Chapter 8 Creating J2EE Applications for BEA WebLogic
WebLogic Server supports transactions that are distributed over multiple data sources; a single database transaction can span multiple EJBs on multiple servers. This can be accomplished explicitly by starting a transaction and invoking several EJBs, or a single EJB can invoke other EJBs that implicitly work within the same transaction context. The following sections describe these scenarios.
In the following code fragment, a client application obtains a UserTransaction object and uses it to begin and commit a transaction. The client invokes two EJBs within the context of the transaction. The transaction attribute for each EJB has been set to Required:
import javax.transaction.*;
...
u = (UserTransaction)
jndiContext.lookup("javax.transaction.UserTransaction");
u.begin();
account1.withdraw(100);
account2.deposit(100);
u.commit();
...
In the above code fragment, updates performed by the "account1" and "account2" EJBs occur within the context of a single UserTransaction. The EJBs commit or roll back as a logical unit. This is true regardless whether "account1" and "account2" reside on the same WebLogic Server, multiple WebLogic Servers, or a WebLogic Server cluster.
The only requirement for wrapping EJB calls in this manner is that both "account1" and "account2" must support the client transaction. The beans' trans-attribute element must be set to Required, Supports, or Mandatory.
You can also use a "wrapper" EJB that encapsulates a transaction. The client calls the wrapper EJB to perform an action such as a bank transfer. The wrapper EJB responds by starting a new transaction and invoking one or more EJBs to do the work of the transaction.
The "wrapper" EJB may explicitly obtain a transaction context before invoking other EJBs, or WebLogic Server may automatically create a new transaction context if the EJB's trans-attribute element is set to Required or RequiresNew. All EJBs invoked by the wrapper EJB must be able to support the transaction context (their trans-attribute elements must be set to Required, Supports, or Mandatory).
WebLogic Server provides additional transaction performance benefits for EJBs that reside in a WebLogic Server cluster. When a single transaction utilizes multiple EJBs, WebLogic Server attempts to use EJB instances from a single WebLogic Server instance, rather than using EJBs from different servers. This approach minimizes network traffic for the transaction.
In some cases, a transaction can utilize EJBs that reside on multiple WebLogic Server instances in a cluster. This can occur in heterogeneous clusters, where not all EJBs have been deployed to all WebLogic Server instances. In these cases, WebLogic Server uses a multi-tier connection to access the datastore, rather than multiple direct connections. This approach uses fewer resources, and yields better performance for the transaction. However, for best performance, the cluster should be homogeneous — all EJBs should reside on all available WebLogic Server instances.
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