Guidelines for Dublin Core™ Application Profiles
Guidelines for Dublin Core™ Application Profiles (Working Draft)
|Is Replaced By:||Not applicable|
|Status of Document:||This is a DCMI Working Draft|
|Description of Document:||This document provides guidelines for the creation of Dublin Core™ Application Profiles. The document explains the key components of a Dublin Core™ Application Profile and walks through the process of developing a profile. The document is aimed at designers of application profiles -- people who will bring together metadata terms for use in a specific context. It does not address the creation of machine-readable implementations of an application profile nor the design of metadata applications in an broader sense. For additional technical detail the reader is pointed to further sources. This document represents work in progress.|
Table of contents
- Framework for Dublin Core™ Application Profiles
- Defining Functional Requirements
- Selecting or Developing a Domain Model
- Selecting and Defining Metadata Terms
- Designing the Metadata Record with a Description Set Profile
- Usage Guidelines
- Syntax Guidelines
- Appendix A: Description Set Model (from DCMI Abstract Model)
- Appendix B: MyBookCase Description Set Profile
- Appendix C: Using RDF properties in profiles: a technical primer
When it comes to metadata, one size does not fit all. In fact, one size often does not even fit many. The metadata needs of particular communities and applications are very diverse. The result is a great proliferation of metadata formats, even across applications that have metadata needs in common. The Dublin Core™ Metadata Initiative has addressed this by providing a framework for designing a Dublin Core™ Application Profile (DCAP) that meets specific application needs while providing semantic interoperability with other applications on the basis of globally defined vocabularies and models.
Note that a DCAP is a generic construct that does not require metadata terms defined by DCMI [DCMI-MT]. A DCAP can use any terms that are defined in accordance with the Resource Description Framework of the World Wide Web Consortium, or RDF [RDF] -- a generalized language for data integration -- combining terms from multiple namespaces as needed.
Although creating an application profile takes effort, that effort results in data designed to fit well with other data in semantic webs of "linked data" [LINKED]. The effort also yields better guidance for metadata creators and clear specifications for metadata developers. By articulating what is intended and can be expected from data, application profiles promote the sharing and linking of data within and between communities.
It is recommended that application profiles be developed as team projects involving, at a minimum, both:
- data content specialists, who are knowledgeable in the resources that need to be described an in the metadata used in the description of those resources, and
- data engineers or architects, who understand how to structure the underlying data for interoperability in a linked data environment.
A DCAP is a document (or set of documents) that specifies and describes the metadata used in a particular application. To accomplish this, a profile:
- describes what a community wants to accomplish with its application (Functional Requirements);
- characterizes the types of things described by the metadata and their relationships (Domain Model);
- enumerates the metadata terms to be used and the rules for their use (Description Set Profile and Usage Guidelines); and
- defines the machine syntax that will be used to encode the data (Syntax Guidelines and Data Formats).
The interoperability of DCAP-based metadata in linked data environments derives from its basis in standards: community domain models (characterizing which things are described in a particular field or application), metadata vocabularies (from which the terms used in the DCAP are chosen), the Dublin Core™ Abstract Model (a generic syntax for metadata records) [DCAM], and DCMI syntax guidelines (which use the generic syntax for concrete implementation encodings) [DCMI-ENCODINGS]. The foundation standard on which these domain standards rest is RDF [RDF].
How these standards fit together is shown in the Singapore Framework for Dublin Core™ Application Profiles [DCMI-SF]. The bottom tier, RDF, provides the foundation standards on which domain standards are built. The upper tier holds the design and documentation components of a metadata application. Taking this upper tier as a roadmap, the sections that follow walk through the process of creating a DCAP, with side trips dipping into some technical details when needed. As an illustration, we create a simple application profile that describes books and authors. We call this example MyBookCase.
The purpose of any metadata is to support an activity. Defining clear goals for the application used in that activity is an essential first step.
Functional requirements guide the development of the application profile by providing goals and boundaries and are an essential component of a successful application profile development process. This development is often a broad community task and may involve managers of services, experts in the materials being used, technical application developers, and potential end-users of the services. In addition to data content specialists, this process should involve at least one expert with a deep understanding of the foundation standards.
There are many methodologies to help in the creation of functional requirements, such as business process modeling, and methods for visualizing requirements, such as the Unified Modeling Language [UML]. Many find that the definition of use cases and scenarios for a particular application helps elicit functional requirements that might otherwise be overlooked.
