HP Integrity NonStop H-Series - Manual (Page 51)

Configuring Automatic Transaction Processing
HP NonStop AutoTMF Software User’s Guide—429952-013
4-7
Transaction Isolation
Automatic transactions are committed by a proprietary set of processing rules, driven
by the following activities and events:
•
Process operations that lock, update, and unlock files and records; such
information determines both the lock protocols that need to be preserved and the
actual lock state of open files.
•
Process operations that signal a requirement or an opportunity to commit an
automatic transaction. For example, when a server process replies to a requestor:
the server has performed database operations for one request and will await a new
request; any outstanding automatic transaction should be committed at this point.
•
Process operations, such as termination, that would cause automatic transactions
to be aborted.
•
Process operations that may require a commit to isolate automatic transactions
from external effects, as described in
Transaction Isolation on page 4-7
•
Time that an automatic transaction has been alive; you can configure the maximum
transaction lifetime to a reasonable interval that is consistent with good
performance
•
Amount of update activity performed under an automatic transaction; you can
configure the maximum number of updates in a transaction to keep the transaction
overhead to a reasonable fraction of the total processing cost.
Transactions that are used for a large number of updates accumulate locks, cause
other resource problems, and have a larger impact if aborted unilaterally.
Transactions tend to increase the time during which locks are held and this increased
time may result in increased lock contention. This phenomenon is common in all
transaction-based database systems. Lock contention can be monitored by using the
NonStop AutoTMF LISTLOCKS command.
Excessive lock contention can be reduced through proper NonStop AutoTMF
configuration, but changes in the configuration may cause more frequent transactions
and a corresponding increase in processing requirements.
Transaction Isolation
Transaction isolation influences the duration of automatic transactions to reduce the
effect of automatic transactions outside the process that started them. There are three
levels of isolation:
•
Weak isolation commits active automatic transactions whenever a server process
waits for or replies to a request on $RECEIVE. The default isolation mode is weak
isolation, which is sufficient for most applications and provides the best
performance; you should use weak isolation unless problems are clearly identified.
•
Normal isolation also commits active automatic transactions whenever a process
sends a request to a server process; normal isolation is required in applications