Concepts/3. Data & Storage/ACID Transactions & Isolation Levels
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3. Data & StorageAdvanced

ACID Transactions & Isolation Levels

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Guarantees Atomicity (all-or-nothing), Consistency (rules preserved), Isolation (concurrent safety), and Durability (WAL commits survive power loss).

STAFF ENGINEER TAKEAWAY: Isolation levels trade performance for anomalies: Read Uncommitted (dirty reads) -> Read Committed -> Repeatable Read -> Serializable (strict 2PL).
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Two-Phase ACID Transaction Commit

1
BEGIN Transaction
Allocates unique TxID in engine
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2
DML Writes & Row Locks
Updates account balances in RAM buffer
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3
WAL fsync Flush
Writes delta log to non-volatile disk
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4
COMMIT & Lock Release
Changes visible to concurrent readers
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✓ ARCHITECTURAL ADVANTAGES

Prevents double-spending and ledger inconsistencies.

✗ TRADE-OFFS & COMPLEXITIES

Higher isolation levels introduce row locking, deadlocks, and reduced write throughput.

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Production Systems Leveraging This Pattern

⚡PostgreSQL⚡Oracle⚡SpannerCockroachDB Architecture↗
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