Functional requirements answer questions such as:
- What do you want to accomplish with your application?
- What are the limits of your application? What will it not attempt to do?
- How do you want the application you create to serve your users?
- Will your application need to perform specific actions, such as sorting alphabetically or downloading data in particular formats?
- What are the key characteristics of your resources, and how does this affect your selection of data elements? For example, do you need to handle a variety of character sets?
- What are the key characteristics of your users? Are they associated with a particular institution or are you serving a general public? Do they all speak the same language? How expert are they in relation to the data your application will manage?
- Are there existing community standards that need to be considered?
Functional requirements can include general goals as well as specific tasks that you need to address. Ideally, functional requirements should address the needs of metadata creators, resource users, and application developers so that the resulting application fully supports the needs of the community.
These are some sample requirements from the Scholarly Works Application Profile (SWAP) [SWAP]:
Facilitate identification of open access materials.
Enable identification of the research funder and project code.
A set of functional requirements may include user tasks that must be supported such as the following from the Functional Requirements for Bibliographic Records (FRBR) [FRBR]:
Use the data to find materials that correspond to the user's stated search criteria.
Use the data retrieved to identify an entity.
For the MyBookCase DCAP our functional requirements are:
Use the data to retrieve books with a title search.
Limit a search to a particular language.
Sort retrieved items by publication date.
Find items about a given subject.
Describe the author as a person with a name and email address.
After defining functional requirements, the next step is to select or develop a domain model. A domain model is a description of what _things_your metadata will describe, and the relationships between those things. The domain model is the basic blueprint for the construction of the application profile.
In the MyBookCase DCAP, our things are books and persons, who can be the authors of the books. We will see below how to describe the book using elements such as title and language, and to describe the _person_with a name and an email address. For now, the domain model for our MyBookCase is simply:
Models can be even simpler than this (e.g., just a book), or they can be more complex. The domain model for the Scholarly Works Dublin Core™ Application Profile, for example, is based on the library-community domain model Functional Requirements for Bibliographic Records (FRBR) [FRBR]. SWAP defines "Scholarly Work" in place of FRBR's more general entity "Work", and introduces new agent relationships beyond those in the FRBR, such as "isFundedBy" and "isSupervisedBy." In this way, SWAP makes use of FRBR but customizes the FRBR model to meet its specific needs:
As explained above, the entities in the domain model -- whether Book and Author, Manifestation and Copy, or just a generic Resource -- are types of things to be described in our metadata. The next step is to choose properties for describing these things. For example, a _book_has a title and author , and a person has a name ; title , author , and name are properties.
The next step, then, is to scan available RDF vocabularies to see whether the properties needed already exist. DCMI Metadata Terms [DCMI-MT] is a good source of properties for describing intellectual resources like documents and web pages; the "Friend of a Friend" vocabulary has useful properties for describing people [FOAF]. If the properties one needs are not already available, it is possible to declare one's own (see Appendix C).
For the MyBookCase application profile, we concluded from our functional requirements that a book should have a title , date , language , subject , and author.
The first consideration in evaluating terms from existing vocabularies is their definition. The Dublin Core™ property "title", for example, is defined as "a name given to the resource". If the definition fits, this property is a candidate for use in your profile. However, the suitability of a property for use in a particular application also depends on data-engineering aspects that may require closer scrutiny. To enable this closer scrutiny, it is useful to ask the following questions about the values you intend to use with each of the properties needed in the profile. Note that for any given property, there may be more than one "yes" answer.
For the value associated with this property:
Do you want to use free text?
Will the free text ever need to follow a pre-defined format such as the W3C format for dates ("YYYY-MM-DD")?
Will you want to select valid values from a controlled list?
- If so, is that list already available somewhere, or will you need to create it?
- Do you want to limit the valid values to a selection from a list, or can unlisted values be used?
Will single value strings suffice (e.g., "1989" or "John Adams"), or is there a need (or potential need) for a more complex structure with multiple components (as when an author has a name , email address , and affiliation )?
Might you ever want to use a URI to identify the value or point to a description of the value?
Answers to these questions inform the metadata engineering step that determines which properties may be used from existing vocabularies and how the corresponding metadata values can be expressed interoperably in the machine-readable format for exchange in a linked data environment. Looking again at our MyBookCase, this is how we might answer the questions about the properties for book:
- The title will be transcribed from the book itself. It will be a free text string.
