[OAI-implementers] Automatically gathering the full-text of
Fri, 19 Mar 2004 16:51:02 +0000 (GMT Standard Time)
On Wed, 17 Mar 2004, herbert van de sompel wrote:
> a. The assumption that a harvester knows that something that is in the
> dc.identifier element of oai_dc points to a - compliant - jump-off page. There
> are two problems with this assumption:
> - lots of things can be in the dc.identifier element both resolvable and
> - lots of things at the end of the thing identified by the content of
> dc.identifier (if resolvable) will not be compliant jump-off pages
thanks for the email. Yes, I completely agree with this analysis. The
proposal is a bit of a hack, and we should perhaps have made this clearer
in the document!
However, I think it is a useful hack :-). Particularly so in the context
of our other recommendations for using simple DC to describe eprints.
Furthermore, I would see it as good practice to embed XHTML <link>
elements into jump-off pages anyway - irrespective of whether the
intention is to ease harvesting by robots or not. So I certainly don't
see our proposal as causing any harm.
The rest of your email raises some quite significant issues - some of
which I suspect are not very easy to discuss by email. I don't propose
giving a detailed response here, but I would like to note a few issues for
Firstly, your comments about the complexity of the objects being described
only goes part-way to describing the problem. The OAI-PMH specification,
rightly, says very little about the nature of the resources that are
described by the records exchanged using the protocol. However,
particular applications of the protocol do need to be clear about the
nature of the resources being described. Furthermore, the complexity of
the problem is not just about whether the resources being described are
aggregations of multiple objects. Part of the complexity arises because
those those resources/objects fit into a model of the real world that
spans both 'conceptual' works and specific digital or physical
'manifestations' of those conceptual works.
Does the oai_dc record that I allow you to harvest describe a conceptual
work (or expression of a work), an article for example, or does it
describe one of the particular manifestations of that work, the PDF copy
of the article for example?
You'll note that I am intentionally using terms from the IFLA FRBR
(Functional Requirements for Bibliographic Records) model here.
In our guidelines for using simple DC to describe eprints we made the
explicit decision to reflect the fact that most implementations of eprint
archives (that we looked at) appeared to be configured to expose oai_dc
metadata about the 'work' rather than about the particular manifestations
of the work (though actually, in many cases (even in our own guidelines
to a certain extent) there is a certain amount of fuzziness going on!).
Unfortunately, there is no real way of indicating in a simple DC record
that the work (as opposed to the manifestation) is being described - this
would be difficult even in qualified DC currently, because the current
DCMI Type vocabulary doesn't allow us to make those distinctions. But, in
principle, the DC model is rich enough to handle this complexity - if
we are prepared to put the effort in to agree how to do it.
But the situation is even more complex than that because it is not clear
to me where OAI resources and records sit within the Web architectural
model of 'resources' and 'representations'. My suspicion is that the FRBR
'manifestation' is the equivalent of the Web architecture
'represresentation' of the FRBR 'work' (if you see what I mean!). The
oai_dc record (and indeed the jump-off page) is a 'representation' of
the 'work' (assuming that is what is being described). But at this point
we almost certainly need a diagram or two! :-(
OK, so on then to the question about whether the protocol can and/or
should be used to exchange 'resources' as well as 'metadata' about
The protocol spec is very explicit in differentiating 'resources' from
'items' and 'records' and makes it very clear that the protocol be used to
exchange 'metadata' between services - I'm thinking of section 2.2 in
particular. Now, with hindsight, I really wish we'd talked instead about
'resources' and 'representations' rather than resources, items, records
and metadata, because that would have given us much more flexibility about
what we do with the protocol. But we didn't - and therefore, I think we
are constrained in terms of what we can do within the semantics of the
This is not just to do with the words being used in the spec. It has to
do with the entities in the model used by the protocol and the identifiers
that are assigned to those entities. An oai-identifier, for exanmple, is
an identifier of an 'item', not of a 'resource' (in terms of the protocol
usage of those words). It seems to me that things are likely to become
very fuzzy if the 'item' or 'record' suddenly becomes the 'resource' and
So, based on this, it seems to me that the protocol will 'break' if we
start using it to carry the 'resource' where the protocol expects to see
the 'record about the resource'.
Now, your complex example of the METS package or the MPEG-21 DIDL is an
interesting case - because those things can be used to carry both the
metadata and the object. Is a METS package the 'resource' or the 'record'
in OAI terms? The answer is that it is somewhere in-between. I certainly
accept that the METS package is a 'representation' of a 'resource' - but,
as I mentioned above, unfortunately we didn't use the words 'resource' and
'representation' in the protocol spec. Yes, the complex package can be
viewed as metadata - but metadata about what - about the 'work' that the
objects in the package 'represent', or about the particular manifestations
contained in the package??!
All in all, I think I'm happy with the case where OAI is used to carry the
METS or DIDL package that contain objects - but I would be much less happy
with a situation where the OAI-PMH is used to carry individual
manifestations (an XHTML document for example). But the fuzziness between
the package and the item worries me and I'm not sure that we are going to
be able to tell them apart very easily in all cases.
Enough for now... I agree with you that much more discussion and thinking
about these issues is required. I'm certainly happy (and indeed
expecting) to be told I'm wrong about any or all of the above! :-)
> * The scenario as described in the propsoal, in which a single metadata record
> corresponds with a single "preprint" is only a special case of - future -
> reality. Increasingly, objects held in and described by repositories will be
> "compound" or "complex", i.e. consisting of multiple datastreams, not just a
> single "preprint". I find that it would be desirable that a solution to get to
> the content would be able to handle such situations. The proposed solution
> could actually accomodate such 'compound' objects, because the mutliple
> datastreams are linked off the jump-off page. There is, however, a problem.
