Two-Phase Locking

Acquire every lock you need, then release them — never the other way round. That one rule makes every schedule serializable, and leaves deadlocks and cascading aborts to deal with.

Shared + exclusive locks

Guarantees conflict-serializability

Deadlocks still possible

Scenario

Protocol

Animation Speed: 1000ms

Step 1 of 0Basic 2PL

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Lock Compatibility

held \ asked	S	X
S
X

Basic 2PL guarantees conflict-serializability but can expose uncommitted writes, so one abort can cascade.

Strict 2PL keeps exclusive locks until commit: no cascading aborts, recoverable schedules. Most systems go further and hold every lock to the end (rigorous 2PL).

Neither prevents deadlock — the database detects cycles in the waits-for graph and aborts a victim.

Knowledge Check

Question 1 of 3

What does the "two phases" of two-phase locking refer to?

DSAverse

Two-Phase Locking

Lock Compatibility

Knowledge Check

See also

Two-Phase Locking

Lock Compatibility

Knowledge Check

See also