- We want to use the date property in various ways in our application, such as sorting a set of retrieved bibliographic records, so we want to be sure that dates are presented in a uniform way as a structured string.
- We want to indicate the language of the book so that users can limit their searches by language. To make sure that languages are always input in the same way, we want to use a controlled list of languages.
- We want to record the subject. In our case, we want to select terms from the Library of Congress Subject headings.
- We know that our author is not a single text string but will be described with several pieces of information, such as email address.
The process of metadata engineering will use these decisions to model the data elements as described in Appendix C. This analysis results in a data model that is coherent with RDF and the DCMI Abstract Model:
- For title we can use the Dublin Core™ property dcterms:title, which has a "literal" range. Properties with literal ranges are often used when single, stand-alone text strings are all that is needed.
- Because we want to perform automated operations like sorting on the date, we can select the Dublin Core™ property dcterms:date. This property has a "literal" range. We can indicate that the value string is formatted in accordance with the W3C Date and Time Formats specification by using syntax encoding scheme dcterms:W3CDTF.
- The language needs to be selected from a controlled list. We achieve this by requiring the use of three-letter codes listed in the international standard ISO 639-3 for the representation of names of languages (such as "eng" for "English") together with the syntax encoding scheme dcterms:ISO639-3 as a datatype. For this, we can use the DCMI property dcterms:language.
- We want to record the subject using the Library of Congress Subject Headings. Typically, we would indicate the subject with a string (e.g., "Islam and Science") together with the vocabulary encoding scheme dcterms:LCSH, which identifies the heading "Islam and Science" as a member of the Library of Congress Subject Headings. Alternately, if the individual terms of the controlled vocabularies have already been given URIs as part of the work to express existing vocabularies using the RDF vocabulary Simple Knowledge Organization System [SKOS], then that URI may be used. In this case, the Library of Congress subject heading "Islam and Science" has been assigned the URI http://lcsh.info/sh85068424#concept. The DCMI property dcterms:subject has a "non-literal" range, which means that it can support the use of value strings, value URIs, and vocabulary encoding scheme URIs as needed.
- Because our author needs to be described with multiple components, such as email address, the author property will need to have a non-literal range so that a separate but linked description can be created in the metadata record. The Dublin Core™ property dcterms:creator is defined with a non-literal range, so we will use this in MyBookCase.
- The person has a name, but we want to record the forename and family name separately rather than as a single string. DCMI Metadata Terms has no such properties, so we will take the properties foaf:firstName and foaf:family_name from the Friend of a Friend vocabulary [FOAF].
- In order to record an email address as contact information for the person, we will use the property foaf:mbox, which has a non-literal range, and use mailto: URIs as values.
- For works of multiple authorship, the order of authors and how many to include (e.g. first 3, or no more than 20)
- How to determine the document type using the prescribed document type vocabulary
- Definitions of minimum data entry
- Character sets, punctuation, and abbreviations to be used in strings
- American Library Association and Library Association, Anglo-American Cataloguing Rules, 2nd ed. (London: Library Association, 1978).
- Dublin Core™ Collection Description Terms.
See also RDF schema < http://dublincore.org/specifications/dublin-core/collection-description/collection-terms/2007-03-09/cldterms.rdf>
- Sauermann, Leo, Richard Cyganiak, eds. Cool URIs for the Semantic Web.
- DCMI Encoding Guidelines
- DCMI Metadata Terms. January, 2008.
See also RDF schema <http://dublincore.org/2008/01/14/dcterms.rdf>
- Powell, Andy, Mikael Nilsson, Ambjoern Naeve, Pete Johnston and Thomas
Baker. DCMI Abstract Model. DCMI Recommendation. June 2007.
- Nilsson, Mikael, Thomas Baker, Pete Johnston. The Singapore Framework for Dublin Core™ Application Profiles.
- Nilsson, Mikael. Description Set Profiles: A constraint language for Dublin Core™ Application Profiles. March, 2008.
- Eprints Terms.
- Brickley, Dan, Libby Miller. FOAF Vocabulary Specification 0.91. November, 2007
- IFLA Study Group on the Functional Requirements for Bibliographic Records. (1998). Functional Requirements for Bibliographic Records - Final Report. Munich: K.G. Saur. Also available at <http://www.ifla.org/VII/s13/frbr/index.htm>
- Linked Data
- World Wide Web Consortium. Resource Description Framework (RDF) <http://www.w3.org/RDF>
- Brickley, Dan and R.V. Guha, editors. RDF Vocabulary Description Language
1.0: RDF Schema. W3C Recommendation. 10 February 2004.
