code: @Override
  	public void update(Object entity) {
  		if (entity == null) {
  			throw new IllegalArgumentException("'entity' must not be [" + entity + "]");
  		}
  		
  		assertTransactional();
  		((AppEngineGlobalTransaction) transaction.get()).coordinator().update(entity);
  	}
docs: Updates entity with the specified entity instance with
  {@code AppEngineCoordinator}.
  This method must be invoked under a transaction.
  
  @param entity The entity instance to be updated.