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Enterprise Beans Tutorial
25
you to permit uncommitted reads by invoking the setTransactionIsolation method: Connection con; ...
con.setTransactionIsolation(TRANSACTION_READ_UNCOMMITTED);
·
Do not change the isolation level in the middle of a transaction. Usually, such a change causes the
DBMS software to issue an implicit commit. Because the isolation levels offered by DBMS vendors
may vary, you should check the DBMS documentation for more information.
Updating Multiple Databases
The J2EE transaction manager controls all enterprise bean transactions except for bean-managed JDBC
transactions.
The J2EE transaction manager allows an enterprise bean to update multiple databases within a transaction.
The figures that follow show two scenarios for updating multiple databases in a single transaction.
In figure 6-3, the client invokes a business method in Bean-A. The business method begins a transaction, updates
Database-X, updates Database-Y, and invokes a business method in Bean-B. The second business method
updates Database-Z and returns control to the the business method in Bean-A, which commits the transaction.
All three database updates occur in the same transaction.
FIGURE 6-3 Updating Multiple Databases
In figure 6-4, the client calls a business method in Bean-A, which begins a transaction and updates Database-X.
Then, Bean-A invokes a method in Bean-B, which resides in a remote J2EE server. The method in Bean-B
updates Database-Y. The transaction managers of the J2EE servers ensure that both databases are updated in the
same transaction.
FIGURE 6-4 Updating Multiple Databases Across J2EE Servers
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