The big idea: A transaction groups several statements into one indivisible unit: either every one takes effect, or none does.
A bank transfer is two updates. Halfway through is a state the world must never see.
BEGIN TRANSACTION;
UPDATE Account SET balance = balance - 500 WHERE id = 'A';
UPDATE Account SET balance = balance + 500 WHERE id = 'B';
COMMIT; -- both take effect, permanently
-- ROLLBACK; -- neither doesWithout it, money is destroyed: If the system fails between the two updates, £500 has left A and never reached B. No error is reported — the first statement succeeded.
The transaction is what makes that outcome impossible rather than merely unlikely.
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| Property | Guarantees | Without it |
|---|---|---|
| Atomicity | All statements or none | Money leaves one account and reaches no other |
| Consistency | Every rule still holds afterwards | A loan row pointing at a deleted customer |
| Isolation | Concurrent transactions do not interfere | Two withdrawals both read the same balance |
| Durability | A committed change survives a crash | A confirmed payment vanishes on reboot |
Isolation is the one worth understanding: Two ATMs read a balance of £100 at the same time. Each checks that £80 is available, each approves, each writes £20 back.
£160 has been withdrawn from £100. Isolation is what makes the second transaction wait until the first has finished — and it is the property exam questions test most.
Durability means written, not remembered: Once COMMIT returns, the change has reached non-volatile storage. Pulling the power a moment later cannot undo it.
That is why a commit is slower than an ordinary write — it must wait for the disc.
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How isolation is achieved
- A transaction takes a lock on the rows it touches
- Another transaction wanting them waits until the first commits or rolls back
- That is what serialises the two ATM withdrawals
- Deadlock: A holds row 1 and wants row 2; B holds row 2 and wants row 1 — neither can proceed
- The database detects this and aborts one, which then retries
Locking costs throughput: Stronger isolation means more waiting, so fewer transactions per second. Some systems deliberately relax it where the risk is acceptable.
Naming that cost — rather than presenting ACID as free — is what a full-mark answer does.
How this is tested — you must trace what goes wrong without a transaction, and name the ACID property that prevents it. It comes up two ways:
Paper 1 Section A
- Describe how transactions maintain integrity, 3-5 marks
- State what ACID stands for
- Explain COMMIT and ROLLBACK
Paper 1 Section B — case study
- Explain a concurrency failure in a scenario
- Identify which property is being relied on
The classic trap: Listing ACID without applying it. The marks are for saying which property prevents the specific failure in the question — and what would happen without it.
IB-style questionExplain[6 marks]
Two people withdraw from the same account of £100 at two cash machines at the same instant, each requesting £80. Explain what could go wrong without transactions, and how ACID prevents it.
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