Download No problem – it`s a URL

OCLC Online Computer Library Center
Erpanet Symposium on
Persistent Identifiers
PURLs
Stuart Weibel
Senior Research Scientist
June 17, 2004
What do we want from Identifiers
 Authority
 Reliability
 Appropriate Functionality (resolution and other
services)
 Persistence – throughout the life cycle of the
information object
 What are the business models to support identifiers?
 Not just a matter of money, but costs are part of
the equation
 PURL: Persistent Uniform Resource Locators
 PURLs look like URLs… they ARE URLs
 PURLs emerged from OCLC’s participation in the
IETF URN activity
 A tool for managing names and namespaces since
1996
The 404 Problem
 Resources disappear…
 Some are actually gone
 Disk reorganizations take place
 Changes in responsibility for resources
occur… bought, sold, abandoned, removed
 URLs serve double duty as names and locators
 Making URLs symbolic names will improve their
usefulness
What is a PURL?
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


PURL: Persistent Uniform Resource Locators
They look like URLs… they ARE URLs
No new technology, no new protocols
A toolset for managing names and namespaces
How PURLs Work
 PURLs take advantage of inherent redirection facility
in the HTTP protocol
 PURLs provide an additional level of indirection that
maps a symbolic identifier to a network location
 PURLs work without plug-ins or other special code in
browsers… they are ‘just’ URLs
 No New Technology added… a feature, not a bug
What does Persistent
mean?
 Not a guarantee of perpetual access
 Not a magic solution to the 404 problem
 Not persistence of resources, but rather of the
names
 PURLs are a toolset that can be used to manage
resource names and locations with greater reliability
Persistence derives from…
 The social or contractual commitments of
organizations responsible for managing information
resources.
 Technology can help, but the problem is, at its heart,
a social one.
Logical Components
of a PURL
http://purl.oclc.org/OCLC/PURL/FAQ
protocol
resolver
address
asset name
PURL Server as a
Redirection Server
http GET
PURL
Server
Client
http redirect
http GET
Resource
Server
resource
PURL Server as a Resource Server
http GET
PURL
Server
Client
resource
Do I have to run my own PURL Server?
 OCLC’s PURL Server is open to all, including the
ability to request domains
 As of Monday, May 24, 2004 :
 PURLs Created =
571 427
 PURLs Resolved =
86 010 679
 Unique Client Systems =
5 763 071
 The PURL server software is available at the
purl.org site for anyone to download and use without
cost or restriction.
PURLs and The Identifier Layer Cake
Social
Business
Policy
Technology
Functionality
The Web: http…TCP/IP…
Functional Layer: Operational characteristics of Identifiers
 Is it globally unique? No problem – it’s a URL
 Matching persistence with the need?
 Organizational commitment
 Can a given identifier be reassigned? No
 Is it resolvable? Yes: To that which is assigned by the
registrant
 How does it ‘behave’? Exactly like a URL, but managed
 Is the ‘name’ portion of the identifier opaque, or can it carry
‘semantics’? Determined by the registrant
 Do humans need to read and transcribe them? Probably
 Do identifiers need to be matched to the characteristics of the
assets they identify? Determined by the registrant
PURL Technical Layer
 What dependencies are assumed?
 http
 What is the nature of the system
 Open Source, public domain
 Are servers centralized? federated? peer to peer?
 Distributed and stand-alone, but could be
federated (see POIs, as an example)
 How is uniqueness assured?
 Inherent in the character of URLs
PURL Policy Layer
 Who has the ‘right’ to assign or distribute Identifiers?
 Anyone can register without cost
 Who has the ‘right’ to resolve them or offer services?
 Unspecified
 What are appropriate assets?
 Determined by the registrant
 Can identifiers be recycled?
 No
 Can ID-Asset bindings be changed?
 Yes, at the discretion of the registrant
 Is there supporting metadata?
 No intrinsic PURL metadata
 Is there a governance model?
 What you do in the privacy of your own PURL server is your own
business
PURL Business model layer
 Who pays the cost?
 PURL.ORG is maintained by OCLC as a free service
 Anyone can run their own PURL server (and pay for it)
 How, and how much?
 Negligible costs
 Who decides?
 The server host
 The problem with identifier business models…
 Those who accrue the value are often not the same as
those who bear the costs. Libraries are in the
business, however, of aggregating costs and making
them look free.
 You can’t collect revenue on resolution
PURL Social Layer: Who do you trust?
 Who do you trust?
 Governments?
 Cultural heritage institutions?
 Commercial entities?
 Non-profit consortia?
 It depends on the context, the service, and the
motivations for the service.
In Summary
 PURLs offer a methodology and tool set for
managing resource names and namespaces
 Neither PURLs nor any other technology are a
replacement for policies or commitments to manage
resource names
 PURLs represent a community-based solution
founded in freely available, widely deployed
technology.
http://purl.org