ACID is a set of four guarantees that relational (SQL) databases provide to ensure data integrity, especially during transactions.
ACID Stands For:
| Letter | Name | Description |
|---|---|---|
| A | Atomicity | All steps in a transaction succeed or none do (it’s “all or nothing”) |
| C | Consistency | Data must remain valid and follow all rules (constraints, types, etc.) |
| I | Isolation | Concurrent transactions don’t interfere with each other |
| D | Durability | Once committed, changes survive crashes and power loss |
🧱 ACID in SQL Databases
Let’s say you’re transferring money between two bank accounts:
BEGIN;
UPDATE accounts SET balance = balance - 100 WHERE id = 1; -- debit
UPDATE accounts SET balance = balance + 100 WHERE id = 2; -- credit
COMMIT;
- If the second update fails (e.g., server crash), Atomicity ensures the first update is also rolled back.
- Consistency ensures no account ends up with negative balance if disallowed.
- Isolation ensures two people don’t overwrite each other’s balances.
- Durability ensures the transaction stays committed even after a crash.
✅ This is critical for systems like banking, inventory, booking, etc.
❄️ Why ACID Is Not Always Important in NoSQL
NoSQL databases often relax ACID to achieve:
- High availability
- Horizontal scalability
- Low latency
Most NoSQL databases follow the BASE model:
| BASE Property | Meaning |
|---|---|
| Basically Available | System works most of the time (not strict) |
| Soft state | State may change even without input |
| Eventually consistent | Consistency is reached eventually |
🔧 Example in NoSQL (MongoDB)
Suppose you’re storing blog posts and comments in MongoDB:
{
"_id": "post123",
"title": "ACID vs BASE",
"comments": [
{ "user": "Alice", "text": "Great article!" },
{ "user": "Bob", "text": "Thanks for the insights!" }
]
}
- MongoDB might store all this in one document.
- You can update it without transactions.
- Even if a user sees old data for a few seconds (due to eventual consistency), that’s acceptable.
✅ No need for strict ACID — performance and scalability are more valuable here.
🎯 So When Is ACID Important?
| Scenario | ACID Needed? |
|---|---|
| Banking / payments | ✅ Yes |
| E-commerce checkout | ✅ Yes |
| Social media feeds | ❌ Not really |
| Product search/autocomplete | ❌ No |
| Logging / analytics | ❌ No |
| Shopping cart session | 🤷 Depends |
✅ Summary
| Feature | SQL (ACID) | NoSQL (BASE) |
|---|---|---|
| Data Integrity | Strong guarantees | Relaxed for performance |
| Scalability | Vertical (harder to scale) | Horizontal (easier to scale) |
| Performance | Slower under load | Fast, low-latency |
| Best for | Financial, ERP, CRM | Big data, social media, analytics |