Interview Question

Explain different transaction isolation levels.

Answer

Transaction isolation levels determine how transactions interact with each other in a database system, particularly when multiple transactions are reading and writing data concurrently.

The Four Standard Isolation Levels

1. Read Uncommitted

The lowest isolation level, allowing transactions to read data that has been modified but not yet committed by other transactions.

using var transaction = await _context.Database.BeginTransactionAsync(
    IsolationLevel.ReadUncommitted);

Characteristics:

• Allows dirty reads (reading uncommitted changes)
• Allows non-repeatable reads (data changing between reads)
• Allows phantom reads (new rows appearing between reads)
• Highest concurrency, lowest consistency

Use Cases:

• Reporting queries where absolute accuracy isn’t critical
• Scenarios where performance is more important than consistency
• Reading approximate counts or statistics

2. Read Committed

Ensures that a transaction only reads data that has been committed by other transactions.

using var transaction = await _context.Database.BeginTransactionAsync(
    IsolationLevel.ReadCommitted);

Characteristics:

• Prevents dirty reads
• Allows non-repeatable reads
• Allows phantom reads
• Good balance between concurrency and consistency

Use Cases:

• Default isolation level in many databases (SQL Server, PostgreSQL)
• General-purpose OLTP applications
• Most business applications where some concurrency anomalies are acceptable

3. Repeatable Read

Ensures that if a transaction reads a row, it will get the same data if it reads that row again, even if other transactions modify the data.

using var transaction = await _context.Database.BeginTransactionAsync(
    IsolationLevel.RepeatableRead);

Characteristics:

• Prevents dirty reads
• Prevents non-repeatable reads
• Allows phantom reads
• Higher consistency, lower concurrency

Use Cases:

• Financial calculations that read the same data multiple times
• Reports that need consistent data throughout the transaction
• Default isolation level in MySQL/MariaDB

4. Serializable

The highest isolation level, ensuring complete isolation from other transactions.

using var transaction = await _context.Database.BeginTransactionAsync(
    IsolationLevel.Serializable);

Characteristics:

• Prevents dirty reads
• Prevents non-repeatable reads
• Prevents phantom reads
• Highest consistency, lowest concurrency
• May cause more deadlocks and timeouts

Use Cases:

• Financial transactions requiring absolute consistency
• Critical data integrity operations
• Scenarios where data must be completely isolated

Additional Isolation Levels (SQL Server Specific)

5. Snapshot

Provides statement-level read consistency without locking.

using var transaction = await _context.Database.BeginTransactionAsync(
    IsolationLevel.Snapshot);

Characteristics:

• Prevents dirty reads
• Prevents non-repeatable reads
• Prevents phantom reads
• Uses row versioning instead of locks
• Good balance between isolation and concurrency

Use Cases:

• Reporting applications that need consistent point-in-time view
• Long-running read operations that shouldn’t block writers
• Applications needing high concurrency with good consistency

Concurrency Phenomena

Key Points 💡

• Higher isolation levels provide more consistency but reduce concurrency
• Lower isolation levels improve performance but may lead to data anomalies
• The default isolation level varies by database system
• SQL Server default: Read Committed
• PostgreSQL default: Read Committed
• MySQL/MariaDB default: Repeatable Read
• Oracle default: Read Committed
• Choose the appropriate isolation level based on your specific requirements
• Consider using optimistic concurrency for better scalability when appropriate
• Snapshot isolation provides a good balance for many applications
• Be aware of deadlock potential at higher isolation levels

Common Follow-up Questions

1. How do you choose the appropriate isolation level for a specific operation?
2. What are the performance implications of different isolation levels?
3. How do isolation levels affect locking behavior in the database?
4. What is the difference between optimistic and pessimistic concurrency control?