a main problem when crawling is long waiting time cuased by crawl-delay
values from robots.txt entries. that attribute is not supported by
google and interpreted by yandex and bing in different ways. In large
crawls there is always one host which blocks the whole crawl with
extreme large values. YaCy now still obeys crawl-delay but limits them
to 10 seconds.
Additionally the blocking logic when loading new robots.txt was analyzed
and a deadlock was removed. Furthermore the construction of new queue
lists was redesigned and it was ensured that always a large list of
different hosts for host-balancing is provided for the loader.
If remote crawl option is not activated, skip init of remoteCrawlJob to save the resources of queue and ideling thread.
Deploy of the remoteCrawlJob deferred on activation of the option.
This organizes all urls to be loaded in separate queues for each host.
Each host separates the crawl depth into it's own queue. The primary
rule for urls taken from any queue is, that the crawl depth is minimal.
This produces a crawl depth which is identical to the clickdepth.
Furthermorem the crawl is able to create a much better balancing over
all hosts which is fair to all hosts that are in the queue.
This process will create a very large number of files for wide crawls in
the QUEUES folder: for each host a directory, for each crawl depth a
file inside the directory. A crawl with maxdepth = 4 will be able to
create 10.000s of files. To be able to use that many file readers, it
was necessary to implement a new index data structure which opens the
file only if an access is wanted (OnDemandOpenFileIndex). The usage of
such on-demand file reader shall prevent that the number of file
pointers is over the system limit, which is usually about 10.000 open
files. Some parts of YaCy had to be adopted to handle the crawl depth
number correctly. The logging and the IndexCreateQueues servlet had to
be adopted to show the crawl queues differently, because the host name
is attached to the port on the host to differentiate between http,
https, and ftp services.
Michael Peter Christen