# Transactional Behaviour

## Target Audience and Content

The target audience for this guide is the Enterprise Bean provider, i.e. the person in charge of developing the software components on the server side. It explains how to define the transactional behaviour of an EJB application.

The content of this guide is the following:

1. Target Audience and Content
2. Declarative Transaction Management
3. Bean-managed Transaction
4. Distributed Transaction Management

## Declarative Transaction Management

In case of container-managed transaction management, the transactional behaviour of an enterprise bean is defined at configuration time, and is part of the assembly-descriptor element of the standard deployment descriptor. It is possible to define a common behaviour for all the methods of the bean, or to define it at the method level. It is achieved by specifying a transactional attribute, which can be one of the following:

- NotSupported: if the method is called within a transaction, this transaction is suspended during the time of the method execution.
- Required: if the method is called within a transaction, the method is executed in the scope of this transaction, else, a new transaction is started for the execution of the method, and committed before the method result is sent to the caller.
- RequiresNew: the method will always be executed within the scope of a new transaction. The new transaction is started for the execution of the method, and committed before the method result is sent to the caller. If the method is called within a transaction, this transaction is suspended before the new one is started, and resumed when the new transaction has completed.
- Mandatory: the method should always be called within the scope of a transaction, else the container will throw the TransactionRequired exception.
- Supports: the method is invoked within the caller transaction scope, if the caller does not have an associated transaction, the method is invoked without a transaction scope.
- Never: The client is required to call the bean without any transaction context; if it is not the case a java.rmi.RemoteException is thrown by the container.

This is illustrated in the following table:

Transaction Attribute | Client transaction | Transaction associated with enterprise Bean's method
NotSupported | - T1 | - -
Required | - T1 | T2 T1
RequiresNew | - T1 | T2 T2
Mandatory | - T1 | error T1
Supports | - T1 | - T1
Never | - T1 | - error

In the deployment descriptor, the specification of the transactional attributes appears in the assembly-descriptor as follows:

      <assembly-descriptor>
        <container-transaction>
          <method>
          <ejb-name>AccountImpl</ejb-name>
          <method-name>*</method-name>
          </method>
          <trans-attribute>Supports</trans-attribute>
        </container-transaction>
        <container-transaction>
          <method>
          <ejb-name>AccountImpl</ejb-name>
          <method-name>getBalance</method-name>
          </method>
          <trans-attribute>Required</trans-attribute>
        </container-transaction>
        <container-transaction>
          <method>
          <ejb-name>AccountImpl</ejb-name>
          <method-name>setBalance</method-name>
          </method>
          <trans-attribute>Mandatory</trans-attribute>
        </container-transaction>
      </assembly-descriptor>

In this example, for all methods of the AccountImpl bean which are not explicitly specified in a container-transaction element, the default transactional attribute is Supports (defined at the bean-level), and for the methods getBalance and setBalance their respective transactional attributes are Required and Mandatory (defined at the method-name level).

## Bean-managed Transaction

A bean that manages its transactions itself must set the `transaction-type` element in its standard deployment descriptor to:

      <transaction-type>Bean</transaction-type>

To demarcate the transaction boundaries in a bean with bean-managed transactions, the bean programmer should use the javax.transaction.UserTransaction interface, defined on an EJB server object that may be obtained using the EJBContext.getUserTransaction() method (the SessionContext object or the EntityContext object depending if the method is defined on a session or an entity bean). The example below shows a session bean method "doTxJob" demarcating the transaction boundaries; the UserTransaction object is obtained from the sessionContext object, that should have been initialized in the setSessionContext method (see the example of the session bean).

    public void doTxJob() throws  RemoteException {
         UserTransaction ut = sessionContext.getUserTransaction();
         ut.begin();
         ... // transactional operations
         ut.commit();
    }

Another way to do this is to use JNDI and to retrieve UserTransaction with the name `java:comp/UserTransaction` in the initial context.

## Distributed Transaction Management

As explained in the previous section, the transactional behaviour of an application can be defined in a declarative way, or coded in the bean and/or the client itself (transaction boundaries demarcation). In any case, the distribution aspects of the transactions are completely transparent to the bean provider and to the application assembler. This means that a transaction may involve beans located on several EJB servers and that the platform will take in charge the management of the global transaction by itself. It will perform the two phase commit protocol between the different servers and the bean programmer will have nothing to do for this purpose.

Once the beans have been developed and the application has been assembled, it is possible for the deployer and for the administrator to configure the distribution of the different beans on one or several machines, and within one or several EJB servers. This may be done without impacting neither the beans code nor their deployment descriptors. The distributed configuration is specified at launch time: in the environment properties of an EJB server, you may specify

- which enterprise beans the EJB server will handle,
- if a Java Transaction Monitor will be located in the same Java Virtual Machine (JVM) or not.

To achieve this goal, two properties must be set in your jonas.properties file, jonas.service.ejb.descriptors and jonas.service.jtm.remote. The first one lists the beans that will be handled on this EJB server (by specifying the name of their deployment descriptors files or the name of the ejb-jar files), and the second one sets the Java Transaction Monitor (JTM) launching mode:

- if set to `true`, the JTM is remote, i.e. the JTM must be launched previously in another JVM,
- if set to `false`, the JTM is local, i.e. it will run into the same JVM as the EJB Server.

Example:

      jonas.service.ejb.descriptors       Bean1.xml, Bean2.xml
      jonas.service.jtm.remote            false

The Java Transaction Monitor may run outside of any EJB server, in this case, it may be launched in a stand alone way using the following command:

    TMServer

Using this configuration facilities, it is possible to adapt the beans distribution to the resources (cpu and data) location, so that performance may be optimal.

The figure below illustrates four cases of distribution configuration of three beans.

1. Case 1: The three beans B1, B2 and B3 are located on the same EJB server, which embeds a Java Transaction Monitor.
2. Case 2: The three beans are located on different EJB servers, one of them running the Java Transaction Monitor, which manages the global transaction.
3. Case 3: The three beans are located on different EJB servers, the Java Transaction Monitor is running outside of any EJB server.
4. Case 4: The three beans are located on different EJB servers. Each EJB server is running a Java Transaction Monitor. One of the JTM acts as the master monitor, while the two others are slaves.

These different configuration cases may be obtained by launching the EJB servers and eventually the JTM (case 3) with the adequate properties. The rational when opting for one of these configurations is resources location and load balancing. However, the following hints may be noted:

- if the beans should run on the same machine, with the same EJB server configuration, case 1 is certainly the more appropriate;
- if the beans should run on different machines, case 4 is the more appropriate, since it favours local transaction management;
- if the beans should run on the same machine, but need different EJB server configurations, case 2 is certainly a good approach.