code: public static Node createUserNode(final String userId) {
  		if (INDEX_USERS.get(IDENTIFIER, userId).getSingle() == null) {
  			final Node user = DATABASE.createNode();
  			user.setProperty(Properties.User.IDENTIFIER, userId);
  			INDEX_USERS.add(user, IDENTIFIER, userId);
  			return user;
  		}
  
  		throw new IllegalArgumentException("user node with identifier \""
  				+ userId + "\" already existing!");
  	}
docs: create a user node in the active database
  
  @param userId
  user identifier
  @return user node having its identifier stored
  @throws IllegalArgumentException
  if the identifier is already in use