- Manola, Frank, Eric Miller. RDF Primer. W3C Recommendation 10 February 2004.
- Berrueta, Diego, Jon Phipps, eds. Best Practice Recipes for Publishing RDF Vocabularies.
- Alvestrand, H. Tags for the Identification of Languages. January, 2001.
- Scholarly Works Application Profile.
- Booch, Grady, James Rumbaugh and Ivar Jacobson. The Unified Modeling Language User Guide. Addison-Wesley, 1998.
- a description set is made up of one or more descriptions
- a description is made up of
- zero or one described resource URI and
- one or more statements
- a statement is made up of
- exactly one property URI and
- exactly one value surrogate
- a value surrogate is either a literal value surrogate or a non-literal value surrogate
- a literal value surrogate is made up of
- exactly one value string
- a non-literal value surrogate is made up of
- zero or one value URIs
- zero or one vocabulary encoding scheme URIs
- zero or more value strings
- a value string is either a plain value string or a typed value string
- a plain value string may be associated with a value string language
- a typed value string is associated with a syntax encoding scheme URI
- a non-literal value may be described by another description.
- Whether free text will be used,
- Whether the free text will ever need follow a pre-defined format,
- Whether valid values should be selected from a controlled list, and
- Whether single strings will suffice or more complex values are needed.
- Simple lists of text strings may be informally documented as usage guidelines in a Description Set Profile.
The selection of properties for describing the author as a person follows the same model as above:
These decisions are summarized in the table below.
|Property||Range||Value String||SES URI||Value URI||VES URI||Related description|
|dcterms:title||literal||YES||no||not applicable ||not applicable||not applicable|
|dcterms:created||literal||YES||YES ||not applicable||not applicable||not applicable|
|foaf:firstName||literal||YES||no||not applicable||not applicable||not applicable|
|foaf:family_name||literal||YES||no||not applicable||not applicable||not applicable|
 These values are not applicable for values with a "literal" range.  http://purl.org/dc/terms/W3CDTF  http://purl.org/dc/terms/ISO639-2  http://purl.org/dc/terms/LCSH  Email addresses can be given using mailto: URIs.
The next step is to describe the metadata record in detail. The DCMI Working Draft "Description Set Profiles: A constraint language for Dublin Core™ Application Profiles" [DSP] provides a method for specifying structural constraints on the descriptions and statements held in a metadata record. Such constraints specify, for example, whether a statement with a given property is repeatable or non-repeatable, optional or required. Description Set Profiles are based on the Description Set Model, which is part of the DCMI Abstract Model [DCAM]. An outline of the Description Set Profile is given in Appendix A. This section presents a simple Description Set Profile for MyBookCase.
A DSP contains one Description Template for each thing in the domain model. The DSP for MyBookCase has two Description Templates: one for book and one for person. Each Description Template has a Statement Template for each of the properties used to describe the book or person.
If each metadata record is to represent exactly one book, a book Description Template will occur once and only once in each Description Set:
DescriptionSet: MyBookCase Description template: Book minimum = 1; maximum = 1
Let us say that each book must have one (and only one) title, which is identified with the property URI http://purl.org/dc/terms/title. Note that title in our case is used in statements with literal values.
