PURLs

[Erik Jul]

Cross-posted to INTERCAT, AUTOCAT, EMEDIA, PACS-L and WEB4LIB.  Please
redistribute as appropriate. --Erik

********************************************************************************

The OCLC Office of Research and the OCLC Internet Cataloging project are 
pleased to announce the introduction PURLs.


                          PURL QUESTIONS AND ANSWERS
                          --------------------------


* What does PURL stand for?

PURL stands for "Persistent Uniform Resource Locator."  


* What is a PURL?

Functionally, a PURL is a URL.


* What does a PURL do?

Instead of pointing directly to the location of an Internet resource, a 
PURL points to an intermediate resolution service.  The resolution service 
associates the PURL with a specific URL and completes the appropriate 
network transfer.

Technically, the PURL resolver does a standard http "redirect" and your 
Web browser completes the network transfer.


                                 ----- 
     PURL---------------------->|     |---------------------->Resource
             points to PURL     |     | associated URL points
             resolver           |     | to resource
                                |     |
                                 ----- 
                          Resolver associates
                          PURL with unique URL;
                          maintenance component
                          facilitates creation of
                          PURLs and modification of
                          associated URLs.
                     

* What does a PURL look like?

PURLs look like URLs.  Here's a PURL taken from a record in the InterCat
Catalog:

             
             http://purl.oclc.org/oclc/oluc/32127398/1
             ^^^^   ------------- --------------------
               /           |               \
          protocol    resolver address     name  


* Why do we need PURLs?

Sometimes URLs do not work because Internet resources move, change names
or method of access, or other reasons.  Once a URL fails, all instances of
that URL (for example, links in a Web document or a bibliographic record) 
become invalid.


* How do PURLs address this problem?

A PURL is a "persistent" URL that uniquely points to a PURL resolver.  
The PURL resolver facilitates the creation, assignment, resolution, and 
modification of PURLs.

The PURL resolver associates each PURL with one URL, and the maintenance
component of the PURL resolver enables authorized users to modify the
URL associated with a PURL. 

Thus, the URL can change; the PURL stays the same.  A PURL can last longer 
than any particular URL that may be associated with it.  PURLs persist, and 
all instances of the PURL (for example, links in a Web document or a
bibliographic record) remain valid.


* What makes PURLs "persistent"?

The maintenance function of the PURL resolver.  

A PURL can be assigned once for any given resource/URL.  A subsequent change 
in the associated URL, when recorded using the maintenance component of the 
PURL resolver, does *not* require a change to the PURL.

A URL associated with a PURL can be modified; PURLs cannot.

Of course, someone has to operate the PURL resolver, and that's the
second source of persistence for PURLs.  OCLC has long been committed to
facilitating access to the world's information, and that commitment stands
behind PURLs, too.


* Can PURLs fail?

Yes.  A PURL is only as good as its associated URL.  If the URLs are not
maintained, PURLs will fail.

The PURL resolver maintenance component is designed to facilitate the 
upkeep of PURLs.


* Does a PURL ever have more than one associated URL?

No.  PURLs are uniquely associated with only one URL. 


* Can anyone modify a PURL's associated URL?

No.  Only authorized users can modify the URL associated with a PURL.


* Who are authorized users?

The PURL maintenance component allows users to designate authorized users.


* How is the name component of a PURL assigned?

For InterCat records, OCLC is automatically generating names that include
the OCLC number of the record containing the PURL.  Other methods of 
assigning names are possible, including automatically generated and 
user-defined names.


* Is this a proprietary or an open solution?

PURLs are based upon widely accepted and deployed, open Internet protocols. 


PURLs in InterCat Records
=========================


* How are PURLs assigned to InterCat records?

OCLC processes InterCat records and transforms the URLs to PURLs.  This
process creates a URL with http protocol, the address for the OCLC PURL
resolver, and a unique name, a component of which is the OCLC number of
the associated bibliographic record.


* Where are PURLs located in InterCat records?

PURLs are located in field 856, subfield $u, "URL," because a PURL is a URL.


* Some records have multiple 856 fields.  Does this mean the record will have
  multiple PURLs?  

Yes.  There is a one-to-one relationship between PURLs and URLs.  If a
bibliographic record describes a resource that is, let's say, available
via both ftp and http and has an 856 field for each access protocol, then
that record will be assigned two PURLs, one for the ftp URL and one for the
http URL.


* What about the URL entered by the cataloger?  What happens to it?

Two things: (1) the URL is entered into the OCLC PURL resolver database, 
and (2) the URL is moved from subfield $u to subfield $z, "Public Note." 
We are doing this so that no information is lost from the bibliographic 
record as a result of PURL processing.


* Will I find PURLs in bibliographic records in PRISM?

No.  At present, PURLs are only being created in InterCat records.


****

The introduction of PURLs into InterCat records is another important 
experimental step in the OCLC Internet Cataloging project, but it is 
only a first step.  

Following our next database update you will begin seeing PURLs in
InterCat records.  These PURLs will be functional, and the OCLC PURL 
resolver will be operational.  Soon I will be announcing the general 
availability of the OCLC PURL service, which will implement the 
creation, assignment, and maintenance of PURLs.

As always, I encourage you to try the InterCat Catalog 
(http://www.oclc.org:6990) and the PURLs and to share your thoughts with 
me and the list.

Stay tuned for more!

Yours truly,

--Erik

Erik Jul
Project Manager
jul at oclc.org


P.S.  Yes, InterCat has its own PURL, <http://purl.oclc.org/net/intercat>.
When the PURL resolver becomes publicly available, you will be able to
resolve that PURL to the current URL, and connect directly to the resource.
Meanwhile, I now have the ability to change the URL, if needed, and any
reference to InterCat that uses the PURL will still be good!

ej