Deadlock detection and prevention in MySQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a deadlock in a database?

A deadlock happens when two or more transactions wait for each other to release locks, so none can proceed. It's like two people blocking each other in a narrow hallway.

intermediate

How does MySQL detect deadlocks?

MySQL detects deadlocks by checking for cycles in the lock wait graph. When it finds a cycle, it chooses one transaction to roll back to break the deadlock.

beginner

Name one common method to prevent deadlocks.

One common method is to always acquire locks in the same order in all transactions. This avoids circular waiting and prevents deadlocks.

intermediate

What is the role of InnoDB in deadlock handling in MySQL?

InnoDB storage engine automatically detects deadlocks and rolls back one transaction to resolve it, allowing others to continue.

beginner

Why is it important to keep transactions short to prevent deadlocks?

Short transactions hold locks for less time, reducing the chance that other transactions will wait and cause deadlocks.

What causes a deadlock in a database?

A A database backup running

B A transaction running too fast

C A query with syntax error

D Two transactions waiting on each other’s locks

Deadlocks happen when transactions wait for each other’s locks, causing a cycle that stops progress.

How does MySQL resolve a detected deadlock?

A It ignores the deadlock

B It rolls back one transaction to break the deadlock

C It restarts the database server

D It pauses all transactions

MySQL chooses one transaction to roll back, freeing locks and resolving the deadlock.

Which practice helps prevent deadlocks?

A Acquiring locks in a consistent order

B Running transactions without locks

C Using very large transactions

D Ignoring error messages

Consistent lock order avoids circular waiting, preventing deadlocks.

What does InnoDB do when it detects a deadlock?

A Rolls back one transaction automatically

B Shuts down the database

C Sends an email alert

D Locks all tables

InnoDB automatically rolls back one transaction to resolve deadlocks.

Why should transactions be kept short to avoid deadlocks?

A Short transactions increase deadlocks

B Short transactions use more CPU

C Short transactions hold locks less time, reducing conflicts

D Short transactions slow down queries

Short transactions reduce lock holding time, lowering deadlock chances.

Explain what a deadlock is and how MySQL detects and resolves it.

Think about transactions waiting for each other and how MySQL breaks the cycle.

Describe two ways to prevent deadlocks in database transactions.

Consider how to avoid circular waiting and reduce lock time.

Practice

1. What is a deadlock in MySQL InnoDB storage engine?

easy

A. A type of backup failure during database replication.

B. A syntax error in SQL queries causing transaction failure.

C. A temporary network disconnection between client and server.

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

Solution

1. Step 1: Understand transaction locking

   Transactions acquire locks on data to ensure consistency. If two transactions wait for each other's locks, progress stops.

2. Step 2: Define deadlock

   This waiting cycle with no resolution is called a deadlock, causing transactions to block indefinitely.

3. Final Answer:

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

4. Quick Check:

   Deadlock = Transactions waiting on each other [OK]

Hint: Deadlock means transactions block each other forever [OK]

Common Mistakes:

• Confusing deadlock with syntax errors
• Thinking deadlock is a network issue
• Assuming deadlock is a backup problem

2. Which MySQL command helps you check the latest deadlock information in InnoDB?

easy

A. SHOW PROCESSLIST;

B. SHOW ENGINE INNODB STATUS;

C. SHOW TRANSACTION LOCKS;

D. SHOW DEADLOCKS;

Solution

1. Step 1: Identify command for deadlock info

   MySQL does not have a direct SHOW DEADLOCKS command, but SHOW ENGINE INNODB STATUS shows detailed InnoDB info including deadlocks.

2. Step 2: Confirm other options

   SHOW PROCESSLIST shows running threads, not deadlocks. SHOW TRANSACTION LOCKS is invalid syntax.