Statement Templates are also created for each of the other properties used to describe a Book (with occurrence options and other constraints as needed):
DescriptionSet: MyBookCase Description template: Book minimum = 1; maximum = 1 Statement template: title minimum = 1; maximum = 1 Property: http://purl.org/dc/terms/title Type of Value = "literal" Statement template: dateCreated minimum = 0; maximum = 1 Property: http://purl.org/dc/terms/created Type of Value = "literal" Syntax Encoding Scheme URI = http://purl.org/dc/terms/W3CDTF Statement template: language minimum = 0; maximum = 3 Property: http://purl.org/dc/terms/language Type of Value = "non-literal" takes list = yes Syntax Encoding Scheme URI = http://purl.org/dc/terms/ISO639-2 Statement template: subject minimum = 0; maximum = unlimited Property: http://purl.org/dc/terms/LCSH Type of Value = "non-literal" takes list = yes Value Encoding Scheme URI = http://lcsh.info/ Statement template: author minimum = 0; maximum = 5 Property: http://purl.org/dc/terms/creator Type of Value = "non-literal" defined as = person
Some of the above properties have a minimum occurrence of 0 (zero). This is a way of saying that these properties are optional in our record and that a record is valid even if these properties are not present. Some of the values are repeatable, such as language, which can occur as many as three times, and author, which can occur as many as five times. We've defined the author as having the value of person, which is described in its own description template:
Description template: Person id=person minimum = 0; maximum = unlimited Statement template: givenName Property: http://xmlns.com/foaf/0.1/givenname minimum = 0; maximum = 1 Type of Value = "literal" Statement template: familyName Property: http://xmlns.com/foaf/0.1/family_name minimum = 0; maximum = 1 Type of Value = "literal" Statement template: email Property: http://xmlns.com/foaf/0.1/mbox minimum = 0; maximum = unlimited Type of Value = "non-literal" value URI = mandatory
A given person can have one optional given name and one optional family name, each of which are literal strings. A person can also have an email address which must be represented by a URI of the form mailto:. Because many of us have more than one email address, we allow this statement to repeat as often as necessary.
We allow our person property to be used any number of times in the metadata record. This may seem to conflict with the fact that person can only be used to represent an author up to five times in the book description, but we anticipate other possible uses for person in our record, such as subjects of a book, so we have chosen not to limit its number in the record in general.
Note that each person description contains data elements for only one person. This also means that an author statement will have only one person value. If there are two authors, then two author statements will be needed in the metadata record, each representing one person. One might allow a single person to have more than one name, such as real names and pseudonyms; however, the metadata would clearly distinguish the case of multiple authors (multiple Description Templates) from that of a single author with multiple names (multiple Statement Templates).
If you wish to include an affiliated institution for the author, you may want to create an institution description that contains the name and location of that institution, which will then link to the author description. You may also have other uses for corporate names and locations such as for recording information about the publisher of the book. As additional descriptions are created to hold the additional information, Description Sets can potentially become quite complex.
This completes the simple Description Set Profile for MyBookCase; see Appendix B for a version of this DSP encoded in XML.
A Description Set Profile defines the "what" of the application profile; usage guidelines provide the "how" and "why". Usage guidelines offer instructions to the people who will create the metadata records, so ideally they should explain each property and anticipate the decisions that must be made in the course of creating a metadata record. Documentation for metadata creators presents some of the same information that is included in the DSP, but in a more human-understandable form. Those inputting metadata will need to know: is this required? is it repeatable? am I limited in the values that I can input in this statement? Oftentimes a user interface can answer these questions, for example by presenting the metadata creator with a list of valid values to choose from.
Some examples of the kinds of rules that might appear in usage guidelines are:
In some cases where usage guidelines are relatively simple, they may be included in the DSP document with the description of the property. The SWAP is an example where the guidance instructions are included in the same document that contains the Description Set Profile definition.
Other communities may have highly complex rules that are best presented as separate documents due to their length and complexity. For example, the Anglo-American Cataloguing Rules used as guidelines by some libraries are recorded in a 600-page book.[AACR2] Instructions relating to titles appear in numerous of the book's chapters and cover many pages of text. Guidelines of this length may not integrate well with the Description Set Profile definition.
The technologies described in this document are syntax neutral; that is, they do not require any particular machine-readable encoding syntax as long as the syntax employed can fully express the values and relationships defined in the DCAP.
To help developers turn their application profiles into functioning applications, DCMI has developed various encoding guidelines [DCMI-ENCODINGS]. Description Set Profiles can be deployed using any concrete implementation syntax for which a mapping to the abstract model has been specified. DCMI has developed or is developing guidelines for encoding DCAM-based metadata in HTML/XHTML, XML, and RDF/XML; others could be added in the future. There is no restriction on use of other types of syntax as long as the resulting data format is compatible with the foundation standards and with the DCMI Abstract Model.
