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\title{RESOURCE IDENTIFIER}

\author{Greg Jan\'ee \\
Institute for Computational Earth System Science \\
University of California at Santa Barbara \\
Santa Barbara, CA 93106-3060 \\
\href{mailto:gjanee@alexandria.ucsb.edu}{\tt gjanee@alexandria.ucsb.edu}}

\date{}

\maketitle

\begin{synonyms}
  \synonym{document identifier}
  \synonym{GUID}
  \synonym{URI}
\end{synonyms}

\vspace{2em}

\begin{definition}

In a networked information system, a {\it resource identifier} is a
compact surrogate for a resource that can be used to identify,
retrieve, and otherwise operate on the resource.  An identifier
typically takes the form of a short textual string.  An identifier
must be {\it resolved\/} to yield the associated resource.

\end{definition}

\vspace{2em}

\begin{scientific}

Resource identifiers can be broadly characterized as being {\it
locations}, which identify resources by where they reside, or as being
{\it names}, which identify resources by intrinsic properties of the
resources.  This distinction is not absolute, and identifiers can
exhibit characteristics of both classes.  Furthermore, whether name or
location, every identifier requires a resolution mechanism to retrieve
the associated resource.  Nevertheless, the distinction between
locations and names is useful in characterizing the properties of
identifiers and the relationships between identifiers and resources.
Consider the answers that might be given to the following questions:

\begin{quote}
{\it Can two distinct, yet identical resources have the same identifier? \\
If a resource changes, must its identifier change?}
\end{quote}

If the answer to these questions is yes, then we would probably
consider the identifiers to be names; if no, locations.  To take two
examples, International Standard Book Numbers (ISBNs) are names: all
copies of a book are identified by the same ISBN, and a revision of a
book causes a new ISBN to be issued.  By contrast, HTTP URLs on the
World Wide Web are locations: two identical but distinct Web resources
must necessarily have different URLs, and a resource's URL is
independent of its potentially changing content.

The distinction between names and locations is codified in the World
Wide Web architecture \cite{webarch}, which roughly partitions the
universe of Uniform Resource Identifiers (URIs) into Uniform Resource
Locators (URLs) and Uniform Resource Names (URNs) \cite{uri}.

Two desirable characteristics of resource identifiers are uniqueness
and persistence.

\vspace{1em}

{\bf Uniqueness}

{\it Uniqueness} is the property that an identifier resolves to a
single resource.  The converse property---that every resource is
identified by a single identifier---is generally desirable, but is
often not enforced (or even enforceable) by systems that allow free
generation of identifiers.  A corollary of Metcalfe's Law on the
``network effect'' states that the value of a given resource can be
measured by the number and value of the other resources that reference
it; duplicate identifiers (or identifier ``aliases'') cause resources
to be undervalued \cite{webarch}.

Schemes for generating universally unique identifiers generally fall
into two categories.  Schemes in the first category guarantee
uniqueness by incorporating into each identifier unique
characteristics of the identified resource, for example a
content-based signature such as an MD5 digest, or
characteristics of the context in which the resource and/or identifier
system reside, for example a network address and timestamp.  UUIDs
\cite{uuid} incorporate both content-based and contextual
characteristics.

Schemes in the second category guarantee uniqueness by acquiring
identifiers from an ``authority'' that maintains a centralized store
of previously-generated identifiers (identifier--resource associations
are often stored as well).  For scalability such systems are often
arranged hierarchically so that a root authority, located at a
well-known address, may delegate identifier generation and resolution
requests to distributed sub-authorities.  DNS and the Handle system
\cite{handle} are two well-known examples of this approach.

\vspace{1em}

{\bf Persistence}

{\it Persistence} is the property that an identifier continues to
reference the associated resource over time.  Persistence is most
visible when it fails, for example, when a URL on the World Wide Web
is ``broken'' and no longer refers to the original resource (or to any
resource at all).

Strictly speaking, persistence is not a property of an identifier;
it's an outcome of the commitment of the operator of the identifier
resolution system to maintain the association between the identifier
and its resource.  But different identifier schemes mitigate to
different degrees known risks to persistence, and in this limited
respect persistence can be thought of as a property of an identifier.

All persistence mechanisms fundamentally employ indirection: an
identifier does not directly identify a resource, but instead
identifies an intermediate quantity which is maintained by the
resource owner to identify the resource even as the latter moves and
changes over time.  In principle this indirection mechanism may be
hidden from the users of resource identifiers, but for scalability
reasons it is typically exposed.  For example, the Persistent Uniform
Resource Locator (PURL) system \cite{purl} employs HTTP's redirection
mechanism.

