Why it matters

Every database claims some level of ACID. Understanding the details lets you evaluate what a database really provides and whether it fits your needs.

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The architecture

Atomicity: transaction either fully commits or fully aborts. Achieved via undo logs or MVCC snapshots.

Consistency: transaction preserves invariants (foreign keys, checks). Application + DB together.

ACID propertiesAtomicity + Durabilitycommit or abort, WALIsolationconcurrent tx don't interfereConsistencyinvariants holdDifferent DBs give different isolation levels — often not fully serializable by default
The four ACID guarantees.
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How it works end to end

Isolation: concurrent transactions behave as if serial. Perfect isolation is serializable; databases offer weaker levels for performance.

Durability: committed data survives crashes. Write-ahead log (WAL) flushed to durable storage before commit ack.

Trade-offs: strict ACID limits throughput. Distributed databases trade some ACID for availability.