Appendix A: Description Set Model (from DCMI Abstract Model)
According to the "Description Set Model" of the DCMI Abstract Model [DCAM], a Dublin Core™ description set has the following structure:
Appendix B: MyBookCase Description Set Profile
<?xml version="1.0" encoding="UTF-8"?> <DescriptionSetTemplate xmlns="http://dublincore.org/xml/dc-dsp/2008/01/14" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://dublincore.org/xml/dc-dsp/2008/01/14"> <DescriptionTemplate ID="Book" minOccurs="1" maxOccurs="1" standalone="yes"> <StatementTemplate ID="title" minOccurs="1" maxOccurs="1" type="literal"> <Property>http://purl.org/dc/terms/title</Property> </StatementTemplate> <StatementTemplate ID="dateCreated" minOccurs="0" maxOccurs="1" type="literal"> <Property>http://purl.org/dc/terms/created</Property> <LiteralConstraint> <SyntaxEncodingScheme>http://purl.org/dc/terms/W3CDTF</SyntaxEncodingScheme> </LiteralConstraint> </StatementTemplate> <StatementTemplate ID="language" minOccurs="0" maxOccurs="3" type="nonliteral"> <Property>http://purl.org/dc/terms/language</Property> <NonLiteralConstraint> <VocabularyEncodingSchemeURI>http://purl.org/dc/terms/ISO639-3</VocabularyEncodingSchemeURI> <ValueStringConstraint minOccurs="1" maxOccurs="1"/> </NonLiteralConstraint> </StatementTemplate> <StatementTemplate ID="subject" minOccurs="0" maxOccurs="infinite" type="nonliteral"> <Property>http://purl.org/dc/terms/LCSH</Property> <NonLiteralConstraint> <VocabularyEncodingSchemeURI>http://lcsh.info</VocabularyEncodingSchemeURI> <ValueStringConstraint minOccurs="1" maxOccurs="1"/> </NonLiteralConstraint> </StatementTemplate> <StatementTemplate ID="author" minOccurs="0" maxOccurs="5" type="nonliteral"> <Property>http://purl.org/dc/terms/creator</Property> <NonLiteralConstraint descriptionTemplateRef="person"/> </StatementTemplate> </DescriptionTemplate> <DescriptionTemplate ID="person" minOccurs="0" standalone="no"> <StatementTemplate ID="givenName" minOccurs="0" maxOccurs="1" type="literal"> <Property>http://xmlns.com/foaf/0.1/givenname</Property> </StatementTemplate> <StatementTemplate ID="familyName" minOccurs="0" maxOccurs="1" type="literal"> <Property>http://xmlns.com/foaf/0.1/family_name</Property> </StatementTemplate> <StatementTemplate ID="email" minOccurs="0" type="nonliteral"> <Property>http://xmlns.com/foaf/0.1/mbox</Property> <NonLiteralConstraint> <ValueURIOccurrence>mandatory</ValueURIOccurrence> </NonLiteralConstraint> </StatementTemplate> </DescriptionTemplate> </DescriptionSetTemplate>
Appendix C: Using RDF properties in profiles: a technical primer
Every application profile design team should include members who understand basic principles for designing metadata on the basis of RDF. This section provides a brief overview of the modeling choices involved in the selection and use of RDF properties in application profiles. The section concludes by relating the technical design choices to the property-by-property requirements with regard to:
The basics of RDF properties
RDF properties are designed to be referenced and processed in a consistent way independently of the contexts in which they appear. The Dublin Core™ element title, for example, can be used in one context for describing books and in another for describing statues. When used correctly and in accordance with the RDF "grammar" for data, such globally defined vocabularies provide a basis for integrating resource descriptions from a variety of sources into coherent wholes.
In order to be usable in RDF-based metadata, properties must be identified with Uniform Resource Identifiers (URIs). The Dublin Core™ element title, for example, is identified with the URI http://purl.org/dc/terms/title (abbreviated here as dcterms:title). Good practice dictates that these URIs be declared and documented somewhere as "RDF properties". This declaration may be made in prose but typically is also made in a machine-readable RDF schema and ideally by the owner of the Internet domain or sub-domain used for the URIs. The URI for dcterms:title, for example, resolves (by redirection) to an RDF schema -- http://dublincore.org/2008/01/14/dcterms.rdf at the time of writing -- which says in a machine-understandable way that dcterms:title is an RDF property. The subdomain http://purl.org/dc/ is "owned" (in the sense of "controlled") by the organization Dublin Core™ Metadata Initiative.
In designing a metadata application, it is for many reasons desirable to use RDF properties that have already been declared somewhere. At a minimum, this is easier than doing the extra work involved in declaring one's own RDF properties. More importantly, the use of known properties provides a basis for semantic interoperability with metadata from other sources. Bear in mind that individuals who create vocabularies may change jobs and move on; research projects finish their work and eventually their servers disappear; and ownership of domain names may lapse, so that URIs which resolve today to an RDF schema might ten years from now resolve to shoe advertisements. It is best to use properties backed by organizations that have made a commitment to their maintenance.