One of the risks to identifier persistence is the inevitable change in
resource ownership and naming, which can induce concomitant changes to
identifiers.  One approach to mitigating this risk is issuing
so-called ``semantics-free'' identifiers.  For example, Digital Object
Identifiers (DOIs) \cite{doi} are sequences of numeric digits having
no external referent.  However, the benefit of mitigating this risk to
persistence must be balanced by the inscrutability of such identifiers
to humans.  Taking the opposite approach, OpenURLs \cite{openurl}
identify resources purely by semantics, specifically, by constraints
placed on resource bibliographic metadata.  For example, an identifier
for an article in a scholarly journal might contain the name of the
journal, the issue number, and the article title, all encoded into a
URI.  Such identifiers can be seen as being names as opposed to
locations.

Robust hyperlinks \cite{robust} provide a different approach to
persistence by combining a location (a URL) with a content-based
signature (a set of words that distinguishes the resource); the
signature can be used to search for the resource should the URL break.
The Archival Resource Key (ARK) scheme \cite{ark} integrates into its
identifier resolver system a protocol for obtaining persistence
guarantees, policies, and other preservation-related metadata.

\vspace{1em}

{\bf Other characteristics}

Additional desirable characteristics of resource identifiers include
global scope, global uniqueness, scalability, extensibility, machine
readability, recognizability in text, and human transcribability
\cite{urn}.  There are substantial benefits to expressing identifiers
as URIs, including hyperlinking, bookmarking, and indexing by Web
search engines \cite{webarch}.  Identifiers that are subject to
transcription errors (credit card numbers, to take one example)
benefit from having error-correcting codes incorporated into them
\cite{ecc}.

\end{scientific}

\vspace{2em}

\begin{thebibliography}{99}

\bibitem{persist} Hans-Werner Hilse and Jochen Kothe (2006).  {\it
Implementing Persistent Identifiers: Overview of concepts, guidelines
and recommendations}. London/Amsterdam: Consortium of European
Libraries and European Commission on Preservation and Access.  ISBN
90-6984-508-3. \\
\url{http://nbn-resolving.de/urn:nbn:de:gbv:7-isbn-90-6984-508-3-8}

\bibitem{webarch} Ian Jacobs and Norman Walsh, eds.\ (2004).  {\it
Architecture of the World Wide Web, Volume One}. \\
\url{http://www.w3.org/TR/webarch/}

\bibitem{uri} Tim Berners-Lee, Roy T. Fielding, and Larry Masinter
(2005).  {\it Uniform Resource Identifier (URI): Generic Syntax}.
IETF RFC 3986. \\
\url{http://www.ietf.org/rfc/rfc3986.txt}

\bibitem{uuid} Paul J. Leach, Michael Mealling, and Rich Salz (2005).
{\it A Universally Unique IDentifier (UUID) URN Namespace}.  IETF RFC
4122. \\
\url{http://www.ietf.org/rfc/rfc4122.txt}

\bibitem{handle} Sam X. Sun, Larry Lannom, and Brian Boesch (2003).
{\it Handle System Overview}.  IETF RFC 3650. \\
\url{http://www.ietf.org/rfc/rfc3650.txt}

\bibitem{purl} Keith Shafer, Stuart Weibel, Erik Jul, and Jon Fausey
(1996).  ``Introduction to Persistent Uniform Resource Locators.''
{\it INET96 Proceedings} (Montreal, Canada; June 24--28, 1996). \\
\url{http://www.isoc.org/inet96/proceedings/a4/a4_1.htm}

\bibitem{doi} Digital Object Identifier System. \\
\url{http://doi.org/}

\bibitem{openurl} Herbert Van de Sompel and Oren Beit-Arie (2001).
``Open Linking in the Scholarly Information Environment Using the
OpenURL Framework.''  {\it D-Lib Magazine} {\bf 7}(3). \\
\url{http://dx.doi.org/10.1045/march2001-vandesompel}

\bibitem{robust} Thomas A. Phelps and Robert Wilensky (2000).  {\it
Robust Hyperlinks Cost Just Five Words Each}.  University of
California, Berkeley Technical Report No.\ UCB/CSD-00-1091. \\
\url{http://www.eecs.berkeley.edu/Pubs/TechRpts/2000/5442.html}

\bibitem{ark} John A. Kunze and R.P.C. Rodgers (2007).  {\it The ARK
Persistent Identifier Scheme}. \\
\url{http://www.cdlib.org/inside/diglib/ark/arkspec.pdf}

\bibitem{urn} Karen Sollins and Larry Masinter (1994).  {\it
Functional Requirements for Uniform Resource Names}.  IETF RFC
1737. \\
\url{http://www.ietf.org/rfc/rfc1737.txt}

\bibitem{ecc} Joseph A. Gallian (1996).  ``Error Detection Methods.''
{\it ACM Computing Surveys} {\bf 28}(3):504--517. \\
\url{http://doi.acm.org/10.1145/243439.243457}

\end{thebibliography}

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