Oh boy are you in for a surprise. Reducers _can_ run with 0 mappers in a job ;-)
/me puts his troll-mask on. ➜ ~HADOOP_HOME hadoop fs -mkdir abc ➜ ~HADOOP_HOME hadoop jar hadoop-examples-0.20.2-cdh3u1.jar wordcount abc out 11/09/07 14:24:14 INFO input.FileInputFormat: Total input paths to process : 0 11/09/07 14:24:14 INFO mapred.JobClient: Running job: job_201109071413_0001 11/09/07 14:24:15 INFO mapred.JobClient: map 0% reduce 0% 11/09/07 14:24:21 INFO mapred.JobClient: map 0% reduce 100% 11/09/07 14:24:22 INFO mapred.JobClient: Job complete: job_201109071413_0001 11/09/07 14:24:22 INFO mapred.JobClient: Counters: 13 11/09/07 14:24:22 INFO mapred.JobClient: Job Counters 11/09/07 14:24:22 INFO mapred.JobClient: Launched reduce tasks=1 11/09/07 14:24:22 INFO mapred.JobClient: SLOTS_MILLIS_MAPS=2209 11/09/07 14:24:22 INFO mapred.JobClient: Total time spent by all reduces waiting after reserving slots (ms)=0 11/09/07 14:24:22 INFO mapred.JobClient: Total time spent by all maps waiting after reserving slots (ms)=0 11/09/07 14:24:22 INFO mapred.JobClient: SLOTS_MILLIS_REDUCES=3113 11/09/07 14:24:22 INFO mapred.JobClient: FileSystemCounters 11/09/07 14:24:22 INFO mapred.JobClient: FILE_BYTES_WRITTEN=59220 11/09/07 14:24:22 INFO mapred.JobClient: Map-Reduce Framework 11/09/07 14:24:22 INFO mapred.JobClient: Reduce input groups=0 11/09/07 14:24:22 INFO mapred.JobClient: Combine output records=0 11/09/07 14:24:22 INFO mapred.JobClient: Reduce shuffle bytes=0 11/09/07 14:24:22 INFO mapred.JobClient: Reduce output records=0 11/09/07 14:24:22 INFO mapred.JobClient: Spilled Records=0 11/09/07 14:24:22 INFO mapred.JobClient: Combine input records=0 11/09/07 14:24:22 INFO mapred.JobClient: Reduce input records=0 /me takes off troll mask. On Wed, Sep 7, 2011 at 1:30 PM, Bejoy KS <bejoy.had...@gmail.com> wrote: > Thanks Sonal. I was just thinking of some weird design and wanted to make > sure whether there is a possibility like that- no maps and all reducers. > > On Wed, Sep 7, 2011 at 1:22 PM, Sonal Goyal <sonalgoy...@gmail.com> wrote: >> >> I dont think that is possible, can you explain in what scenario you want >> to have no mappers, only reducers? >> Best Regards, >> Sonal >> Crux: Reporting for HBase >> Nube Technologies >> >> >> >> >> >> >> >> On Wed, Sep 7, 2011 at 1:18 PM, Bejoy KS <bejoy.had...@gmail.com> wrote: >>> >>> Hi >>> I'm having a query here. Is it possible to have no mappers but >>> reducers alone? AFAIK If we need to avoid the tyriggering of reducers we can >>> set numReduceTasks to zero but such a setting on mapper wont work. So how >>> can it be achieved if possible? >>> >>> Thank You >>> >>> Regards >>> Bejoy.K.S >> > > -- Harsh J
