WriteHdfsP expects items of type Map.Entry<K,V> and writes the key
and value parts to HDFS. Each processor instance creates a single file
in the output path identified by the member ID and the processor ID.
Unlike in MapReduce, the output files are not sorted by key and are
written in the order they are received by the processor.
A similar transformation to ReadHdfsP can be
done for WriteHdfsP to map the incoming items into the required
format. For example:
Vertex sink = dag.newVertex("sink", WriteHdfsP.writeHdfs(jobConf, (String k) -> new Text(k),
(Long c) -> new LongWritable(c)));
This will transform the key and value to their Writable equivalents
which can be required for certain OutputFormat implementations.