Morning,
first of all, thanks for your input on that issue. I've started this
thread because it is always one of the first of questions I get from
potential content providers. Especially from institutions that must
guarantee a certain kind of quality in their data, such as libraries.
If, for instance they link to a LOD-published concept in a thesaurus
or a DBPedia resource, and these resources change / disappear over
time it is difficult to provide that kind of quality.
I partly agree with Kingsley's answer "You have to test for Null
Pointers (URIs) when programming for the Linked Data Web too" - this
is of course true. But imagine programming against a DB which does not
provide referential integrity - a nightmare. Besides that I am not
sure if that is the answer those institutions might expect. Of course,
in an open world we cannot provide the "quality" features DBMS
provide, but we can at least provide some mechanism that helps solving
broken link issues.
Michael, in my opinion this should be an automated process which can
1.) discover broken links - this could be the LOD source itself,
SINDICE, or any other client. If an LOD source "knows" that all the
links/references are OK, it could publish that info using voiD -
@Michael: do you think that makes sense? Maybe introduce
"void:numberOfTanglingLinks" in the dataset statistics?
2.) notify other clients / datasources about broken links - here I
thought about a kind of iNotify [1] service for LOD sources.
3.) fix the problem, if possible, by directing to alternative link
targets if there are any
Since we are already working on a service which should provide (2) and
(3), I would be happy to contribute to a kind of "REVIVAL" thing, or
whatever you call it ;-)
Best,
Bernhard
[1] http://en.wikipedia.org/wiki/Inotify
On Feb 12, 2009, at 8:08 AM, Michael Hausenblas wrote:
Bernhard, All,
So, another take on how to deal with broken links: couple of days
ago I
reported two broken links in a TAG finding [1] which was (quickly and
pragmatically, bravo, TG!) addressed [2], recently.
Let's abstract this away and apply to data rather than documents. The
mechanism could work as follows:
1. A *human* (e.g. Through a built-in feature in a Web of Data
browser such
as Tabulator) encounters a broken link an reports it to the respective
dataset publisher (the authoritative one who 'owns' it)
OR
1. A machine encounters a broken link (should it then directly ping
the
dataset publisher or first 'ask' its master for permission?)
2. The dataset publisher acknowledges the broken link and creates
according
triples as done in the case for documents (cf. [2])
In case anyone wants to pick that up, I'm happy to contribute. The
name?
Well, a straw-man proposal could be called *re*pairing *vi*ntage link
*val*ues (REVIVAL) - anyone? :)
Cheers,
Michael
[1] http://lists.w3.org/Archives/Public/www-tag/2009Jan/0118.html
[2] http://lists.w3.org/Archives/Public/www-tag/2009Feb/0068.html
--
Dr. Michael Hausenblas
DERI - Digital Enterprise Research Institute
National University of Ireland, Lower Dangan,
Galway, Ireland, Europe
Tel. +353 91 495730
http://sw-app.org/about.html
From: Bernhard Haslhofer <[email protected]>
Date: Thu, 5 Feb 2009 16:35:35 +0100
To: Linked Data community <[email protected]>
Subject: Broken Links in LOD Data Sets
Resent-From: Linked Data community <[email protected]>
Resent-Date: Thu, 05 Feb 2009 15:36:13 +0000
Hi all,
we are currently working on the question how to deal with broken
links/
references between resources in (distinct) LOD data sets and would
like to know your opinion on that issue. If there is some work going
on into this direction, please let me know.
I think I do not really need to explain the problem. Everybody knows
it from the "human" Web when you follow a link and you get an
annoying
404 response.
If we assume that the consumers of LOD data are not humans but
applications, broken links/references are not only "annoying" but
could lead to severe processing errors if an application relies on a
kind of "referential integrity".
Assume we have an LOD data source X exposing resources that describe
images and these images are linked with resources in DBPedia (e.g.,
http://dbpedia.org/resource/Berlin)
. An application built on-top of X follows links to retrieve the geo-
coordinates in order to display the images on a virtual map. If now,
for some reason, the URL of the linked DB-Pedia resource changes
either because DBPedia is moved or re-organized, which I guess could
happen to any LOD source in a long-term perspective, the application
might crash if doesn't consider that referenced resources might move
or become unavailable.
I know that "cool URIs don't change" but I am not sure if this
assumption holds in practice, especially in a long-term perspective.
For the "human" Web several solutions have been proposed, e.g.,
1.) PURL and DOI services for translating URNs into resolvable URLs
2.) forward references
3.) robust link implementations, i.e., with each link you keep a set
of related search terms to retrieve moved / changed resources
4.) observer / notification mechanisms
X.) ?
I guess (1) is not really applicable for LOD resources because of
scalability and single-point of failure issues. (2) would require
that
LOD providers take care of setting up HTTP redirects for their moved
resources - no idea if anybody will do that in reality and how this
can scale. (3) could help to re-locate moved resources via search
engines like Sindice but not really fully automatically. (4) could at
least inform a data source that certain references are broken and it
could remove them.
Another alternative is of course to completely leave the problem to
the application developers, which means that they must consider
that a
referenced resource might exist or not. I am not sure about the
practical consequences of that approach, especially if several data
sources are involved, but I have the feeling that it is getting
really
complicated if one cannot rely on any kind of referential integrity.
Are there any existing mechanism that can give us at least some basic
feedback about the "quality" of an LOD data source? I think, the
referential integrity could be such a quality property...
Thanks for your input on that issue,
Bernhard
______________________________________________________
Research Group Multimedia Information Systems
Department of Distributed and Multimedia Systems
Faculty of Computer Science
University of Vienna
Postal Address: Liebiggasse 4/3-4, 1010 Vienna, Austria
Phone: +43 1 42 77 39635 Fax: +43 1 4277 39649
E-Mail: [email protected]
WWW: http://www.cs.univie.ac.at/bernhard.haslhofer
______________________________________________________
Research Group Multimedia Information Systems
Department of Distributed and Multimedia Systems
Faculty of Computer Science
University of Vienna
Postal Address: Liebiggasse 4/3-4, 1010 Vienna, Austria
Phone: +43 1 42 77 39635 Fax: +43 1 4277 39649
E-Mail: [email protected]
WWW: http://www.cs.univie.ac.at/bernhard.haslhofer
