Transactions ensure data integrity when multiple operations must succeed or fail together. Understanding ACID properties and isolation levels is essential for reliable applications.
ACID Properties#
Atomicity
- All operations succeed or all fail
- No partial updates
Consistency
- Database moves from valid state to valid state
- Constraints are maintained
Isolation
- Concurrent transactions don't interfere
- Each transaction sees consistent data
Durability
- Committed changes persist
- Survives system failures
Basic Transactions#
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Isolation Levels#
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Isolation Level Problems#
Dirty Read:
- Reading uncommitted changes
- Prevented by: Read Committed and above
Non-Repeatable Read:
- Same query returns different results
- Prevented by: Repeatable Read and above
Phantom Read:
- New rows appear between queries
- Prevented by: Serializable
| Level | Dirty Read | Non-Repeatable | Phantom |
|-----------------|------------|----------------|---------|
| Read Uncommitted| Yes | Yes | Yes |
| Read Committed | No | Yes | Yes |
| Repeatable Read | No | No | Yes |
| Serializable | No | No | No |
Optimistic vs Pessimistic Locking#
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Deadlock Prevention#
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Savepoints#
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Distributed Transactions#
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Best Practices#
DO:
✓ Keep transactions short
✓ Lock in consistent order
✓ Use appropriate isolation level
✓ Handle deadlocks with retry
✓ Use optimistic locking for low-contention
DON'T:
✗ Hold transactions during external calls
✗ Use serializable for everything
✗ Ignore transaction timeouts
✗ Nest transactions unnecessarily
Conclusion#
Transactions are your safety net for data integrity. Choose the right isolation level, prevent deadlocks through consistent ordering, and keep transactions as short as possible.
For distributed systems, consider saga patterns instead of distributed transactions.