ACID Transactions: The Foundation of Data Integrity
The Reader's Dilemma
Dear Marilyn,I've been working with databases for years, but I still get confused about ACID properties. My manager keeps asking about "transaction isolation levels" and "consistency guarantees," and I just nod along. Can you explain ACID in a way that actually makes sense?
Marilyn's Reply
ACID isn't just a chemistry term—it's the contract that databases make with you about how they'll protect your data. Think of it as the four promises every reliable database makes. Let me break down each one with examples that will make you the expert in your next meeting.
The Spark: Understanding ACID
A - Atomicity: All or Nothing
Atomicity means a transaction is indivisible—either all operations succeed, or none do. There's no partial execution.
Real-World Example: Bank Transfer
Transferring $100 from Account A to Account B involves two operations:
- Debit $100 from Account A
- Credit $100 to Account B
Without atomicity, a crash after step 1 would lose $100 forever. Atomicity ensures both happen or neither does.
Quick Check
What happens if a system crash occurs in the middle of an atomic transaction?
C - Consistency: Rules Are Rules
Consistency ensures that every transaction moves the database from one valid state to another. All defined rules, constraints, and triggers must be satisfied.
| Constraint Type | Example | Violation Behavior |
|---|---|---|
| Primary Key | user_id must be unique | Transaction rejected |
| Foreign Key | order.user_id must exist in users | Transaction rejected |
| Check Constraint | balance >= 0 | Transaction rejected |
| Trigger | Update audit log on change | Trigger executes atomically |
Quick Check
If a transaction would result in a negative bank balance (violating a CHECK constraint), what happens?
I - Isolation: No Peeking
Isolation ensures that concurrent transactions don't interfere with each other. Each transaction should appear to run in complete isolation, even when many run simultaneously.
Isolation Levels (Weakest to Strongest):
- Read Uncommitted: Can see uncommitted changes (dirty reads)
- Read Committed: Only sees committed data, but may see different data on re-read
- Repeatable Read: Same query returns same results within a transaction
- Serializable: Transactions execute as if they were serial (one at a time)
Quick Check
Which isolation level provides the strongest guarantees but potentially the lowest performance?
D - Durability: Written in Stone
Durability guarantees that once a transaction is committed, it survives any subsequent failure—power outage, crash, or hardware failure.
How Databases Achieve Durability:
- Write-Ahead Logging (WAL): Changes written to log before data files
- Checkpointing: Periodic snapshots of database state
- Replication: Data copied to multiple servers
- fsync: Forces OS to flush data to physical disk
Quick Check
What is the primary purpose of Write-Ahead Logging (WAL)?