RFC 2731
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.
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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.
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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
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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!
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.
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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:
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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)
-----
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Creator (entity that created the content)
-------
Subject (topic or keyword)
-------
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Description (account, summary, or abstract of the content)
-----------
Publisher (entity that made the resource available)
---------
Contributor (other entity that made a contribution)
-----------
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Date (of an event in the life of the resource; [WTN8601] recommended)
----
Type (nature, genre, or category; [DCT1] recommended)
----
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Format (physical or digital data format, plus optional dimensions)
------
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Identifier (of the resource)
----------
Source (reference to the resource's origin)
------
Language (of the content of the resource; [RFC1766] recommended)
--------
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Relation (reference to a related resource)
--------
Coverage (extent or scope of the content)
--------
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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.
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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)
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{ $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
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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:
(--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
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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:
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 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
$filesize = "(--mbfilesize)"; # replaced late (separate pass)
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($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;
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}
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 ----
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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/
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[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
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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.
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