$Isolation

Isolation is the "I" in ACID - it governs how concurrent transactions interact with each other in a database.

Nov 16, 2025

Isolation ensures that concurrent transactions don’t interfere with each other. Each transaction should execute as if it’s the only one running on the database.

Why it matters: Without isolation, system will face problems ike:

• Dirty reads: reading uncommitted data from another transaction
• Non repeatable reads: same query returns different results within one transaction
• Phantom reads: new rows appear between queries in the same transaction

Imagine you have £1000 in your account. Two transactions happen simultaneously:

• Transaction A: You withdraw £100 at an ATM
• Transaction B: Your paycheck of £2000 deposits

Without proper isolation, Transaction B might read your balance (£1000) while Transaction A is updating it, leading to lost updates. With isolation, each transaction completes independently.

Isolation levels (from weakest to strongest):

• Read Uncommitted: No isolation, allows dirty reads
• Read Committed: Can’t read uncommitted data (most common default), but allows non-repeatable reads
• Repeatable Read: Same reads return same results, but allows new rows
• Serializable: Complete isolation, as if transactions ran sequentially

Higher isolation = better consistency but lower performance. Most databases default to Read Committed as a practical middle ground.

Further Reading

• Designing Data-Intensive Applications