@Override
  public long writeTo(WritableByteChannel channel) throws IOException {
    long written = 0;
    if (currentIndexInProgress < compositeSendList.size()) {
      written = compositeSendList.get(currentIndexInProgress).writeTo(channel);
      if (compositeSendList.get(currentIndexInProgress).isSendComplete()) {
        currentIndexInProgress++;
      }
    }
    return written;
  }
Holds multiple Send instances and sends them over the network
ambry-protocol/src/main/java/com/github/ambry/protocol/CompositeSend.java