Strict Two-Phase Locking
Notation
- S(a): Lock a in shared mode.
- X(a): Lock a in exclusive mode.
- U(a): Unlock a.
Two-phase
All locks are obtained before the first lock is released.
- Two-phase: S(A), S(B), X(C), U(A), U(B), U(C).
- Not two-phase: S(A), S(B), U(B),
X(C), U(A), U(C).
Without two-phase locking, serializability cannot be guaranteed.
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