Locks 
The concept of locks is
closely related to the concept of transaction isolation. Transaction isolation
makes sure that two simultaneously running transactions do not affect each
other. Transaction isolation levels are
realized by setting
locks on read or changed rows
in a database table.
Databases provide
multiple isolation levels that
determine how strong the isolation is between two simultaneously running
transactions.
Experience shows
that if the isolation is stronger, throughput and scalability
decrease.
For more
information, see 
Connection Transaction
Isolation in the Architecture Manual.
We recommend that you choose a locking strategy and apply it consistently throughout all application components. You can choose from two available locking techniques:
· Database locks
With database locks, you apply the locking technique provided by your database vendor. For more information, see the documentation of your database.
Database vendors do not offer uniform semantics for locks. For portability reasons, we recommend that you use logical locks.
· Logical locks
With logical locks, you use a locking technique
provided and managed centrally by the Web AS Java. Logical locks are managed
by the Enqueue Server via a central lock table. J2EE applications use the
LogicalLocking and TableLocking interfaces
provided by the Locking Adapter Service. These interfaces access the
Locking Manager, which in turn communicates with the Enqueue Server. For more
information, see Architecture of the
Locking Adapter Service in the Architecture Manual.
For more information about logical locks, see: