Java – Writing Hadoop reduces output to Elasticsearch

I had some trouble understanding how to write the output of a simple Hadoop back to Elasticsearch.

The job configuration is:

job.setOutputFormatClass(EsOutputFormat.class);
job.setOutputKeyClass(NullWritable.class);
job.setOutputValueClass(MapWritable.class);

Reducer does:

final DoubleWritable average = new DoubleWritable(sum / size);

final MapWritable output = new MapWritable();
output.put(key, average);
context.write(NullWritable.get(), output);

And yet I got this (for me) inexplicable anomaly :

14/08/15 16:59:54 INFO mapreduce. Job: Task Id : attempt_1408106733881_0013_r_000000_2, Status : FAILED Error: 
  org.elasticsearch.hadoop.EsHadoopIllegalArgumentException: 
  [org.elasticsearch.hadoop.serialization.field.MapWritableFieldExtractor@5796fabe] cannot extract value from object [org.apache.hadoop.io.MapWritable@dcdb8e97]    
    at org.elasticsearch.hadoop.serialization.bulk.TemplatedBulk$FieldWriter.write(TemplatedBulk.java:49)   
    at org.elasticsearch.hadoop.serialization.bulk.TemplatedBulk.writeTemplate(TemplatedBulk.java:101)  
    at org.elasticsearch.hadoop.serialization.bulk.TemplatedBulk.write(TemplatedBulk.java:77)   
    at org.elasticsearch.hadoop.rest.RestRepository.writeToIndex(RestRepository.java:130)   
    at org.elasticsearch.hadoop.mr.EsOutputFormat$EsRecordWriter.write(EsOutputFormat.java:161)     
    at org.apache.hadoop.mapred.ReduceTask$NewTrackingRecordWriter.write(ReduceTask.java:558)

In the context.write() call.

I’m a little confused, any ideas?

Turns out I made a mistake in the job configuration. I added the following:

configuration.set("es.mapping.id", "_id");

The _id field is not actually added in the outgoing Mapwritable; This causes ES to throw an exception.

This is useful if MapWritableFleldExtractor logs the field in which it failed.