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Intermediate Topic 28 of 51 Databases

Locking and Two-Phase Locking

Using locks to serialize conflicting operations.

How it works

Databases lock data items to control access: shared locks allow concurrent reads and exclusive locks allow a single writer. Locks may cover rows, pages, tables or ranges, and intention locks mark higher-level interest. Two-phase locking splits a transaction into a growing phase, where it only acquires locks, and a shrinking phase, where it only releases them; this guarantees serializable schedules. Strict 2PL holds exclusive locks until commit to prevent cascading aborts.

Step-by-step walkthrough

  1. A transaction acquires locks as it works, shared for reads and exclusive for writes.
  2. In two-phase locking it may not take new locks after releasing one.
  3. It holds all locks until commit.
  4. Then all locks are released together.
  5. This guarantees serializability but can deadlock, so the manager detects cycles.

Where it is used

Relational databases, lock managers in distributed systems and SELECT FOR UPDATE row locking in applications use these protocols.

Common pitfall

Two-phase locking can deadlock, so the database detects cycles or uses timeouts and aborts a victim. Coarse locks reduce concurrency and fine ones increase overhead. Lock escalation can unexpectedly block many users. Long transactions hold locks and cause pile-ups. Acquire locks in a consistent order and keep transactions short.

Where it fits in the course

Locking and Two-Phase Locking is topic 28 of 51 in Databases, rated Intermediate. It follows Concurrency Control. The next topic is MVCC.

Practice and explore

How to study this topic

  1. Read the explanation above and restate the definition in your own words.
  2. Open the interactive lesson and answer its mini quiz.
  3. Read the common pitfall, then write a short example that avoids it.
  4. Check your understanding in the Databases quiz, and revisit the topic if you miss a question.

Frequently asked questions

What is Locking and Two-Phase Locking?
Using locks to serialize conflicting operations. It belongs to Databases and is an intermediate-level topic.
Where do people use Locking and Two-Phase Locking?
Relational databases, lock managers in distributed systems and SELECT FOR UPDATE row locking in applications use these protocols.
What is the most common mistake with Locking and Two-Phase Locking?
Two-phase locking can deadlock, so the database detects cycles or uses timeouts and aborts a victim.
What should I learn before this topic?
Start with Concurrency Control, the topic just before it in Databases.
How can I practice it?
Answer the mini quiz in the interactive lesson, then take the Databases quiz.
Is Locking and Two-Phase Locking hard to learn?
It is an Intermediate topic. Be comfortable with the Beginner topics in this area first, then read the common pitfall carefully, since it is where most mistakes happen.
How long does it take to learn Locking and Two-Phase Locking?
Most learners can read the explanation and finish the interactive lesson in under an hour. Fluency comes from applying the idea, so plan a few short practice sessions with the quiz.
Which topics are related to Locking and Two-Phase Locking?
Concurrency Control, MVCC, Isolation Levels are closely connected. The full list is in the related topics section of this page.
What should I study after Locking and Two-Phase Locking?
Continue with MVCC, the next topic in Databases.

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