Two Phase Locking High Priority 2PL-HP Protocol

Two Phase Locking High Priority (2PL-HP) Protocol, 2PL-HP Concurrency Control Protocol for real-time database


2PL High-Priority (2PL-HP)


It is one of the pessimistic concurrency control protocols. It modifies the basic 2PL protocol by incorporating high priority conflict resolution scheme. It works as follows;

When a higher priority transaction (requester) is requesting a lock on an object held by lower priority transactions (lock holder) in a conflicting mode, the lock holders are aborted or restarted and the requester is granted the lock.
If the requester’s priority is lower than the lock holders’ priority then requester will wait for the object to be released.
A new reader can join the group of lock-holding readers only if its priority is higher than that of all the writers waiting for the lock.

This can be represented in pseudo code as follows;
/* P(T) denotes the priority of a transaction T.
Tr is the requesting transaction with HIGHER PRIORITY.
Th is the lock holding transaction with LOWER PRIORITY. */
if (P(Tr)>P(Th)) then
          Th ABORTs;
Tr continues;
else
          Tr WAIT;
endif

Discussion:
Let us assume a transaction T1 with higher priority is requesting a data item held by a transaction T2 with lower priority. The 2PL-HP forces (wound) the lower priority transaction T2 to restart or abort. This action will release the lock held by T2, then T1 acquires the lock and continue.

If T1’s priority is lower than that of T2’s priority, then 2PL-HP forces T1 to wait for the object to be released by T2.

2PL-HP shows the properties of “priority based wound-wait” under conflict resolution.

Merits:
  • 2PL-HP serves a deadlock prevention mechanism.
  • 2PL-HP ensures urgent higher priority transactions to meet their deadlines.

Demerits:
  • A transaction restarted by a higher-priority transaction may miss its deadline and discarded.
  • Lower priority transactions always restarted due to the higher priority transactions end in wastage of system resources used by lower priority transactions.
  • More number of lower priority transactions with conflicting locks will lead to wasted restarts thus results in performance degradation of the system.

Comparison:

The experimental results showed real-time concurrency control algorithms based on High-Priority perform considerably better than those based on Priority-Inheritance. Furthermore, it was found that the 2PL-HP outperforms the 2PL-WP scheme in the context of a RTDB environment, and it was concluded that the basic priority-inheritance is inappropriate for conflict resolution under 2PL.

References:
Abbott, R., and Garcia-Molina, H. 1988. Scheduling real-time transactions: a performance evaluation, Proc. of 14th Intl. Conf. on Very Large DataBases. August




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