@Override
  public synchronized void begin() throws NotSupportedException, SystemException {
    LogWriter log = InternalDistributedSystem.getLogger();
    if (log.fineEnabled()) {
      log.fine("UserTransactionImpl starting JTA transaction");
    }
    int temp = storedTimeOut;
    storedTimeOut = TransactionManagerImpl.DEFAULT_TRANSACTION_TIMEOUT;
    tm.begin();
    tm.setTransactionTimeout(temp);
  }
Calls begin() of the transaction manager owning this user transaction

@see javax.transaction.UserTransaction#begin()
geode-core/src/main/java/org/apache/geode/internal/jta/UserTransactionImpl.java