hadoop-common-user mailing list archives

Site index · List index
Message view « Date » · « Thread »
Top « Date » · « Thread »
From Dave Shine <Dave.Sh...@channelintelligence.com>
Subject RE: Counting records
Date Mon, 23 Jul 2012 14:34:47 GMT
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()

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.Marron@trilliumsoftware.com]
Sent: Monday, July 23, 2012 10:25 AM
To: common-user@hadoop.apache.org
Subject: Counting records


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?


Peter Marron
Trillium Software UK Limited

The information contained in this email message is considered confidential and proprietary
to the sender and is intended solely for review and use by the named recipient. Any unauthorized
review, use or distribution is strictly prohibited. If you have received this message in error,
please advise the sender by reply email and delete the message.

View raw message