Library Cataloging

Persistent Identifiers

My summary of the PURLs/Handles aspect of the creation of persistent identifiers:

When deciding on what system to use to help manage collections pointed at with URIs, there are two major systems people use: Handles and PURLs. I have looked at several comparisons of the two systems. Most of them are now getting dated and do not summarize the issues I see as important, so I'm offering my own.

This may be somewhat biased towards the PURL system. It is my staff that maintains the PURL code here at OCLC, but I think this lays out the main points that need to be considered:

Either system will work. Both have been successfully employed in helping to maintain persistent identifiers to digital items

The systems are not quite direct competitors. Handles are built at a slightly higher level than PURLs (more on this below). PURLs have no pretensions other than a way to maintain URIs, and to do this with URLs. Handles have a definition outside the current URL implementation of them.

Handles have several advantages

More active software development
More active promotion of use
Java implementation
Migration of a collection of handles from one resolution server to another is 'built in'
Defined outside DNS and URLs

PURL advantages

Open Source software license, OSI compliant
Lower overhead: no centralized server involved in resolution unless wanted
Simpler
Partial redirects

Other differences

Handles can resolve to several versions of an object (whether this is an advantage depends on your point-of-view and application)
A Handle system could probably be built on top of PURLs, while PURLs on top of handles would be more difficult. This shows that the systems are operating at slightly different levels. If the Handle system's functionality is just what you need, then it will probably be simpler to use. If not, the PURL system offers more flexibility.

Partial redirects, mentioned as a PURL advantage, need more explanation than there is room for here. Partial redirects are a way of using the PURL resolution server to resolve only the beginning of a PURL, allowing the migration of whole collections from one place to another by changing a single PURL. I am planning a paper describing how this can be used, but for now see the PURL documentation for more information.

In general, if you are worried about being able to resolve DNS names and whether HTTP is going to be supported long-term, then Handles promises to be independent of those protocols (although the current implementation uses them). If you are more interested in the lowest-overhead method, or plan to use partial redirects, then PURLs are probably more suitable.

In any case, you can't make an informed decision without understanding how both the Handle registry system and PURL partial redirects work.

Links

A good bibliography of some of these issues
Some practical guidelines on creating links with some persistence even without PURLs or Handles ('Cool URIs')
Clifford Lynch's 'five questions' about identifier systems
Papers about the Handle System
Handle System home
PURL home

Thom Hickey
2003 March--Posted with permission. Appeared on DC-General mail list.

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Library Cataloging