Re: Think before you write Semantic Web crawlers

Hello!

> 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.

Hmm, I would strongly argue it is not the case (and stale datasets are
a bit issue in LOD imho!). The data on the BBC website, for example,
changes approximately 10 times a second.

We've also been hit in the past (and still now, to a lesser extent) by
badly behaving crawlers. I agree that, as we don't provide dumps, it
is the only way to generate an aggregation of BBC data, but we've had
downtime in the past caused by crawlers. After that happened, it
caused lots of discussions on whether we should publish RDF data at
all (thankfully, we succeeded to argue that we should keep it - but
that's a lot of time spent arguing instead of publishing new juicy RDF
data!)

I also want to point out (in response to Andreas's email) that HTTP
caches are *completely* inefficient to protect a dataset against that,
as crawlers tend to be exhaustive. ETags and Expiry headers are
helpful, but chances are that 1) you don't know when the data will
change, you can just make a wild guess based on previous behavior 2)
the cache would have expired the time the crawler requests a document
a second time, as it has ~100M (in our case) documents to crawl
through.

Request throttling would work, but you would have to find a way to
identify crawlers, which is tricky: most of them use multiple IPs and
don't set appropriate user agents (the crawlers that currently hit us
the most are wget and Java 1.6 :/ ).

So overall, there is no excuse for badly behaving crawlers!

Cheers,
y

>
> 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/
>

Received on Wednesday, 22 June 2011 09:37:42 UTC