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