I would think most users would expect a unique hit, yeah, the 
expectation is generally that a link like that is an 'identifier' 
representing a single record.  That's even kind of implied by the word 
"permalink" to me, my assumption is that a "permalink" is a link to a 
particular record, rather than to "search results".

The most inconvenient case is going to be when there are two (or more) 
hits for a given LCCN, and one of them represents a correct match, and 
one a cancelled/invalid match.   The client software in question may end 
up using the 'wrong' hit.

It's not a problem unless an 010$z on one record has not been re-used as 
an 010$a on a different record.  But it's not entirely clear to me in 
what cases this may occur.

Perhaps the documentation could discuss this issue, and make it clear 
that multiple record can come back from a permalink, and give some 
examples of cases where this might occur, especially cases where an 
010$z on one (or more) record is also an 010$a on one (and only ever 
one, I think? There can't be the same 010$a on two different records can 
there?) record.

On 9/7/2011 5:27 PM, Della Porta, Ann wrote:
> Jonathan,
>
>       When LC first released the LCCN Permalink service in 2008, we considered whether to index the canceled LCCNs -- and decided yes, we needed to provide access to both 010a and 010z.  Here's the rational:  LCCNs are an identifier that dates back to 1898. Most cancelled LCCNs represent a record that was previously distributed by LC's Catalog Distribution Service.
>
>       For bibliographic records, sometimes these older LCCN appeared on printed cards -- and newer versions of the record have been assigned a different LCCN.  LC has to assume that other libraries may have entered these older (and now cancelled) LCCNs into their catalog records in the 010a field.  In other cases, publishers and others print invalid or cancelled LCCNs in their CIP data (sometimes a publisher may apply for both PCN and CIP data for the same content; sometimes the same LCCN is simply used on different titles).  In addition, there are rare cases where the same LCCN has inadvertently been assigned to two records (an error LC corrects as soon as we are aware of the situation).
>
>       For authority records, we face an additional issue.  There are times when records for the same heading enter the master name authority file from multiple nodes on the same day. In this case, one record is retained and the duplicate cancelled. For subject authorities, we have situations where one heading is split into two or more separate headings. In this case, the split records receive new LCCNs and the older LCCN is entered as an 010z on the new heading records.  In addition, we have situations where a name authority might be moved to subjects or vice versa; in this case, the original LCCN is again often retained in 010z.
>
>       We still assume that some folks want to be able to retrieve something. Therefore, for the LCCN Permalink service, LC decided to cast the LCCN net more broadly than the 010a. In the infrequent situation where multiple records are retrieved, all hits will be displayed on the LCCN Permalink presentations for that identifier.
>
>       Do you think that most users expect a "unique" hit?
>
>            Ann
>
> -----Original Message-----
> From: Jonathan Rochkind [mailto:[log in to unmask]]
> Sent: Wednesday, September 07, 2011 2:16 PM
> To: Next generation catalogs for libraries
> Cc: Della Porta, Ann; Code for Libraries
> Subject: Re: [NGC4LIB] Permalink service for authority data now available at LC
>
> Very nice, thanks.
>
> I wonder the rationale behind searching both valid and cancelled LCCNs.
> This has caused me trouble in the past in similar systems, because a cancelled LCCN seems in some cases to duplicate a different valid LCCN, so you search on an LCCN, and get, in this case, both the correct record and and an incorrect record -- the requirement that software take account of this and look into the records to see which is the valid one adds significant complexity to the software, and is also a detail likely to be overlooked in client software implementation.
>
> I wonder if any alternatives were considered. I'm not exactly sure what the solution is though -- if it wasn't for the _collision_ issue, then we could say requesting a permalink for a cancelled LCCN should result in an HTTP redirect to the permalink for the correct LCCN.  But a cancelled LCCN on one record can be identical to a correct LCCN on another, or theoretically the same cancelled LCCN could exist on more than one record -- making it unclear what design could be better.
>
> On 9/7/2011 1:55 PM, Della Porta, Ann wrote:
>> Apologies for cross posting.
>>
>> The Library of Congress is pleased to announce an expansion of its LCCN Permalink Service for the Library's name and subject authority records. These persistent URLs are based on the Library of Congress Control Number (LCCN). As with bibliographic records, LCCN Permalinks are displayed on authority record entries in LC Authorities<http://authorities.loc.gov>.
>>
>> Create an LCCN Permalink
>> Simply begin your URL with the LCCN Permalink domain name -- http://lccn.loc.gov/ -- then add an LCCN.
>>          Examples:  http://lccn.loc.gov/n79018774   or  http://lccn.loc.gov/sh85026371
>>
>> LCCNs should be formatted according to the info:lccn<http://info-uri.info/registry/OAIHandler?verb=GetRecord&metadataPrefix=reg&identifier=info:lccn/>   URI specification. Instructions are also available in the LCCN Permalink FAQ<http://lccn.loc.gov/lccnperm-faq.html>.
>>
>> How LCCN Permalink Works
>> An LCCN Permalink retrieves a MARCXML-formatted record using the Z39.50/SRU protocol. Both valid and cancelled LCCNs (MARC 21 fields 010a and 010z) are searched. Authority record displays for LCCN Permalink follow the labeled display found in LC Authorities. MARCXML and MADS versions of the records are also available. Displays link to entries in LC Authorities<http://authorities.loc.gov>   and the LC Online Catalog<http://catalog.loc.gov>   -- and, where appropriate, to entries in the Virtual International Authority File<http://viaf.org>   and LC Authorities and Vocabularies<http://id.loc.gov/>.
>>
>> More Information
>> Additional information on this service is available on the LCCN Permalink FAQ<http://lccn.loc.gov/lccnperm-faq.html>. Specific questions can also be sent to the Library of Congress through Ask-A-Librarian<http://www.loc.gov/rr/askalib/ask-digital.html>.
>>
>> Ann Della Porta
>> Chief, Integrated Library System Program Office Library of Congress
>> Washington, DC 20540-4010 [log in to unmask]
>>