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Transaction manager services
Transaction Manager for two-phase commit, you must set the “Allow unrecoverable
completion” property to true in “Properties” for the Transaction Manager as
displayed under the Partition in the BES Management Console. Alternatively, you
can set system property EJBAllowUnrecoverableCompletion for the partition.
The OTS engine exists in a separate address space. It provides a complete solution for
distributed transactional CORBA applications. Implemented on top of the VisiBroker
ORB, the OTS engine simplifies the complexity of distributed transactions by providing
an essential set of services - including a transaction service, recovery and logging,
integration with databases, and administration facilities - within one, integrated
architecture.
Distributed transactions and two-phase commit
Note
The Borland EJB Container supports distributed transactions. Distributed transactions
are those transactions that cross systems, platforms, and Java Virtual Machines
(JVMs).
Transactions that manipulate data across multiple resources use a two-phase commit
process. This process ensures that the transaction correctly updates all resources
involved in the transaction. If it cannot update all resources, then it updates none of the
resources.
Although support is provided by BES for two-phase commit transactions, they are
inherently expensive due to number of remote procedure calls (RPCs) and should be
used only when needed. See “When to use two-phase commit transactions” on
page 170.
There are two steps to a two-phase commit. The first step is the preparation phase. In
this phase the transaction service requests that each resource involved in the
transaction readies its updates and signal to the transaction service whether it can
commit the updates. The second step is the commit phase. The transaction service
initiates the actual resource updates only when all resources have signaled that they
can complete the update process. Should any resource signal they cannot perform
updates, the transaction service instructs all other resources to rollback all updates
involved in the transaction.
The Transaction Manager and OTS engine support both heterogeneous distributed
(two-phase commit) transactions and two-phase commit for homogeneous resources.
By default, the Transaction Manager does not allow multiple resources to participate in
a global transaction, but it can be configured to allow multiple resource participation
through its support for unrecoverable transaction completion. This can be enabled on
the Transaction Manager by setting either “Allow unrecoverable completion” option
from the Management Console (right-click the Transaction Manager and select
“Properties”), or the Partition system property EJBAllowUnrecoverableCompletion. When
unrecoverable transaction completion is enabled, the container makes a one-phase
commit call on each participating resource during the transaction commit process.
Care must taken when enabling unrecoverable transaction completion; as the name
suggests, no recovery is available when a failure occurs prior to transaction
completion, which may lead to inconsistent states in participating resources.
To support heterogeneous two-phase commit transactions, the OTS engine must
integrate with XA support in the underlying resources. With availability of XA-enabled
JDBC drivers from DBMS vendors and JMS support provided by message service
providers, the EJB container and OTS engine allow multiple resources to participate in
a single transaction.
Two-phase commit for homogeneous databases requires some configuration of the
DBMS servers. While the container controls the commit to the first database, the
DBMS server controls the commits to the subsequent databases using the DBMS's
built-in transaction coordinator. For more information, see your vendor's manual for the
DBMS server.
Chapter 19: Transaction management 169