@Override
  public boolean isOpen() {
    return open;
  }
Internal method for writing to the channel that allows for a pluggable action to be taken to produce a
retained {@link ByteBuf} that can be added to the composite buffer.
@param retainedBufSupplier creates a retained {@link ByteBuf} that can be freely added to a
                           {@link CompositeByteBuf}.
@param callback called once the buffer has been added.
@return a future that is completed once the buffer has been added.
ambry-commons/src/main/java/com/github/ambry/commons/RetainingAsyncWritableChannel.java