ByteString contentType = meta.getContentType();
if (contentType == null) {
throw new IllegalArgumentException("no content type");
}
RpbMapRedReq req = RPB.RpbMapRedReq.newBuilder().setRequest(request)
.setContentType(meta.getContentType()).build();
c.send(MSG_MapRedReq, req);
return new MapReduceResponseSource(this, c, contentType);