The difference between these two scenarios is that there's almost no CPU
involvement in serving the PDF file, but naive RDF sites use lots of
cycles to generate the response to a query for an RDF document.
Right now queries to data.southampton.ac.uk (eg.
http://data.southampton.ac.uk/products-and-services/CupCake.rdf ) are
made live, but this is not efficient. My colleague, Dave Challis, has
prepared a SPARQL endpoint which caches results which we can turn on if
the load gets too high, which should at least mitigate the problem. Very
few datasets change in a 24 hours period.
Martin Hepp wrote:
Hi Daniel,
Thanks for the link! I will relay this to relevant site-owners.
However, I still challenge Andreas' statement that the site-owners are to blame
for publishing large amounts of data on small servers.
One can publish 10,000 PDF documents on a tiny server without being hit by
DoS-style crazy crawlers. Why should the same not hold if I publish RDF?
But for sure, it is necessary to advise all publishers of large RDF datasets to
protect themselves against hungry crawlers and actual DoS attacks.
Imagine if a large site was brought down by a botnet that is exploiting Semantic Sitemap information for DoS attacks, focussing on the large dump files.
This could end LOD experiments for that site.
Best
Martin
On Jun 21, 2011, at 10:24 AM, Daniel Herzig wrote:
Hi Martin,
Have you tried to put a Squid [1] as reverse proxy in front of your servers
and use delay pools [2] to catch hungry crawlers?
Cheers,
Daniel
[1] http://www.squid-cache.org/
[2] http://wiki.squid-cache.org/Features/DelayPools
On 21.06.2011, at 09:49, Martin Hepp wrote:
Hi all:
For the third time in a few weeks, we had massive complaints from site-owners
that Semantic Web crawlers from Universities visited their sites in a way close
to a denial-of-service attack, i.e., crawling data with maximum bandwidth in a
parallelized approach.
It's clear that a single, stupidly written crawler script, run from a powerful University network, can quickly create terrible traffic load.
Many of the scripts we saw
- ignored robots.txt,
- ignored clear crawling speed limitations in robots.txt,
- did not identify themselves properly in the HTTP request header or lacked contact information therein,
- used no mechanisms at all for limiting the default crawling speed and re-crawling delays.
This irresponsible behavior can be the final reason for site-owners to say
farewell to academic/W3C-sponsored semantic technology.
So please, please - advise all of your colleagues and students to NOT write simple
crawler scripts for the billion triples challenge or whatsoever without familiarizing
themselves with the state of the art in "friendly crawling".
Best wishes
Martin Hepp
--
Christopher Gutteridge -- http://id.ecs.soton.ac.uk/person/1248
You should read the ECS Web Team blog: http://blogs.ecs.soton.ac.uk/webteam/
