Deadlock handling in databases in DBMS Theory - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a deadlock in a database system?

A deadlock occurs when two or more transactions wait indefinitely for each other to release locks on resources, causing all involved transactions to stop progressing.

intermediate

Name one common method used to detect deadlocks in databases.

One common method is the wait-for graph, where transactions are nodes and edges represent waiting for locks. A cycle in this graph indicates a deadlock.

intermediate

What is deadlock prevention in databases?

Deadlock prevention involves designing the system to avoid conditions that can cause deadlocks, such as by ordering resource requests or using timeouts.

intermediate

How does deadlock resolution typically work in databases?

Deadlock resolution usually involves choosing one transaction as a victim and rolling it back to break the deadlock, allowing other transactions to continue.

beginner

Why is deadlock handling important in database management?

Deadlock handling is important to ensure that transactions do not get stuck forever, which would block other operations and reduce system performance and reliability.

What indicates a deadlock in a wait-for graph?

A A cycle in the graph

B A disconnected node

C A node with no edges

D A linear chain of nodes

A cycle in the wait-for graph means transactions are waiting on each other in a loop, which is a deadlock.

Which of the following is NOT a deadlock handling technique?

A Deadlock detection

B Deadlock prevention

C Deadlock avoidance

D Deadlock ignoring

Ignoring deadlocks is not a handling technique because it causes the system to hang indefinitely.

What action is taken to resolve a deadlock?

A Commit all transactions

B Restart the database server

C Rollback one or more transactions

D Increase lock timeout

Rolling back one or more transactions breaks the deadlock cycle and allows others to proceed.

Deadlock prevention can be achieved by:

A Allowing transactions to wait indefinitely

B Requesting all locks at once in a fixed order

C Ignoring lock requests

D Using random lock granting

Requesting locks in a fixed order prevents circular wait, a necessary condition for deadlocks.

Why is deadlock detection preferred over prevention in some systems?

A Prevention can reduce concurrency

B Detection avoids all deadlocks

C Detection is simpler to implement

D Prevention is slower

Prevention techniques can limit how transactions access resources, reducing system concurrency and performance.

Explain what a deadlock is in a database and why it happens.

Think about transactions waiting for locks held by each other.

Describe the main strategies used to handle deadlocks in databases.

Consider how systems avoid, find, or fix deadlocks.

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