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Container-Managed Persistence in Borland Enterprise Server
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VerifyModifiedFields
VerifyAllFields
The behavior exhibited by the container corresponds to the value of the
optimisticConcurrencyBehavior table properties. See Chapter 16, “Using BES
Properties for CMP 2.x.”
Pessimistic Behavior
In this mode, the container will allow only one transaction at a time to access the data
held by the entity bean. Other transactions seeking the same data will block until the
first transaction has committed or rolled back. This is achieved by setting the
SelectForUpdate table property and issuing a tuned SQL statement with the FOR UPDATE
statement included. You can issue this SQL by overriding the SQL generated by the
CMP engine. See Chapter 17, “EJB-QL and Data Access Support.” Other selects on
the row are blocked until then. The tuned SQL generated looks like this:
SELECT ID, NAME FROM EMP_TABLE WHERE ID=? FOR UPDATE
You can also specify the SelectForUpdateNoWAIT table property. Doing so instructs the
database again to lock the row until the current transaction is committed or rolled back.
However, other selects on the row will fail (rather than blocking). The following SQL
illustrates a SELECT statement for the above:
SELECT ID, NAME FROM EMP_TABLE WHERE ID=? FOR UPDATE NOWAIT
These options should be used with caution. Although it does ensure the integrity of the
data, your application's performance could suffer considerably. This option will also not
function if you are using the Option A cache, since the entity bean remains in memory
in this mode and calls to ejbLoad() are not made between transactions.
SelectForUpdate
SelectForUpdateNoWA
IT
UpdateAllFields
Optimistic Concurrency
This mode permits the container to allow multiple transactions to operate on the same
data at the same time. While this mode is superior in performance, there is the
possibility that data integrity could be compromised.
The Borland Enterprise Server has four optimistic concurrency behaviors which are
specified as Table Properties. These behaviors are:
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SelectForUpdate
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SelectForUpdateNoWAIT
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UpdateAllFields
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UpdateModifiedFields
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VerifyModifiedFields
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VerifyAllFields
Use this option for pessimistic concurrency. With this option specified, the database
locks the row until the current transaction is committed or rolled back. Other selects on
the row are blocked until then.
Use this option for pessimistic concurrency. With this option specified, the database
locks the row until the current transaction is committed or rolled back. Other selects on
the row will fail.
With this option specified, the container issues an update on all fields, regardless of
whether or not they were modified. For example, consider a CMP entity bean with three
fields, KEY, VALUE1, and VALUE2. The following update will be issued at the terminus
of every transaction, regardless of whether or not the bean was modified:
UPDATE MyTable SET (VALUE1 = value1, VALUE2 = value2) WHERE KEY = key
Chapter 15: Entity Beans and Table Mapping for CMP 2.0 131