Deadlock handling in databases in DBMS Theory - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

What is a deadlock in database systems?

Which of the following best describes a deadlock situation in a database?

A A database server crashes due to hardware failure.

B Two or more transactions wait indefinitely for each other to release locks.

C A transaction fails due to syntax errors in SQL statements.

D A transaction commits successfully without any conflicts.

💡 Hint

Think about transactions waiting for resources held by others.

A deadlock occurs when two or more transactions are waiting for each other to release locks, causing an indefinite wait.

📋 Factual

intermediate

Which method is commonly used to detect deadlocks?

Which technique is typically used by database systems to detect deadlocks?

A Timeout mechanism that aborts transactions after a fixed time.

B Periodic backup of database logs.

C Wait-for graph analysis to find cycles indicating deadlocks.

D Indexing tables to speed up queries.

💡 Hint

Think about graphs representing waiting relationships.

Databases use wait-for graphs to detect cycles, which indicate deadlocks.

🚀 Application

advanced

What happens when a deadlock is detected?

When a deadlock is detected in a database, what is the usual action taken by the system?

A The system duplicates the locked data to avoid waiting.

B The system shuts down to prevent data corruption.

C The system ignores the deadlock and continues processing.

D The system rolls back one or more transactions to break the deadlock.

💡 Hint

Consider how the system can resolve waiting cycles.

To resolve deadlocks, the system aborts one or more transactions to free locks and break the cycle.

🔍 Analysis

advanced

Which scenario is an example of deadlock?

Consider two transactions T1 and T2. T1 holds a lock on resource A and waits for resource B. T2 holds a lock on resource B and waits for resource A. What does this scenario represent?

A Deadlock, because both transactions wait for each other’s locked resources.

B Normal transaction processing with no issues.

C Transaction starvation due to priority inversion.

D Data inconsistency caused by concurrent updates.

💡 Hint

Think about circular waiting conditions.

This is a classic deadlock where two transactions wait indefinitely for each other’s locks.

❓ Reasoning

expert

Why is deadlock prevention harder than deadlock detection?

Why is it generally more difficult for database systems to prevent deadlocks than to detect and resolve them?

A Prevention requires restricting transaction behavior upfront, which can reduce concurrency and performance.

B Detection algorithms are slower and less reliable than prevention methods.

C Prevention needs hardware support that is rarely available.

D Detection causes permanent data loss, while prevention does not.

💡 Hint

Consider the trade-offs between safety and performance.

Deadlock prevention restricts how transactions acquire locks, limiting concurrency and reducing system throughput, making it harder to implement effectively.

Practice

1. What is a deadlock in a database system?

easy

A. A situation where two or more transactions wait indefinitely for each other to release locks

B. A method to speed up database queries

C. A backup process for database recovery

D. A type of database index

Solution

1. Step 1: Understand the concept of deadlock

   A deadlock occurs when transactions wait for resources locked by each other, causing a cycle of waiting.

2. Step 2: Identify the correct description

   Only a situation where two or more transactions wait indefinitely for each other to release locks describes this waiting cycle causing indefinite blocking.

3. Final Answer:

   A situation where two or more transactions wait indefinitely for each other to release locks -> Option A

4. Quick Check:

   Deadlock = indefinite waiting cycle [OK]

Hint: Deadlock means transactions stuck waiting on each other [OK]

Common Mistakes:

• Confusing deadlock with query optimization
• Thinking deadlock is a backup process
• Mixing deadlock with indexing

2. Which of the following is a common method to handle deadlocks in databases?

easy

A. Ignoring the deadlock and letting transactions wait

B. Deadlock detection and transaction rollback

C. Increasing the database size

D. Using more indexes

Solution

1. Step 1: Review deadlock handling methods

   Deadlock handling usually involves detecting the deadlock and aborting one transaction to break the cycle.

2. Step 2: Match the correct method

   Deadlock detection and transaction rollback correctly states deadlock detection and rollback, which is standard practice.

3. Final Answer:

   Deadlock detection and transaction rollback -> Option B

4. Quick Check:

   Deadlock handling = detect and rollback [OK]

Hint: Deadlocks are handled by detection and rollback [OK]

Common Mistakes:

• Thinking deadlocks can be ignored
• Believing increasing size fixes deadlocks
• Confusing indexing with deadlock handling

3. Consider two transactions T1 and T2. T1 holds a lock on resource A and waits for resource B. T2 holds a lock on resource B and waits for resource A. What will happen if the database uses deadlock detection?

medium

A. Both transactions will complete successfully without waiting

B. The transactions will wait forever causing a system crash

C. The database will automatically grant both resources to both transactions

D. The database will detect the deadlock and abort one transaction

Solution

1. Step 1: Analyze the resource waiting scenario

   T1 and T2 each hold one resource and wait for the other, creating a circular wait (deadlock).

2. Step 2: Understand deadlock detection response

   The database detects this cycle and aborts one transaction to break the deadlock.

3. Final Answer:

   The database will detect the deadlock and abort one transaction -> Option D

4. Quick Check:

   Deadlock detection aborts one transaction [OK]

Hint: Deadlock detection aborts one waiting transaction [OK]

Common Mistakes:

• Assuming both transactions finish without waiting
• Thinking system crashes always occur
• Believing resources are shared automatically

4. A database system uses timeout to handle deadlocks. A transaction waits for a lock for 10 seconds before aborting. What is the problem if the timeout is set too low?

medium

A. Transactions may abort unnecessarily even if no deadlock exists

B. Deadlocks will never be detected

C. The database will run out of memory

D. All transactions will complete faster

Solution

1. Step 1: Understand timeout-based deadlock handling

   Timeout aborts transactions waiting too long for locks, assuming deadlock.

2. Step 2: Analyze low timeout effect

   If timeout is too low, transactions may abort even when just waiting normally, causing unnecessary rollbacks.

3. Final Answer:

   Transactions may abort unnecessarily even if no deadlock exists -> Option A

4. Quick Check:

   Low timeout causes false aborts [OK]

Hint: Too low timeout causes false deadlock aborts [OK]

Common Mistakes:

• Thinking deadlocks won't be detected at all
• Assuming database memory is affected
• Believing all transactions speed up

5. In a database with multiple transactions, which strategy best reduces the chance of deadlocks?

hard

A. Allowing transactions to lock resources randomly

B. Increasing the number of concurrent transactions

C. Ensuring all transactions lock resources in the same order

D. Disabling deadlock detection

Solution

1. Step 1: Understand deadlock cause

   Deadlocks happen when transactions lock resources in different orders, causing circular waits.

2. Step 2: Identify prevention strategy

   Forcing all transactions to lock resources in the same order prevents cycles, reducing deadlocks.

3. Final Answer:

   Ensuring all transactions lock resources in the same order -> Option C

4. Quick Check:

   Same lock order prevents deadlocks [OK]

Hint: Lock resources in same order to avoid deadlocks [OK]

Common Mistakes:

• Thinking random locking reduces deadlocks
• Believing more transactions help
• Disabling detection solves deadlocks