1. Introduction to Persistent Identifiers
Summary
Definition of persistent identifiers
Which objects – physical / digital
Proliferation of identifiers
What do identifiers allow us to do
The challenge of real persistence
Social contracts
Present, real examples
Course on persistent identifiers, Madrid (Spain)
Type of Persistent Identifiers
Kevin Richards
Software Developer
Landcare Research
February 8th, 2012

2. Introduction
This presentation explains some characteristics of a typical identifier scheme, some examples of types of identifiers and briefly looks at some issues that may arise when the information associated with an identifier changes
This session is presented by Kevin Richards, Software Developer, at Landcare Research in New Zealand.

3. Summary
Types of Persistent Identifiers
Characteristics of an ideal identifier scheme
Commonly used identifier types
Points for consideration
Events that impact the maintenance of identifiers
Discussion

4. Characteristics of an Identifier scheme
Universally unique
Unchanging
Independent generation
Opaque
Associated services

5. Characteristics of an Identifier
Universally Unique
A mechanism is required to ensure global uniqueness
Integers obviously not good here
Internet example – domain names, URI
UUID ensures uniqueness with no tie to resolution protocol

6. Characteristics of an Identifier
Unchanging
The resource the identifier refers to must remain the same
The link between the identifier itself and what it identifies is the key point
Draw diagram to illustrate – eg ID – refers to this object, etc

7. Characteristics of an Identifier Scheme
Independent generation, ideally
It should be possible to create identifiers without relying on a central service
If a generation mechanism has been defined to ensure uniqueness, then the creation of identifiers can be done by anyone, anywhere
c.f. DOI
We are mobilising millions of objects

8. Characteristics of an Identifier Scheme
Opaque, ideally
In theory, should not be possible to determine any information about a resource by looking at the identifier
Often not easy to achieve
May be more of a sociological issue

9. Characteristics of an Identifier
Associated Services
Not required for a persistent identifier but makes the identifier more useful
Resolution of identifier
Other functions may include metadata requests and generation
Explain uselessness of a UUID by itself

10. Resolution of Identifiers
URLs, URIs, URN
E.g. http, lsid, mailto, ftp
HTTP
Protocol for resolving a web resource which may have many formats, e.g. image, html page, text, xml
May have redirects, HTTP 303
Resolution proxies

11. Types of Identifier Schemes
UUID (sometimes GUID)
Assured unique
E.g. d e1-b86c c9a66
Hard to type in
Not resolvable
Not always DB friendly
Opaque

12. Types of Identifier Schemes
HTTP URI
Uniform Resource Identifier
Web based – uses HTTP and DNS
In common use
Promoted by Linked Data advocates
May not be opaque due to semantics of domain names
E.g.
A generalisation of URLs.

13. Types of Identifiers PURL Persistent Uniform Resource LocatorEg
Web based using HTTP and HTTP redirect
Resolved through PURL resolver
May not be opaque due to domain names and paths

14. Types of Identifiers DOI Digital Object Identifier
Eg doi: /182
Managed by DOI Foundation (commercial)
Generated by DOI Foundation
Resolved through DOI resolution service
Very opaque

15. Types of Identifiers LSID Life Science Identifier (URN)
Developed by OMG for “name” based identification
E.g. urn:lsid:example.org:specimen:12921
Resolution protocol independent (i.e. does not rely upon HTTP)
3 step resolution mechanism
May not be opaque due to domain names

16. When things change Change of institution name (domain name for URLs)
E.g. old ID new ID
Broken Identifiers ?
Avoid by avoiding institution names for authority, use Project names. E.g use NZspecimens.org.nz instead

17. When things change Transfer of dataset (or split)
New owner for dataset
Eg specimens of NHM given permanently to regional herbarium -> new institution name
Similar solution to last problem, but if project name changes too, then still an issue, e.g. NZspecimens.org.nz project is migrated to wellingtonSpecimens.org.nz
2 options
new owner maintains old Identifiers
Old Identifiers redirected to new identifiers

18. When things change Resource destroyed
When the resource the identifier refers to has been deleted or removed from the system
Preferred results is to maintain the resolution of the identifier and state it is deprecated in the metadata for that resource

19. Summary/conclusions Characteristics of an Identifier
Universally unique, independent generation, unchanging, opaque and actionable
Types of Identifiers (some of them)
URI, PURL, DOI, LSID
When things change
Split or transfer of dataset, change of institution name, destroyed resource

20. Introduction to Persistent Identifiers
Summary
Definition of persistent identifiers
Which objects – physical / digital
Proliferation of identifiers
What do identifiers allow us to do
The challenge of real persistence
Social contracts
Present, real examples
Course on persistent identifiers, Madrid (Spain)
Type of Persistent Identifiers
Kevin Richards
Software Developer
Landcare Research
February 8th, 2012