> Let's presume we have a situation in which an object is deposited in an
> institutional repository that has 2 datastreams, each of which actually has a
> unique identifier, say a doi or something. Thinking of a - future -
> self-archiving scenario and the trend to accord identifiers at finer levels of
> granularity, this is not unlikely at all. Now we get 3 things in dc.identifier
> (2 doi's and a link to a jump-off page), and 2 things in the jump-off page
> (links to the 2 datastreams). How do I know which doi goes with which
> datastream? Information that - I hope we will all agree - is rather significant.
> OK. The point I am trying to make is that the described scenario and its more
> general problem domain (beyond eprints, and into the realm of objects with
> multiple datastreams) may call for another approach. Our research has shown
> that such an approach can remain 100% OAI-PMH-based if a complex object format
> such as METS, MPEG-21 DIDL or SCORM is used. These formats can be "parallel"
> OAI-PMH "metadata formats" through which harvesters can get to the content
> without running into issues such as the ones mentioned above. Content can be
> embedded in the XML wrappers or pointed at by them. Identifiers can be
> unambiguously connected to content. If content changes, the datstamp of the
> "conplex" record changes.
> I anticipate concerns re the overhead of introducing a solution based on a
> complex object format. At this point, I would like to say 2 things with this
> * It took 2 people on my team about 2 days to create a prototype plug-in that
> enables OAI-PMH harvesting of content from DSpace repositories. Our plug-in
> rendered content using the MPEG-21 DIDL XML wrapper format. Most of the time
> invested in this plug-in was spent figuring out the DSpace API and a sensible
> way to map the DSpace data model to the DIDL data model. The prototype was
> demonstrated at the DSpace federation meeting, last week. Although
> questions/issues did arise in the course of our work, non seemed unsolvable.
> But it is my impression that the very fast delivery of a prototype indicates the
> feasibility of the complex format approach.
> * I would personally be very willing to spend time with the apporpiate
> representatives of the community - including yourself - to work towards a
> solution that is future-proof and provides adequate guarantees regarding
> perceived requirements of a content-harvesting solution. I would actually
> prefer that over going for a solution which is attractive at first glance
> because of its obvious simplicity, but which seems to raise some relevant
> questions upon closer inspection.
> To end, I would like to thank you for bringing this topic to the list. I have
> had many private email exchanges over the last few months especially with
> representatives from DARE and DINI about this and related problem domains. I
> hope that your mail can be another impulse towards a joint action in this realm.
> The problem is very real, and I would love our community to jointly create a
> really good solution to it.
> many greetings
> Andy Powell wrote:
> > The JISC-funded ePrints UK project has a requirement to automatically
> > harvest both metadata and full-text from the eprint archives within UK
> > academia (and potentially elsewhere). This is so that we can pass both
> > metadata and full-text to the various 'enhancement' Web services offered
> > by our partners.
> > http://www.rdn.ac.uk/projects/eprints-uk/
> > In order for our harvesting robot to be able to do this, it must be able
> > to reliably (and automatically) determine the correct URL(s) for the
> > various full-text manifestation(s) (HTML, PDF, RTF, etc.) of each eprint.
> > Our "Using simple Dublin Core to describe eprints" guidelines are intended
> > to encourage greater consistency in the metadata that is exposed by eprint
> > archives using the 'oai_dc' format within the OAI Protocol for Metadata
> > Harvesting (OAI-PMH). Somewhat perversely, because we stick rigidly to
> > the semantics of the DC element set, our guidelines make determining the
> > URL of each manifestation that is available quite difficult. (This is
> > largely a consequence of the 'simple' nature of 'simple DC'!). In
> > general, the URL in the <dc:identifier> element of the oai_dc record is
> > the URL of a jump-off page, rather than a direct link to the full-text.
> > We would like to suggest a new proposal for unambiguously embedding the
> > URL for each manifestation of an eprint into the (X)HTML jump-off page for
> > that eprint. Since the jump-off page is generated automatically by the
> > eprint archive software, doing this shouldn't be too difficult (in fact,
> > we would hope that archive software, such as eprints.org, will be
> > configured to do this out of the box).
> > If this proposal is adopted, it will make it much easier to write OAI
> > service provider software that can reliably gather the full-text of an
> > eprint, given only the oai_dc record for that eprint.
> > The proposal is at
> > http://www.rdn.ac.uk/projects/eprints-uk/docs/encoding-fulltext-links/
> > Comments are welcome,
> > Andy
> > --
> > Distributed Systems, UKOLN, University of Bath, Bath, BA2 7AY, UK
> > http://www.ukoln.ac.uk/ukoln/staff/a.powell/ +44 1225 383933
> > Resource Discovery Network http://www.rdn.ac.uk/
> > ECDL 2004, Bath, UK - 12-17 Sept 2004 - http://www.ecdl2004.org/
> > _______________________________________________
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> Herbert Van de Sompel
> digital library research & prototyping
> Los Alamos National Laboratory - Research Library
> + 1 (505) 667 1267 / http://lib-www.lanl.gov/~herbertv/
> "met gestreken jeans de dansvloer penetreren"
Distributed Systems, UKOLN, University of Bath, Bath, BA2 7AY, UK
http://www.ukoln.ac.uk/ukoln/staff/a.powell/ +44 1225 383933
Resource Discovery Network http://www.rdn.ac.uk/
ECDL 2004, Bath, UK - 12-17 Sept 2004 - http://www.ecdl2004.org/