Do NoSQL databases support ACID?

Some do, with limitations. MongoDB supports ACID transactions within a replica set. DynamoDB supports transactions within a single region. But many NoSQL databases trade ACID for scalability, offering eventual consistency instead.

What are isolation levels?

Isolation levels control how much concurrent transactions can see each other's changes: Read Uncommitted (least strict), Read Committed, Repeatable Read, and Serializable (most strict). PostgreSQL defaults to Read Committed.

What is a write-ahead log?

A write-ahead log (WAL) records changes before applying them to the database. If the system crashes, the WAL is replayed to recover committed transactions. This is the primary mechanism for durability in relational databases.

ACID Transactions Explained

The four guarantees that make database transactions reliable — ensuring data integrity even through errors, crashes, and concurrent access.

ACID Transactions

ACID is an acronym for Atomicity, Consistency, Isolation, and Durability — the four properties that guarantee database transactions are processed reliably, even in the face of errors, crashes, or concurrent access.

Explanation

Consider a bank transfer: debit $100 from Account A and credit $100 to Account B. If the system crashes after the debit but before the credit, $100 vanishes. ACID properties prevent this. Atomicity ensures both operations succeed or both fail — no partial transfers. Consistency ensures the total balance across accounts remains constant. Isolation ensures concurrent transfers do not interfere with each other. Durability ensures committed transactions survive power failures. Relational databases (PostgreSQL, MySQL, Oracle) provide ACID guarantees through mechanisms like write-ahead logging (durability), locking or MVCC (isolation), constraint checking (consistency), and rollback support (atomicity). Different isolation levels (Read Uncommitted, Read Committed, Repeatable Read, Serializable) offer trade-offs between consistency and performance — higher isolation prevents more anomalies but reduces concurrency. NoSQL databases often relax ACID guarantees for performance and scalability. Many offer "eventual consistency" instead of immediate consistency, and some provide ACID only within a single document or partition. Understanding these trade-offs is essential when choosing a database for a given use case.

Bookuvai Implementation

Bookuvai uses ACID-compliant databases (PostgreSQL) for all transactional data — financial records, user accounts, and business-critical operations. We configure appropriate isolation levels based on concurrency requirements and use database transactions to ensure data integrity across related operations. For non-transactional data, we may use eventually consistent stores with explicit conflict resolution.

Key Facts

Atomicity: all operations in a transaction succeed or all fail
Consistency: transactions move the database between valid states
Isolation: concurrent transactions do not interfere with each other
Durability: committed data survives crashes and power failures
Higher isolation levels reduce concurrency but prevent more anomalies

Related Terms

Frequently Asked Questions

Do NoSQL databases support ACID?: Some do, with limitations. MongoDB supports ACID transactions within a replica set. DynamoDB supports transactions within a single region. But many NoSQL databases trade ACID for scalability, offering eventual consistency instead.
What are isolation levels?: Isolation levels control how much concurrent transactions can see each other's changes: Read Uncommitted (least strict), Read Committed, Repeatable Read, and Serializable (most strict). PostgreSQL defaults to Read Committed.
What is a write-ahead log?: A write-ahead log (WAL) records changes before applying them to the database. If the system crashes, the WAL is replayed to recover committed transactions. This is the primary mechanism for durability in relational databases.