Isolation Levels

Overview

Isolation levels define the degree to which transactions are isolated from each other in concurrent database systems. They represent different trade-offs between consistency guarantees and system performance, allowing applications to choose the appropriate level based on their requirements.

The Four Standard Isolation Levels

Read Uncommitted (Level 0)

Lowest isolation, highest performance

• Transactions can read uncommitted changes from other transactions
• Allows dirty reads, non-repeatable reads, and phantom reads
• Rarely used due to data integrity concerns
• Useful for approximate counts or analytics where exact precision isn't critical

Read Committed (Level 1)

Most commonly used isolation level

• Transactions can only read committed data
• Prevents dirty reads but allows non-repeatable reads and phantom reads
• Default in most database systems (PostgreSQL, Oracle, SQL Server)
• Good balance between consistency and performance

Repeatable Read (Level 2)

Stronger consistency guarantees

• Same data read multiple times within a transaction returns same values
• Prevents dirty reads and non-repeatable reads
• May still allow phantom reads (new rows appearing)
• Default in MySQL/InnoDB

Serializable (Level 3)

Highest isolation, strictest consistency

• Transactions execute as if they were run serially, one after another
• Prevents all read anomalies: dirty reads, non-repeatable reads, phantom reads
• Highest performance cost due to increased locking/blocking
• Guarantees complete isolation

Read Anomalies

Dirty Read

Reading uncommitted data from another transaction that might be rolled back.

Non-Repeatable Read

Same query returns different results within the same transaction due to committed updates by other transactions.

Phantom Read

New rows appear in query results due to committed inserts by other transactions.

Implementation Approaches

Locking-Based

• Use shared and exclusive locks
• Two-phase locking protocol
• Risk of deadlocks
• Used in traditional databases

Multiversion Concurrency Control (MVCC)

• Multiple versions of data maintained
• Readers don't block writers
• Snapshot isolation
• Used in PostgreSQL, Oracle

Optimistic Concurrency Control

• Detect conflicts at commit time
• Higher concurrency but potential for aborts
• Suitable for low-conflict workloads

Choosing Isolation Levels

Performance vs Consistency Trade-off

• Higher isolation = lower performance
• More locking/blocking with stronger isolation
• Consider application requirements

Common Use Cases

Read Uncommitted:

• Analytics and reporting where approximate data is acceptable
• Bulk data loading scenarios

Read Committed:

• Most OLTP applications
• Web applications with moderate consistency needs
• Default choice for most systems

Repeatable Read:

• Financial applications requiring consistent views
• Report generation requiring stable snapshots
• Applications sensitive to non-repeatable reads

Serializable:

• Critical financial transactions
• Systems requiring absolute consistency
• Applications where correctness is more important than performance

Database-Specific Behavior

PostgreSQL

• Implements true Serializable isolation using predicate locking
• MVCC-based implementation
• Serialization failures can occur

MySQL (InnoDB)

• Repeatable Read is default
• Uses gap locking to prevent phantom reads
• Consistent non-locking reads

Oracle

• Read Committed is default
• Strong MVCC implementation
• Serializable implemented as Snapshot Isolation

Understanding isolation levels is crucial for designing applications that balance consistency requirements with performance needs in concurrent database environments.