code: @Nonnull
  	public <N> byte[] buildCompositeKeyNamespace(@Nonnull N namespace, @Nonnull TypeSerializer<N> namespaceSerializer) {
  		try {
  			serializeNamespace(namespace, namespaceSerializer);
  			final byte[] result = keyOutView.getCopyOfBuffer();
  			resetToKey();
  			return result;
  		} catch (IOException shouldNeverHappen) {
  			throw new FlinkRuntimeException(shouldNeverHappen);
  		}
  	}
docs: Returns a serialized composite key, from the key and key-group provided in a previous call to
  {@link #setKeyAndKeyGroup(Object, int)} and the given namespace.
  
  @param namespace           the namespace to concatenate for the serialized composite key bytes.
  @param namespaceSerializer the serializer to obtain the serialized form of the namespace.
  @param <N>                 the type of the namespace.
  @return the bytes for the serialized composite key of key-group, key, namespace.