC 2731 Encoding Dublin Core Metadata in HTML December
1999 9. 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) Kunze Informational [Page 14] RFC 2731 Encoding
Dublin Core Metadata in HTML December 1999 { $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 4), 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
Kunze Informational [Page 15] RFC 2731 Encoding Dublin Core
Metadata in HTML December 1999 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 within the document. Our
variable references have the form "(--mbVARNAME)", and here is what
they look like inside a template: 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 Kunze
Informational [Page 16] RFC 2731 Encoding Dublin Core Metadata in
HTML December 1999 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 48.2.4.4.1c 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: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 48.2.4.4.1c 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 "foo.work". # The size of foo.work 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\""); $workfile = "$infile.work"; 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 = "http://moes.bar.com/doh"; # pre-set base URL here also $filename = "$infile.html"; # final output filename $filesize = "(--mbfilesize)"; # replaced late (separate pass) Kunze Informational [Page 18] RFC 2731 Encoding Dublin Core Metadata in HTML December 1999 ($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; Kunze Informational [Page 19] RFC 2731 Encoding Dublin Core Metadata in HTML December 1999 } 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 ---- Kunze Informational [Page 20] RFC 2731 Encoding Dublin Core Metadata in HTML December 1999 10. 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 Fax: +1 415-476-4653 EMail: jak@ckm.ucsf.edu 11. References [AAT] Art and Architecture Thesaurus, Getty Information Institute. http://shiva.pub.getty.edu/aat_browser/ [AC] The A-Core: Metadata about Content Metadata, (in progress) http://metadata.net/ac/draft-iannella-admin-01.txt [DC1] Weibel, S., Kunze, J., Lagoze, C. and M. Wolf, "Dublin Core Metadata for Resource Discovery", RFC 2413, September 1998. ftp://ftp.isi.edu/in-notes/rfc2413.txt [DCHOME] Dublin Core Initiative Home Page. http://purl.org/DC/ [DCPROJECTS] Projects Using Dublin Core Metadata. http://purl.org/DC/projects/index.htm [DCT1] Dublin Core Type List 1, DC Type Working Group, March 1999. http://www.loc.gov/marc/typelist.html [freeWAIS-sf2.0] The enhanced freeWAIS distribution, February 1999. http://ls6-www.cs.uni- dortmund.de/ir/projects/freeWAIS-sf/ [GLIMPSE] Glimpse Home Page. http://glimpse.cs.arizona.edu/ [HARVEST] Harvest Web Indexing. http://www.tardis.ed.ac.uk/harvest/ Kunze Informational [Page 21] RFC 2731 Encoding Dublin Core Metadata in HTML December 1999 [HTML4.0] Hypertext Markup Language 4.0 Specification, April 1998. http://www.w3.org/TR/REC-html40/ [ISEARCH] Isearch Resources Page. http://www.etymon.com/Isearch/ [ISO639-2] Code for the representation of names of languages, 1996. http://www.indigo.ie/egt/standards/iso639/iso639-2- en.html [ISO8601] ISO 8601:1988(E), Data elements and interchange formats -- Information interchange -- Representation of dates and times, International Organization for Standardization, June 1988. http://www.iso.ch/markete/8601.pdf [MARC] USMARC Format for Bibliographic Data, US Library of Congress. http://lcweb.loc.gov/marc/marc.html [PERL] L. Wall, T. Christiansen, R. Schwartz, Programming Perl, Second Edition, O'Reilly, 1996. [RDF] Resource Description Framework Model and Syntax Specification, February 1999. http://www.w3.org/TR/REC-rdf-syntax/ [RFC1766] Alvestrand, H., "Tags for the Identification of Languages", RFC 1766, March 1996. ftp://ftp.isi.edu/in-notes/rfc1766.txt [SWISH-E] Simple Web Indexing System for Humans - Enhanced. http://sunsite.Berkeley.EDU/SWISH-E/ [TGN] Thesaurus of Geographic Names, Getty Information Institute. http://shiva.pub.getty.edu/tgn_browser/ [WTN8601] W3C Technical Note - Profile of ISO 8601 Date and Time Formats. http://www.w3.org/TR/NOTE-datetime [XML] Extensible Markup Language (XML). http://www.w3.org/TR/REC-xml Kunze Informational [Page 22] RFC 2731 Encoding Dublin Core Metadata in HTML December 1999 12. Full Copyright Statement Copyright (C) The Internet Society (1999). 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 implementation 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. Acknowledgement Funding for the RFC Editor function is currently provided by the Internet Society. Kunze Informational [Page 23]