RDF property semantics
RDF properties are usually provided with natural-language definitions. Designers of application profiles should take care to use the properties in ways that are compatible with these definitions. Designers may add technical constraints on use of properties (such as repeatability), or provide more narrow interpretations of definitions for particular purposes, but they should not contradict the meaning of the properties intended by their maintainers.
The intended meaning of a property is determined not just by natural-language definitions but also by formally declared relationships of the given property to other properties. Definitions typically specify a formal "domain" (the class of things that can be described by the property) and a "range" (a class of things that can be values). This additional information improves the utility of RDF properties by enabling inferences about the things they are used to describe. The property foaf:img (image), for example, has a domain of foaf:Person and a range of foaf:Image, so that when metadata-consuming applications find metadata using the property foaf:img, they can automatically infer that the thing being described with this property is a person and that the value being referred to by the property is an image. Properties may also be semantic refinements of other properties. The property dcterms:abstract, for example, is a sub-property of dcterms:description, meaning that anything which is said to have an abstract also may be said to have a description.
For the purposes of re-using properties in application profiles, it is especially important to check whether or not the properties are intended to be used with values that are literals. Properties that are intended to be used with values that are literals -- i.e., with values that by definition may consist of just one value string, optionally augmented with a language tag (in a "plain value string") or a datatype identifier (in a "typed value string") -- are said to have a "literal" range. Examples of properties with a "literal" range are dcterms:date, which is declared with a range of rdfs:Literal, and foaf:firstName, which is defined as being an owl:DatatypeProperty. The advantage of properties with a "literal" range is simplicity. The metadata carries -- and metadata-consuming applications expect -- just one plain or typed value string, making the metadata simple to encode and simple to process.
Properties with anything other than a "literal" range are said to have a "non-literal" range. Examples of properties with a "non-literal" range include dcterms:license, with the range dcterms:LicenseDocument, and foaf:holdsAccount, with the range foaf:OnlineAccount. Where literal-range properties may be simpler to process, non-literal-range properties are more flexible and extensible. In descriptive metadata, literal values constitutes "terminals" (in the sense of "end point"); the value string "Mary Jones" cannot itself be the starting point for any further description of the person Mary Jones. A non-literal value, in contrast, has hooks to which one may attach any number of additional pieces of information about the person Mary Jones, such as her email address, institutional affiliation, and date of birth. Potentially, non-literal values can be represented by any combination of the following:
<ul> <li>A plain or typed value string (Value String in the DCMI Abstract Model) -- and not just one, but potentially several in parallel, as in the case of a title rendered in English, French, and Japanese.</li> <li>A URI identifying the value resource (Value URI).</li> <li>A URI identifying an enumerated set (or controlled vocabulary) of which the value is a member (Vocabulary Encoding Scheme URI).</li> </ul>
Note that the difference between literal-range and non-literal-range properties is primarily a modeling issue. The type of property determines how the metadata will be encoded machine-processably for exchange and interpreted by applications that consume the metadata. End-users need not necessarily see the difference. When displayed in a search result, a value string looks the same regardless of whether it is directly a literal value or a value string attached to a non-literal value.
When using an existing property, the choice between a literal and non-literal range will usually be mandated by the official definition. If that mandated choice is not sufficient (e.g., the dcterms:date property has a literal range and a more complex value is needed), or if a property with the needed semantics cannot be found anywhere, then a new property (with a new URI) must be coined.
Coining new RDF properties
By definition, Dublin Core™ application profiles "use" properties that have been defined somewhere -- i.e., somewhere outside of the profile itself. If no existing property can be found among any of the well-known vocabularies, then the designers of an application profile will need to declare one themselves.
Declaring a new property is in itself not a difficult task. One gives it a name, formulates a definition, decides whether it takes a literal or non-literal range, and coins a URI for the property under a namespace to which one has access (and not, for example, under http://microsoft.com or http://amazon.de). Services such as http://purl.org, which is used for identifying DCMI properties, provide "persistent" URIs that can be redirected to documentation at more temporary locations. Guidance for creating and publishing RDF vocabularies can be found in "Cool URIs for the Semantic Web" [COOLURIS], the RDF Primer [RDF-PRIMER], and "Best Practice Recipes for Publishing RDF Vocabularies" [RECIPES]. Best-practice examples include DCMI Metadata Terms [DCMI-MT], Dublin Core™ Collection Description Terms [CTERMS], and Eprints Terms [ETERMS], It is good practice for terms also to be published in RDF schemas; for examples, see see the schemas associated with DCMI Metadata Terms [DCMI-MT] and Dublin Core™ Collection Description Terms [CTERMS].
