code: private void doBatchStart(final JsonObject message) {
      if (currentBatch != null) {
        currentBatch.handleEnd(null);
      }
      String batchID = message.getString("batch");
      Object args = deserializer.deserialize(message);
      if (log.isDebugEnabled()) {
        log.debug(String.format("%s - Batch started: Batch[batch=%s, args=%s]", this, batchID, args));
      }
      currentBatch = new DefaultConnectionInputBatch(batchID, this);
      if (batchHandler != null) {
        batchHandler.handle(currentBatch);
      }
      currentBatch.handleStart(args);
    }
docs: Handles a batch start.