code: private BufferBuilder getBufferBuilder(int targetChannel) throws IOException, InterruptedException {
  		if (bufferBuilders[targetChannel].isPresent()) {
  			return bufferBuilders[targetChannel].get();
  		} else {
  			return requestNewBufferBuilder(targetChannel);
  		}
  	}
docs: The {@link BufferBuilder} may already exist if not filled up last time, otherwise we need
  request a new one for this target channel.