🔒Isolation (ACID)

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Also known as: Transaction Isolation, Concurrency Control

The ACID property ensuring concurrently executed transactions behave as if executed serially without interfering.

DESCRIPTION

Isolation guarantees transactional independence under concurrency, preventing dirty reads, non-repeatable reads, and phantom reads. Databases implement isolation via locks, MVCC, and serializable schedules. SQL defines levels like Read Uncommitted, Read Committed, Repeatable Read, and Serializable. Strong isolation improves correctness but can reduce throughput due to contention. System designs balance isolation with availability depending on workload and consistency needs.

ALTERNATIVE DEFINITIONS

Wikipedia

Isolation ensures that the concurrent execution of transactions results in a system state that would be obtained if transactions were executed serially.

Defines isolation as equivalence to serial execution outcomes.

Source

PostgreSQL Documentation

The isolation level defines the degree to which a transaction must be isolated from data modifications made by other transactions.

Focuses on standardized isolation levels affecting visibility of concurrent changes.

Source

Oracle Database Concepts

Isolation ensures that the effects of one transaction are not visible to other transactions until the transaction commits.

Emphasizes commit boundaries for visibility of changes.

Source

RELATED TERMS

ACID Atomicity Consistency Durability Serializability Multiversion Concurrency Control Locking Transaction

COMPETING TERMS

BASE Eventual Consistency

RELATIONS

• Broader: ACID

KNOWLEDGE AREAS

Data Storage & Operations Data Engineering Data Architecture Software Engineering

TARGET AUDIENCE

Data Engineer Software Developer Data Architect Cloud Architect

EXTERNAL SOURCES

• Wikipedia - ACID
• MySQL - Isolation levels in InnoDB
• Microsoft SQL Server - SQL Server isolation levels overview

TOP QUESTIONS

Why is Isolation (ACID) important in OLTP systems?

It preserves correctness under concurrency, preventing inconsistent reads and write conflicts in transactional workloads.

Category: Relevancy

What are the standard Isolation levels?

Read Uncommitted, Read Committed, Repeatable Read, and Serializable each control read phenomena and concurrency trade-offs.

Category: Overview

How does MVCC implement Isolation?

MVCC maintains multiple versions of rows so readers see a consistent snapshot while writers proceed without blocking reads.

Category: Example

What risks arise from weak Isolation?

Dirty reads, non-repeatable reads, and phantom reads can lead to incorrect business decisions or integrity violations.

Category: Risks

Is Serializable always the best Isolation level?

It’s strongest but may reduce throughput due to contention. Choose based on consistency needs and workload characteristics.

Category: Opinion

How does Isolation relate to BASE?

ACID Isolation prioritizes consistency; BASE favors availability with eventual consistency, often in distributed systems.

Category: Topic

Can indexes affect Isolation behavior?

Yes, index range scans can introduce phantom reads under weaker isolation levels without predicate locking.

Category: Example

Where to start learning Isolation (ACID)?

Study database concurrency control, SQL specs, and vendor docs on locks, MVCC, and isolation levels.

Category: Requirements

What is a common mistake with Isolation settings?

Defaulting to Read Uncommitted for speed, which allows dirty reads and risks inconsistent outcomes.

Category: Mistakes