Whether to assign a literal or a non-literal range is essentially a choice between simplicity and extensibility. Value strings alone may suffice for recording a date ("2008-10-31") or a title ("Gone with the Wind"), but for authors, one may need to record more than just a name. When in doubt, it is wise to assign a non-literal range. In the case of authors, for example, the non-literal range provides a hook for adding email address, affiliation, and date of birth or for using a URI to point to a description of the author somewhere outside one's own application. Because they support the use of URIs (i.e., the use of URIs "as URIs" and not just "as strings"), non-literal values are crucial in achieving the ideal of linked metadata -- descriptions that are cross-referenced using globally valid identifiers.
Translating user-defined data requirements into design decisions
We can now return to the questions asked of data content experts about each potential property with regard to potential values.
Do you want to use free text? "Free text" (i.e., strings of characters) is called a Value String in the DCMI Abstract Model and can be used with properties of either a literal or non-literal range. Note that in some cases, there may be a requirement to use multiple value strings, in parallel, in a single statement, for example in the case of values that are represented in multiple languages, which can only be done in conjunction with non-literal-range properties.
Will the free text ever need to follow a pre-defined format? If so, then the Value String can be used with a Syntax Encoding Scheme (datatype).
Will single value strings suffice or is there a need (or potential need) for a more complex structure with multiple components? If anything more than a single value string is needed for the value, then the property used must have a non-literal range. If by chance you have found a property with the right natural-language definition but the wrong range -- for example, with the more limiting literal range -- you may need to coin your own property, with its own URI, using that definition with a non-literal range.
Might you ever want to use a URI to identify the value or point to a description of the value? The DCMI Abstract Model defines a Value URI as a syntactic construct separate from a Value String. Value URIs cannot be used to describe literal values; they must be used with properties that have a non-literal range. It is of course possible to record a URI as a Value String -- a URI is, after all, a string -- but applications consuming the metadata on this basis will have no reliable way to distinguish that string from other strings in order to interpret it as an identifier.
Will you want to select valid values from a controlled list? If so, then the following are possible:
<li>Simple lists of text strings may be more formally defined as a <i>Syntax Encoding Scheme</i> (<i>SES</i>, or <i>datatype</i>), with a URI, making the list citable and available for use in many application profiles.</li> <li>A list of strings may be interpreted as labels for a list of concepts -- a <i>Vocabulary Encoding Scheme</i> (<i>VES</i>). Note that in contrast to the individual text strings listed in an SES, the individual concepts of a VES may also, or alternatively, be identified using URIs.</li> <li>If the list of values is already available somewhere and has already been identified (e.g., by DCMI) as a <i>Syntax Encoding Scheme</i> or <i>Vocabulary Encoding Scheme</i>, then use that URI with the proper modeling construct.</li> <li>If the list of values is already available somewhere but has not yet been identified as an SES or VES, you may need to interpret which model it more closely fits. Sometimes either interpretation is defensible. Whichever way you decide, it is important that the URI you coin for the encoding scheme be clearly declared as one or the other in order to avoid any ambiguity in the metadata.</li> <li>If you want to restrict the set of valid values to a fixed list (as opposed to allowing the use of unlisted values), this restriction can be declared and documented in a <i>Description Set Profile</i>.</li> </ul>
In general, the use of formally defined values, such as controlled lists, adds precision to metadata and thus increases its suitability for automatic processing. The use of SES and VES, as appropriate, is an important step in this direction. Increasingly, however, URIs are being assigned to individual terms in controlled vocabularies using the RDF vocabulary Simple Knowledge Organization System [SKOS]. The concept "World Wide Web" in the Library of Congress Subject Headings, for example, has recently been assigned the URI http://lcsh.info/sh95000541#concept. As controlled vocabularies become increasingly "SKOSified", it will become easier to use those vocabularies to find and integrate access to resources from multiple sources on the open Web. The VES construct can flexibly accommodate the transition from Value Strings alone, to Value Strings with VES URIs, and from there to Value URIs (with or without VES URIs).