TMF Introduction (G06.26+)

TMF Application Programming
HP NonStop TMF Introduction—522414-002
5-7
Managing Deadlock
After the UNLOCKRECORD request, the locks are released immediately, but only for
the unchanged records; locks on updated records are held until the transaction is
either committed or backed out.
Using File Locks for Repeatable Access
If an application issues LOCKFILE in a transaction, updates records while it holds the
lock granted by LOCKFILE, and then issues UNLOCKFILE before issuing END-
TRANSACTION, the file lock is held until the transaction commits.
Managing Deadlock
Transaction deadlock occurs when two transactions cannot proceed because they are
each waiting for the other to release a lock. Planning applications requires considering
how locks affect concurrency and how to avoid deadlock. If deadlock occurs, the
application needs to detect it and take appropriate action.
In Figure 5-1, deadlock results from this sequence of row (record) locking operations:
1. Transaction T1 accesses and locks row A.
2. Transaction T2 accesses and locks row B.
3. Transaction T1 attempts to access and lock row B and has to wait.
4. Transaction T2 attempts to access and lock row A and has to wait.
In this example, neither transaction can proceed. This situation can be avoided by
setting a time limit on how long a statement should wait before being granted a lock
request for access: if a request exceeds the time limit, the statement times out.
Figure 5-1. Deadlock Example
T2
Debits Balance B $500,00
T1
Debits Balance A $100,00
CHECKING
BAL
A
Deadlock
BAL
B
Credit Balance ACredit Balance B
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