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:
1. Shared Lock (S-lock): Allows multiple transactions to read a data item.
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:
1. Growing phase: Locks are acquired and no locks are released.
2. Shrinking phase: Locks are released and no new locks are acquired.
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.

Practice

1. What is the main purpose of lock-based protocols in a database management system?

easy

A. To control concurrent access to data and prevent conflicts

B. To speed up database queries by caching results

C. To backup data automatically at regular intervals

D. To encrypt data for security purposes

Solution

1. Step 1: Understand the role of lock-based protocols

   Lock-based protocols manage how multiple users access data simultaneously to avoid conflicts.

2. Step 2: Identify the main goal

   The main goal is to control concurrent access and maintain data consistency.

3. Final Answer:

   To control concurrent access to data and prevent conflicts -> Option A

4. Quick Check:

   Lock-based protocols = control access [OK]

Hint: Locks manage who reads/writes data safely [OK]

Common Mistakes:

• Confusing locking with data backup
• Thinking locks speed up queries
• Mixing locking with encryption

2. Which of the following is the correct syntax to request a shared lock on a data item X in a lock-based protocol?

easy

A. lock_shared(X)

B. lock(X, shared)

C. lock_exclusive(X)

D. lock(X, exclusive)

Solution

1. Step 1: Understand shared lock syntax

   Shared locks allow multiple readers and are usually requested with a 'shared' parameter.

2. Step 2: Identify correct syntax format

   Among options, lock(X, shared) correctly requests a shared lock on X.

3. Final Answer:

   lock(X, shared) -> Option B

4. Quick Check:

   Shared lock syntax = lock(X, shared) [OK]

Hint: Shared lock uses 'shared' keyword in lock call [OK]

Common Mistakes:

• Using exclusive lock syntax for shared lock
• Confusing function names
• Omitting lock type parameter

3. Consider two transactions T1 and T2 accessing data item A. T1 requests a shared lock on A, then T2 requests an exclusive lock on A. What will happen next?

medium

A. Both T1 and T2 will hold locks simultaneously

B. T2 will immediately get the exclusive lock

C. T1 will be aborted due to conflict

D. T2 will wait until T1 releases the shared lock

Solution

1. Step 1: Analyze lock compatibility

   Shared locks allow multiple readers, but exclusive locks require exclusive access.

2. Step 2: Understand lock waiting rules

   T2's exclusive lock request conflicts with T1's shared lock, so T2 must wait until T1 releases the lock.

3. Final Answer:

   T2 will wait until T1 releases the shared lock -> Option D

4. Quick Check:

   Exclusive lock waits for shared lock release [OK]

Hint: Exclusive lock waits if shared lock exists [OK]

Common Mistakes:

• Assuming exclusive lock is granted immediately
• Thinking both locks can coexist
• Believing transaction abort happens automatically

4. A transaction T tries to acquire an exclusive lock on data item B but the system throws a deadlock error. What is the most likely cause?

medium

A. T is waiting for another transaction holding a conflicting lock on B

B. T requested a shared lock instead of exclusive lock

C. T has already released all its locks

D. The data item B does not exist in the database

Solution

1. Step 1: Understand deadlock in locking

   Deadlock occurs when two or more transactions wait indefinitely for locks held by each other.

2. Step 2: Identify cause of deadlock error

   T's exclusive lock request is blocked because another transaction holds a conflicting lock, causing deadlock.

3. Final Answer:

   T is waiting for another transaction holding a conflicting lock on B -> Option A

4. Quick Check:

   Deadlock = waiting on conflicting locks [OK]

Hint: Deadlock means waiting on each other's locks [OK]

Common Mistakes:

• Confusing deadlock with syntax errors
• Assuming deadlock means missing data
• Thinking deadlock occurs if locks are released

5. In a system using strict two-phase locking (2PL), which of the following is true about when locks are released?

hard

A. Locks are released randomly during the transaction

B. Locks are released immediately after each data item is accessed

C. All locks are released only after the transaction commits or aborts

D. Shared locks are released before exclusive locks

Solution

1. Step 1: Understand strict two-phase locking

   Strict 2PL requires holding all locks until the transaction finishes (commit or abort) to ensure serializability.

2. Step 2: Identify lock release timing

   Locks are not released during the transaction but only after it ends, preventing conflicts.

3. Final Answer:

   All locks are released only after the transaction commits or aborts -> Option C

4. Quick Check:

   Strict 2PL releases locks after commit/abort [OK]

Hint: Strict 2PL holds locks until transaction ends [OK]

Common Mistakes:

• Releasing locks too early during transaction
• Confusing strict 2PL with basic locking
• Assuming random lock release