- Two Phase Locking (2PL):
- Two Phase Locking =
a ordering of
locking operations in a
transaction
where:
- All the lock operations precedes all the unlock operations
- Two Phase Locking =
a ordering of
locking operations in a
transaction
where:
- Schematically:
Transaction: lock(A) ... lock(B) ... ... lock(X) ... ... unlock(B) ... unlock(X) ... unlock(A) ...The 2 phases:
- Phase 1:
- Acquire locks
- Phase 2:
- Release locks
- Phase 1:
- Example:
Notice:
- All lock operations precedes all unlock operations !!!
We saw in the previous example that execution using 2-phase locking results in a consistent database state:
- Recall:
- Transactions using non 2-phase locking can result in a inconsistsent DB state
We have also seen an example previously:
Notice:
- The transactions are
not
two-phase !!!
- The unlock operation ui(A) happens before an lock operation (ℓi(B)
Execution results in INconsistent database state:
