Running multiple Apache Nutch fetch map tasks on a Hadoop Cluster - nutch

Question

I am unable to run multiple fetch Map taks for Nutch 1.7 on Hadoop YARN.
I am using the bin/crawl script and did the following tweaks to trigger a fetch with multiple map tasks , however I am unable to do so.
Added maxNumSegments and numFetchers parameters to the generate phase.
$bin/nutch generate $commonOptions $CRAWL_PATH/crawldb $CRAWL_PATH/segments -maxNumSegments $numFetchers -numFetchers $numFetchers -noFilter
Removed the topN paramter and removed the noParsing parameter because I want the parsing to happen at the time of fetch.
$bin/nutch fetch $commonOptions -D fetcher.timelimit.mins=$timeLimitFetch $CRAWL_PATH/segments/$SEGMENT -threads $numThreads #-noParsing#
The generate phase is not generating more than one segment.
And as a result the fetch phase is not creating multiple map tasks, also I belive the script is written it does not allow the fecth to fecth multiple segemnts even if the generate were to generate multiple segments.
Can someone please let me know , how they go the script to run in a distributed Hadoop cluster ? Or if there is a different version of script that should be used?
Thanks.

Answer 1

Are you using Nutch 1.xx for this? In this case, the Generator class looks for a flag called "mapred.job.tracker" and tries to see if it is local. This property has been deprecated in Hadoop2 and the default value is set to local. You will have to overwrite the value of the property to something other than local and the Generator will generate multiple partitions for the segments.

Answer 2

I've recently faced this problem and thought it'd be a good idea to build upon Keith's answer to provide a more thorough explanation about how to solve this issue.
I've tested this with Nutch 1.10 and Hadoop 2.4.0.
As Keith said the if block on line 542 in Generator.java reads the mapred.job.tracker property and sets the value of variable numLists to 1 if the property is local. This variable seems to control the number of reduce tasks and has influence in the number of map tasks.
Overwriting the value of said property in mapred-site.xml fixes this:
<property>
    <name>mapred.job.tracker</name>
    <value>distributed</value>
</property>
(Or any other value you like except local).
The problem is this wasn't enough in my case to generate more than one fetch map task. I also had to update the value of the numSlaves parameter in the runtime/deploy/bin/crawl script. I didn't find any mentions of this parameter in the Nutch 1.x docs so I stumbled upon it after a bit of trial and error.
#############################################
# MODIFY THE PARAMETERS BELOW TO YOUR NEEDS #
#############################################
# set the number of slaves nodes
numSlaves=3
# and the total number of available tasks
# sets Hadoop parameter "mapred.reduce.tasks"
numTasks=`expr $numSlaves \* 2`
...