docs: function(params, cb) {
      var correlationId = _cbt.put(offCommitResponse(cb));
      var msg = envelope(offCommit.encode()
                                  .correlation(correlationId)
                                  .client(_options.clientId)
                                  .group(params.group)
                                  .topic(params.topic)
                                  .partition(params.partition)
                                  .offset(params.offset)
                                  // .when(params.when) //disabled in api ver1
                                  .meta(params.meta)
                                  .end());
  
      sendMsg(msg, correlationId, cb);
    }