Transaction basics: read/write, commit/abort, ACID, transaction states

0 · Recall Warm-up

Answer from memory first. Answers are in the Answer Vault at the bottom.

1. What is the cost unit used for disk-based query cost? from Lesson 5.1.1
2. Which DBMS job undoes half-finished tasks after a crash? from Lesson 0.1.2
3. What does "atomic" mean for a task? from Lesson 0.1.1

1 · The Hook

Asha pays ₹500 of hostel fees by bank transfer. Two updates must happen: ₹500 leaves her account, ₹500 reaches the college. If the server crashes between them, the money has left one account and reached nobody. A DBMS groups such steps into a transaction and guarantees four properties, ACID, so that this can never be seen.

2 · Predict First

The transfer is: read A; A = A − 500; write A; read B; B = B + 500; write B. The system crashes right after "write A". After restart, what should the database show for A and B?

Make a guess in your head. The answer opens Section 3.

3 · Core Concept

Predict First — the answer

The original values of both (as if the transfer never started). A transaction is all or nothing: since it did not finish, its write to A must be undone. This is atomicity, enforced by the recovery manager.

Transaction

A transaction is a sequence of database operations that forms one logical unit of work. In the abstract model only two data operations matter:

• read(X) (R(X)): copy item X from the database into a local variable;
• write(X) (W(X)): copy the local variable back into item X.

Computation on local variables (A = A − 500) is invisible to the database. A transaction ends with commit (all changes become permanent) or abort (all changes are undone, also called rollback).

ACID

Transaction states

• Active: executing operations.
• Partially committed: the last operation has executed; changes may still be only in memory.
• Committed: changes are made durable (commit record safely written).
• Failed: normal execution cannot continue (error, crash, conflict).
• Aborted: rolled back to the state before it started; it may then be restarted or killed.
• Terminated: committed or aborted.

Transitions: active → partially committed → committed; active → failed; partially committed → failed; failed → aborted.

4 · Visual

Transaction states. Only a committed transaction's changes become permanent.

5 · Worked Examples

Example 1 Easy

Which ACID property is violated if a committed transfer disappears after a power cut?

Solution

Durability.

Example 2 Medium

A + B = 2000 before the transfer. During the transfer, after write(A) and before write(B), another transaction computes A + B and gets 1500. Which property failed?

Solution

Isolation: the other transaction saw an intermediate state of the transfer.

Example 3 GATE

Which ACID property is the responsibility of the application programmer rather than the DBMS?

Solution

Consistency (beyond declared constraints, only the programmer knows that a transfer must preserve A + B). Atomicity and durability are the recovery manager's; isolation is concurrency control's.

8 · Connect the Dots

9 · Self-Test

GATE format. Try every question before opening the vault.

Q1 · MCQ · 1 mark(s) — ACID

Which component ensures durability?

1. (A) Application programmer
2. (B) Concurrency control manager
3. (C) Query optimiser
4. (D) Recovery manager

Q2 · MCQ · 1 mark(s) — States

After its last operation executes but before its changes are made permanent, a transaction is:

1. (A) committed
2. (B) aborted
3. (C) terminated
4. (D) partially committed

Q3 · MSQ · 2 mark(s) — ACID

Which properties are the DBMS's responsibility (not mainly the programmer's)?

1. (A) Consistency
2. (B) Isolation
3. (C) Atomicity
4. (D) Durability

Q4 · MCQ · 1 mark(s) — Operations

In the transaction model, A = A − 500 (on a local copy) is:

1. (A) not a database operation
2. (B) a read operation
3. (C) a commit
4. (D) a write operation

Q5 · MCQ · 2 mark(s) — Atomicity

A transaction executes write(A) and then the system crashes before commit. On restart the recovery manager must:

1. (A) commit the transaction
2. (B) undo write(A)
3. (C) leave A as it is
4. (D) redo write(A)

11 · Concept Ledger

Unlocked in this lesson. From here on, these terms may be used.

transactionread(X)write(X)commitabort (rollback)ACIDatomicityconsistencyisolationdurabilitytransaction statesrecovery managerconcurrency control manager