The "C" in ACID and CAP Theorem Are Completely Different Concepts

While learning about CAP theorem, I noticed “Wait, isn’t the ‘C’ in ACID different from the ‘C’ in CAP?” After investigating, they were indeed completely different.

ACID Consistency (What I Initially Thought)

This is about “whether data satisfies business rules after a transaction”.

For example, in a bank transfer:

// Begin transaction
BEGIN;

// Transfer 1000 from A to B
UPDATE accounts SET balance = balance - 1000 WHERE user = 'A';
UPDATE accounts SET balance = balance + 1000 WHERE user = 'B';

// Commit
COMMIT;

After this transaction completes, does it maintain these rules? That’s ACID Consistency:

• Is A’s balance not negative?
• Are foreign key constraints not violated?
• Are application-defined business rules satisfied?

This is exactly what I initially thought.

CAP Consistency (What CAP Theorem Refers To)

This is about “whether all nodes see the same data”.

Using the same transfer example:

// Write to Tokyo server
await tokyo.update({ user: 'A', balance: 9000 });

// Immediately read from Osaka server
const balance = await osaka.select({ user: 'A' });
// → Might still return 10000 (old value)

Can you read the same value from Osaka server immediately after writing to Tokyo server? That’s CAP Consistency.

This is a completely different dimension - it’s about synchronization between replicas over a network, not about ACID.

My biggest discovery was these two points:

ACID assumes a single node, CAP assumes multiple nodes - this was my biggest finding. Despite using the same word “consistency”, the target is completely different.

While learning about distributed systems, I was confused by the meaning of “consistency”. I discovered that the “C” in ACID and CAP are the same word but entirely different concepts.

Specifically, ACID assumes a single node, CAP assumes multiple nodes, and the targets of inconsistency differ (business rule violations vs replica lag). Being aware of these two differences makes learning distributed systems much smoother.