Lock-based protocols in DBMS Theory - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a lock-based protocol in database management?

A lock-based protocol is a method used to control access to data in a database by locking data items. It ensures that transactions do not interfere with each other, maintaining data consistency and preventing conflicts.

beginner

What are the two main types of locks used in lock-based protocols?

The two main types of locks are:<br>1. Shared Lock (S-lock): Allows multiple transactions to read a data item.<br>2. Exclusive Lock (X-lock): Allows a transaction to write or modify a data item exclusively.

intermediate

Explain the purpose of the Two-Phase Locking (2PL) protocol.

Two-Phase Locking (2PL) ensures serializability by dividing a transaction's locking actions into two phases:<br>1. Growing phase: Locks are acquired and no locks are released.<br>2. Shrinking phase: Locks are released and no new locks are acquired.<br>This prevents conflicts and ensures correct transaction order.

intermediate

What problem can occur if locks are not managed properly in lock-based protocols?

Deadlock can occur when two or more transactions wait indefinitely for locks held by each other, causing the system to freeze or stall.

beginner

How does a shared lock differ from an exclusive lock in terms of transaction access?

A shared lock allows multiple transactions to read the same data simultaneously but prevents writing. An exclusive lock allows only one transaction to write or modify the data and prevents others from reading or writing until the lock is released.

What does an exclusive lock allow a transaction to do?

A Write or modify data exclusively

B Read data only

C Read data with other transactions

D Release locks early

An exclusive lock allows a transaction to write or modify data exclusively, preventing other transactions from accessing it simultaneously.

Which phase in Two-Phase Locking is when locks are acquired?

A Shrinking phase

B Waiting phase

C Growing phase

D Releasing phase

In the growing phase, a transaction acquires all the locks it needs and does not release any.

What problem arises when two transactions wait for each other's locks indefinitely?

A Deadlock

B Rollback

C Starvation

D Commit failure

Deadlock occurs when transactions wait forever for locks held by each other, causing a standstill.

Which lock type allows multiple transactions to read the same data simultaneously?

A None of the above

B Exclusive lock

C Write lock

D Shared lock

Shared locks allow multiple transactions to read the same data at the same time.

Why are lock-based protocols important in databases?

A To speed up data entry

B To prevent data conflicts and ensure consistency

C To delete old data

D To backup data automatically

Lock-based protocols prevent conflicts between transactions and keep data consistent.

Describe how Two-Phase Locking (2PL) works and why it is important.

Think about the order of locking and unlocking in a transaction.

Explain the difference between shared and exclusive locks with examples.

Consider who can access the data and how.