static class SideOutputMapper extends Mapper<Long, String, Void> {
transient BlobFileOutputWriter sideOutput;
@Override
public void beginSlice() {
sideOutput = new BlobFileOutputWriter("test file", "application/octet-stream");
try {
sideOutput.beginShard();
sideOutput.beginSlice();
} catch (IOException ex) {
throw new RuntimeException(ex);