3. Final Answer:

   SHOW ENGINE INNODB STATUS; -> Option B

4. Quick Check:

   SHOW ENGINE INNODB STATUS = Deadlock info [OK]

Hint: Use SHOW ENGINE INNODB STATUS for deadlock details [OK]

Common Mistakes:

• Using non-existent SHOW DEADLOCKS command
• Confusing PROCESSLIST with deadlock info
• Trying SHOW TRANSACTION LOCKS which is invalid

3. Consider two transactions T1 and T2 running concurrently:
-- T1: START TRANSACTION;
UPDATE accounts SET balance = balance - 100 WHERE id = 1;
UPDATE accounts SET balance = balance + 100 WHERE id = 2;
-- T2: START TRANSACTION;
UPDATE accounts SET balance = balance + 100 WHERE id = 2;
UPDATE accounts SET balance = balance - 100 WHERE id = 1;
What is the likely outcome if both run at the same time without deadlock prevention?

medium

A. A deadlock occurs causing one transaction to be rolled back.

B. Both transactions complete successfully without waiting.

C. Both transactions fail with syntax errors.

D. Transactions run sequentially without any lock conflicts.

Solution

1. Step 1: Analyze lock acquisition order

   T1 locks account id=1 first, then id=2. T2 locks id=2 first, then id=1. This creates a circular wait.

2. Step 2: Identify deadlock

   Because each transaction waits for the other's locked row, a deadlock occurs. MySQL detects this and rolls back one transaction.

3. Final Answer:

   A deadlock occurs causing one transaction to be rolled back. -> Option A

4. Quick Check:

   Opposite lock order = Deadlock and rollback [OK]

Hint: Opposite lock order in transactions causes deadlock [OK]

Common Mistakes:

• Assuming both transactions succeed without conflict
• Thinking syntax errors cause failure
• Believing transactions run sequentially automatically

4. You have this MySQL transaction code causing deadlocks:
START TRANSACTION;
UPDATE products SET stock = stock - 1 WHERE id = 10;
UPDATE orders SET status = 'processed' WHERE id = 100;
COMMIT;
How can you fix deadlocks by changing the transaction?

medium

A. Keep transactions short and access tables in the same order everywhere.

B. Add a LOCK TABLES statement before updates.

C. Use multiple COMMIT statements inside the transaction.

D. Disable autocommit mode.

Solution

1. Step 1: Understand deadlock causes

   Deadlocks often happen when transactions lock tables in different orders or run too long holding locks.

2. Step 2: Apply prevention best practice

   Keeping transactions short and always locking tables in the same order prevents circular waits and deadlocks.

3. Final Answer:

   Keep transactions short and access tables in the same order everywhere. -> Option A

4. Quick Check:

   Short, ordered transactions prevent deadlocks [OK]

Hint: Short, consistent lock order avoids deadlocks [OK]

Common Mistakes:

• Thinking LOCK TABLES solves deadlocks alone
• Using multiple COMMITs inside one transaction
• Confusing autocommit disabling with deadlock fix

5. You want to prevent deadlocks in a high-traffic MySQL application. Which combination of strategies is best?

• 1. Keep transactions short
• 2. Set innodb_lock_wait_timeout to a low value
• 3. Use consistent lock order in all transactions
• 4. Disable foreign key constraints

hard

A. 1 and 4 only

B. 2 and 4 only

C. 1, 2, and 3 only

D. All four strategies

Solution

1. Step 1: Evaluate each strategy

   Keeping transactions short and consistent lock order reduce deadlocks. Setting a low innodb_lock_wait_timeout helps detect and abort deadlocks quickly.

2. Step 2: Assess disabling foreign keys

   Disabling foreign keys can cause data integrity issues and does not prevent deadlocks effectively.

3. Final Answer:

   1, 2, and 3 only -> Option C

4. Quick Check:

   Short, timeout, order = Deadlock prevention [OK]

Hint: Short transactions + timeout + lock order prevent deadlocks [OK]

Common Mistakes:

• Disabling foreign keys to prevent deadlocks
• Ignoring lock wait timeout settings
• Not enforcing consistent lock order