BMT Transaction Propagation

[Luciano A. Pozzo]

Hi all,

I have a few questions about BMT transaction propagation of EJB version 2.1. So, first I will explain the situation.


If I have two beans configured as BMT, and the Bean 1 access Bean 2 as below:

BEAN 1 (BMT)

    
UserTransaction ut = context.getUserTransaction();
ut.begin();
// do some data source updates 
// call the Stateless BMT nr 2
ut.commit();

BEAN 2 (BMT)

    
UserTransaction ut = context.getUserTransaction();
ut.begin();
// do more data source access and updates
ut.commit();

So, the questions are:
1 - The EJB2 method run in same transaction of EJB1?
2 - Or everytime that I do ut.begin() a new transaction is started?
3 - And, if a new transaction is started, how can I do for run in same transaction of EJB1 (like Required option of CMT)?

Thank's

[ramprasad madathil]

Before begining, I assume you have sound reasons not to use CMTs.

1 - The EJB2 method run in same transaction of EJB1?

In your example - yes. ut.begin() starts a new transaction. The rule with BMT UserTransactions is that it always suspends the incoming transaction and starts a new one with the call to begin(). For stateless, the transaction should finish at the end of the method. This is true irrespective of whether the incoming transaction is BMT or CMT.

2 - Or everytime that I do ut.begin() a new transaction is started?

Yes, see above.

3 - And, if a new transaction is started, how can I do for run in same transaction of EJB1 (like Required option of CMT)?

You should get a handle to the existing transaction, but should not call begin(). Its the begin() that starts the transaction. And since you havent called begin(), you shouldn't commit() or rollback() either from within EJB2.
This is handled by EJB1 which began the transaction. The commit flow is normal and you neednt signal back to the caller that all is well. However if you have an exception in EJB2, you should convey this back to the caller and he would rollback the transaction.

To mark the transaction for rollback, you would call setRollbackOnly() on the UT object from EJB2.

In EJB1, you would check the status of transaction before committing or rolling back. Use the javax.transaction.Status class for checking transaction status.

EJB2

    UserTransaction ut = context.getUserTransaction();
    if(ut.getStatus() != Status.ACTIVE){
       //oh-oh no transaction
       //throw new TransactionRequiredException(); //this mimics mandatory; we will look at Required later
    }
    //do not begin transaction here - just use the existing one
    try{
        //more updates
    }catch(Exception e){
        //log the error
        ut.setRollbackOnly();//set it immediately for rollback; take no chances
    }

EJB1

    UserTransaction ut = context.getUserTransaction();
    try{
       ut.begin();
       // do some data source updates 
       // call the Stateless BMT nr 2
    }catch(Exception e){
        ut.setRollbackOnly(); //set it immediately for rollback; take no chances
       //log
       //some other exception handling stuff
    }finally{
        if(ut.getStatus() == Status.ACTIVE){
           ut.commit();
        }
         else{
           ut.rollback();
        }
    }
    ut.commit();
 

I am sure you can tweak EJB2 to mimic the Required behaviour of CMT transactions. Does this help?

cheers,
ram.

[ August 18, 2007: Message edited by: ramprasad madathil ]
[ August 18, 2007: Message edited by: ramprasad madathil ]

[Luciano A. Pozzo]

Hi Ramprasad,

Thank-you for the reply. It was what I wanted to know

[Luciano A. Pozzo]

Ramprasad,

A question about the snippet below (of your code) appeared:

 
    UserTransaction ut = context.getUserTransaction();
    if(ut.getStatus() != Status.ACTIVE){
       //oh-oh no transaction
       //throw new TransactionRequiredException(); //this mimics mandatory; we will look at Required later
    }
    //do not begin transaction here - just use the existing one
    try{
        //more updates
    }catch(Exception e){
        //log the error
        ut.setRollbackOnly();//set it immediately for rollback; take no chances
    }

Is it really possibly access the EJB1 transaction status through context.getUserTransaction()? Because according HFEJB (pages 482 and 490):

If you write a BMT bean, nobody else can ever include your bean in their transaction!

.
[ August 19, 2007: Message edited by: Luciano A. Pozzo ]

[ramprasad madathil]

You are right. My pervious reply was incorrect. I am sorry.
The container suspends an incoming transaction (BMT or CMT) when it invokes another BMT - in short a BMT can never take part in another bean's transaction. In stateful session beans, you can have transactions spanning multiple method calls - but it would take a very brave person to do that.

ram.