[OAI-implementers] Requesting a part of a record possible wit h OAI-OMH?

Joachim Wackerow wackerow@zuma-mannheim.de
Thu, 22 Jan 2004 11:10:29 +0100


Sorry about the late answer.

Thank you very much for all your comments and the summary, your input 
helped me very much.

Here is the summary from my perspective:

I understand, that the actual version of OAI-PMH can request a whole 
record (XML document), but not a part of record (a fragment of XML 
document).

The proposal from Naomi (1) is a workaround, which will work if it's 
only necessary to request only a few defined parts of a record.

Jeffs proposal (2a) has the attractive idea to request a part of a 
record by the means of XPATH. In my eye's it's a drawback to depend on a 
external service for the reason, that the "main" protocol (OAI-PMH) is 
lacking this feature (nothing against external services). I would like 
to think more about a general solution (for DDI). But nevertheless, 
Jeff, thank you very much for your offer.

Hussein's proposal (2b) of a new protocol on top of or parallel to 
OAI-PMH sounds very good. The proposed way to add this new protocol 
seems to be very flexible also for other new protocols, it would be a 
way of a general extensibility of OAI-PMH. The question seems to me: is 
  "requesting a part of record" not a candidate of a new version of 
OAI-PMH? "GetRecordPart" and "ListRecordParts" were the tools I'm 
looking for.

The actual version of OAI-PMH has the intention of requesting whole 
records (primary use of harvesting protocol). The question of requesting 
a part of a record by the means of XPATH will arise with large XML 
documents. The service provider could be interested only in a part of 
the information. Large XML documents have usually DTDs/Schemas with a 
deeply nested structure and a lot of elements or have repeatable 
elements which could be repeated very often.

The size of DDI files reach often 500-1000 KB. The description of a 
social survey consists often in several hundred questions/variables and 
in an extensive general study description.

I see one problem with the idea to request fragments of a XML document 
stored in a record: the client would not be able to validate the 
response, because itís only a fragment of a XML document. A better 
solution would be to embed the fragment in a frame of a valid XML 
document (like a "condensed version" of a XML document). That would be 
dependent from the XML DTD/schema (the question would be, which elements 
are required).

Right now we are just thinking about using OAI-PMH to offer DDI-files. 
Perhaps there is another way to offer XML files for service providers, 
Hussein mentioned SOAP.

I will try to give the DDI community 
(http://www.icpsr.umich.edu/DDI/users/listserv.html) a hint on this 
discussion thread.

Achim