ACID and CAP both contain a “C,” but they refer to very different concepts in the context of databases and distributed systems. Let’s break them down:
ACID (Atomicity, Consistency, Isolation, Durability)
- C = Consistency
- This means that a database must remain in a valid state before and after a transaction.
- If a transaction starts with a valid state and follows the rules (constraints, referential integrity), then after the transaction, the database must still be valid.
- Example: If you transfer money from Account A to Account B, the total balance across both should not change unexpectedly.
CAP (Consistency, Availability, Partition Tolerance)
- C = Consistency
- In the CAP theorem, consistency means that every read gets the most recent write (or an error).
- If multiple nodes exist in a distributed system, all nodes must return the same up-to-date data.
- Example: If you update a value on Node A, then Node B should immediately reflect the same update before any read operation.
Key Difference in “C”
| Feature | ACID Consistency | CAP Consistency |
|---|---|---|
| Scope | Single database transactions | Distributed systems |
| Meaning | Ensures the database follows integrity rules before and after a transaction | Ensures all replicas of data return the same latest value |
| Enforcement | Enforced by strict rules and constraints | Enforced by synchronization across nodes |
Summary
- ACID C (Consistency) = Ensuring database integrity and rule enforcement.
- CAP C (Consistency) = Ensuring synchronized data across distributed nodes.