You could just use a counter and never emit anything from the Map(). Use the
getCounter("MyRecords", "RecordTypeToCount").increment(1) whenever you find the
type of record you are looking for. Never call output.collect(). Call the job
with reduceTasks(0). When the job finishes, you can programmatically get the
values of all counters including the one you create in the Map() method.
Dave Shine
Sr. Software Engineer
321.939.5093 direct | 407.314.0122 mobile
CI Boost(tm) Clients Outperform Online(tm) www.ciboost.com
-----Original Message-----
From: Peter Marron [mailto:peter.mar...@trilliumsoftware.com]
Sent: Monday, July 23, 2012 10:25 AM
To: common-user@hadoop.apache.org
Subject: Counting records
Hi,
I am a complete noob with Hadoop and MapReduce and I have a question that is
probably silly, but I still don't know the answer.
For the purposes of discussion I'll assume that I'm using a standard
TextInputFormat.
(I don't think that this changes things too much.)
To simplify (a fair bit) I want to count all the records that meet specific
criteria.
I would like to use MapReduce because I anticipate large sources and I want to
get the performance and reliability that MapReduce offers.
So the obvious and simple approach is to have my Mapper check whether each
record meets the criteria and emit a 0 or a 1. Then I could use a combiner
which accumulates (like a LongSumReducer) and use this as a reducer as well,
and I am sure that that would work fine.
However it seems massive overkill to have all those "1"s and "0"s emitted and
stored on disc.
It seems tempting to have the Mapper accumulate the count for all of the
records that it sees and then just emit once at the end the total value. This
seems simple enough, except that the Mapper doesn't seem to have any easy way
to know when it is presented with the last record.
Now I could just make the Mapper take a copy of the OutputCollector for each
record called and then in the close method it could do a single emit. However,
although, this looks like it would work with the current implementation, there
seem to be no guarantees that the collector is valid at the time that the close
is called. This just seems ugly.
Or I could get the Mapper to record the first offset that it sees and read the
split length using report.getInputSplit().getLength() and then it could monitor
how far it is through the split and it should be able to detect the last
record. It looks like the MapRunner class creates a Mapper object and uses it
to process a split, and so it looks like it's safe to store state in the mapper
class between invocations of the map method. (But is this just an
implementation artefact? Is the mapper class supposed to be completely
stateless?)
Maybe I should have a custom InputFormat class and have it flag the last record
by placing some extra information in the key? (Assuming that the InputFormant
has enough information from the split to be able to detect the last record,
which seems reasonable enough.)
Is there some "blessed" way to do this? Or am I barking up the wrong tree
because I should really just generate all those "1"s and "0"s and accept the
overhead?
Regards,
Peter Marron
Trillium Software UK Limited
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