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TMF Application Programming
Controlling Transaction Isolation
processes to read the same data, but prevents any process from writing to the locked
data or reading it without intent to rewrite. An exclusive lock allows only the
transaction that holds the lock to access the data. Using shared locks improves
concurrency; using exclusive locks improves accuracy.
Locks on updated, inserted, or deleted data are retained until the transaction either
commits or is backed out. For read-only access, a lock is held on the current position
(row or record being read) and is released when the next row or record becomes the
current position. Locks on data read are typically released by the application before
the transaction commits or aborts.
This locking provides average transaction isolation and average concurrency.
To lock SQL data for read and update, the application specifies STABLE ACCESS. To
lock Enscribe data for read and update, the application specifies both shared locks and
read statements with locks.
Locking for Repeatable Access
When locking for repeatable access, the transaction has exclusive use of data and
complete isolation from concurrent transactions. This type of access provides
maximum insulation from the effects of other transactions.
With repeatable access enabled, a transaction can run as though it were the only
active transaction in the system. The transaction will not read uncommitted changes
made by other transactions. Furthermore, other transactions cannot alter any
uncommitted changes made by this transaction. If your transaction has updated a set
of detail items, subtotals, and totals so that they are consistent with one another, no
other transaction can disturb that consistency. Other transactions cannot update,
delete, or insert any data within the entire range of rows or records accessed by your
transaction. This guarantees your transaction the opportunity to reread previously read
rows or records and see exactly the same data values each time.
For audited tables in SQL, the transaction acquires shared locks on all accessed data
and exclusive locks on any inserted or updated data. All locks are held until the end of
the transaction. This isolation level prevents other transactions from inserting or
modifying rows in the range of rows examined by the transaction. Repeatable access
provides a high level of isolation; this access has a lower level of concurrency than
stable access, but a much higher level of concurrency than file locks.
To lock SQL data for repeatable access, the application specifies SERIALIZABLE or
REPEATABLE READ.
For Enscribe data, the application must specify file locks to gain exclusive use of entire
files. The application-specified file locks should be released after all transaction
operations are completed. File locks give an application nearly complete isolation, but
they can greatly impact concurrency.
HP NonStop TMF Introduction — 540134-001
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