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The application servers can direct client service calls
to the least-loaded server and can spawn additional shared servers as needed.
Architectures for OLTP | 209
Fault-tolerance
The application server acts as a transaction manager; the monitor performs the
commit or rollback of the transaction.* The underlying database becomes a
resource manager, but doesn??™t control the transaction. If the database server fails
while executing some transaction, the application server can resubmit the transaction
to a surviving database server, as control of the transaction lies with the
application server.
This type of transaction resiliency is a hallmark of the older TP monitors such as
Tuxedo, and the newer application servers and standards offer similar features.
Transaction routing
The logic in the middle tier can direct transactions to specific database servers,
increasing scalability.
Heterogeneous transactions
Application servers can manage transactions across multiple heterogeneous database
servers??”for example, a transaction that updates data in Oracle and DB2.


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