JDO : Transactions

A Transaction forms a unit of work. The Transaction manages what happens within that unit of work, and when an error occurs the Transaction can roll back any changes performed. Transactions can be managed by the users application, or can be managed by a framework (such as Spring), or can be managed by a J2EE container. These are described below.

See also :-

Locally Managed Transactions

When using a JDO implementation such as DataNucleus in a J2SE environment, the transactions are by default Locally Managed Transactions . The users code will manage the transactions by starting, and commiting the transaction itself. With these transactions with JDO you would do something like

PersistenceManager pm = pmf.getPersistenceManager();
Transaction tx = pm.currentTransaction();
try
{
    tx.begin();
    
    {users code to persist objects}
    
    tx.commit();
}
finally
{
    if (tx.isActive())
    {
        tx.rollback();
    }
}
pm.close();

When you use a framework like Spring you would not need to specify the tx.begin(), tx.commit(), tx.rollback() since that would be done for you. The basic idea with Locally Managed transactions is that you are managing the transaction start and end.



JTA Transactions

When using a JDO implementation such as DataNucleus in a J2SE environment, you can also make use of JTA Transactions . You define the persistence property javax.jdo.option.TransactionType setting it to "JTA" . Then you make use of JTA (or JDO) to demarcate the transactions. So you could do something like

UserTransaction ut = (UserTransaction)
    new InitialContext().lookup("java:comp/UserTransaction");
PersistenceManager pm = pmf.getPersistenceManager();
try
{
    ut.begin();
    
    {users code to persist objects}
    
    ut.commit();
}
finally
{
    pm.close();
}

So here we used the JTA API to begin/commit the controlling ( javax.transaction.UserTransaction ).

An alternative is where you don't have a UserTransaction started and just use the JDO API, which will start the UserTransaction for you.

UserTransaction ut = (UserTransaction)
    new InitialContext().lookup("java:comp/UserTransaction");
PersistenceManager pm = pmf.getPersistenceManager();
Transaction tx = pm.currentTransaction();
try
{
    tx.begin(); // Starts the UserTransaction
    
    {users code to persist objects}
    
    tx.commit(); // Commits the UserTransaction
}
finally
{
    pm.close();
}

Important : please note that you need to set both transactional and nontransactional datasources, and the nontransactional cannot be JTA .



Container Managed Transactions

When using a J2EE container you are giving over control of the transactions to the container. Here you have Container Managed Transactions . In terms of your code, you would do like the previous example except that you would OMIT the tx.begin(), tx.commit(), tx.rollback() since the J2EE container will be doing this for you.



Transaction Isolation

JDO 2.2 provides a mechanism for specification of the transaction isolation level. This can be specified globally via the PersistenceManagerFactory property datanucleus.transactionIsolation (javax.jdo.option.TransactionIsolationLevel). It accepts the following values

  • read-uncommitted : dirty reads, non-repeatable reads and phantom reads can occur
  • read-committed : dirty reads are prevented; non-repeatable reads and phantom reads can occur
  • repeatable-read : dirty reads and non-repeatable reads are prevented; phantom reads can occur
  • serializable : dirty reads, non-repeatable reads and phantom reads are prevented

The default (in DataNucleus) is read-committed . An attempt to set the isolation level to an unsupported value (for the datastore) will throw a JDOUserException. For example DB4O only supports read-committed so will always use that. As an alternative you can also specify it on a per-transaction basis as follows (using the names above).

Transaction tx = pm.currentTransaction();
...
tx.setIsolationLevel("read-committed");


JDO Transaction Synchronisation

There are situations where you may want to get notified that a transaction is in course of being committed or rolling back. To make that happen, you would do something like

PersistenceManager pm = pmf.getPersistenceManager();
Transaction tx = pm.currentTransaction();
try
{
    tx.begin();

    tx.setSynchronization(new javax.transaction.Synchronization()
    {
        public void beforeCompletion()
        {
             // before commit or rollback
        }

        public void afterCompletion(int status)
        {
            if (status == javax.transaction.Status.STATUS_ROLLEDBACK)
            {
                // rollback
            }
            else if (status == javax.transaction.Status.STATUS_COMMITTED)
            {
                // commit
            }
        }
    });
    
    tx.commit();
}
finally
{
    if (tx.isActive())
    {
        tx.rollback();
    }
}
pm.close();