code: @Override
  	public AppEngineTransaction beginTransaction() {
  		if (isTransactional()) {
  			throw new IllegalStateException("Transaction has already started ["
  					+ transaction.get().id() + "]");
  		}
  		
  		AppEngineTransaction tx = new AppEngineTransaction(this);
  		transaction.set(tx);
  		logger.fine("Transaction [" + transaction.get().id() + "] started");
  		return tx;
  	}
docs: Begins transaction. You have to do transactional operation after calling
  this method.
  
  @return The transaction begun.