docs/wordcount.md

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# Apache Accumulo Word Count example

The WordCount example ([WordCount.java]) uses MapReduce and Accumulo to compute
word counts for a set of documents. This is accomplished using a map-only MapReduce
job and an Accumulo table with combiners.

To run this example, create a directory in HDFS containing text files. You can
use the Accumulo README for data:

    $ hdfs dfs -mkdir /wc
    $ hdfs dfs -copyFromLocal /path/to/accumulo/README.md /wc/README.md

Verify that the file was created:

    $ hdfs dfs -ls /wc

After creating the table, run the WordCount MapReduce job with your HDFS input directory:

    $ ./bin/runmr mapreduce.WordCount -i /wc

[WordCount.java] creates an Accumulo table named with a SummingCombiner iterator
attached to it. It runs a map-only M/R job that reads the specified HDFS directory containing text files and
writes word counts to Accumulo table.

After the MapReduce job completes, query the Accumulo table to see word counts.

    $ accumulo shell
    username@instance> table examples.wordcount
    username@instance examples.wordcount> scan -b the
    the count:20080906 []    75
    their count:20080906 []    2
    them count:20080906 []    1
    then count:20080906 []    1
    ...

When the WordCount MapReduce job was run above, the client properties were serialized
into the MapReduce configuration.  This is insecure if the properties contain sensitive 
information like passwords. A more secure option is store accumulo-client.properties
in HDFS and run the job with the `-D` options.  This will configure the MapReduce job
to obtain the client properties from HDFS:

    $ hdfs dfs -mkdir /user
    $ hdfs dfs -mkdir /user/myuser
    $ hdfs dfs -copyFromLocal /path/to/accumulo/conf/accumulo-client.properties /user/myuser/
    $ ./bin/runmr mapreduce.WordCount -i /wc -t examples.wordcount2 -d /user/myuser/accumulo-client.properties

After the MapReduce job completes, query the `examples.wordcount2` table. The results should
be the same as before:

    $ accumulo shell
    username@instance> table examples.wordcount2
    username@instance examples.wordcount2> scan -b the
    the count:20080906 []    75
    their count:20080906 []    2
    ...


[WordCount.java]: ../src/main/java/org/apache/accumulo/examples/mapreduce/WordCount.java