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C

ACID

ACID

Definition

ACID is the acronym for the four properties a reliable transaction must guarantee: Atomicity, Consistency, Isolation, Durability. Each is covered in depth in its own topic (22.5-22.8); this topic introduces them together as a unified framework.

How It Works

LetterPropertyOne-line meaning
AAtomicityAll-or-nothing — no partial completion
CConsistencyThe database moves from one valid state to another valid state
IIsolationConcurrent transactions don't interfere with each other's intermediate state
DDurabilityOnce committed, changes survive even a crash

These four properties together are WHY a transaction can be trusted as a reliable building block for correct application logic, even in the presence of concurrent users, hardware failures, and bugs.

Edge Cases and Pitfalls

  • The four letters are often taught as if they're independent, but they interact: Atomicity is partly what makes Consistency achievable (an incomplete update could leave data inconsistent), and Isolation exists specifically to prevent concurrent transactions from seeing each other's not-yet-atomic, in-progress state.
  • Not every storage engine provides full ACID guarantees — as discovered practically in this course, MySQL's default MyISAM engine does NOT support transactions at all (no real Atomicity/Isolation/Durability for multi-statement operations), while InnoDB does; choosing a transactional storage engine is a prerequisite for ACID to mean anything.
  • "Consistency" in ACID is sometimes confused with the "C" in the CAP theorem (a distributed-systems concept covered in Chapter 38) — they are related but NOT the same concept; ACID's Consistency is about respecting the database's own constraints/rules, while CAP's Consistency is about all nodes seeing the same data at the same time in a distributed system.

Key Takeaways

  • ACID = Atomicity, Consistency, Isolation, Durability — the four guarantees a reliable transaction provides.
  • These properties depend on the storage engine actually supporting transactions (e.g. InnoDB, not MyISAM).
  • ACID's "Consistency" and the CAP theorem's "Consistency" (Chapter 38) are related but distinct concepts — don't conflate them.

Mock Test

  • ACID - Quick Test

    8 questions on ACID.

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