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DBMS/Phase 6 · Transactions & Concurrency

Checkpoint CP-6 · Item 113 of 119

Phase 6 Checkpoint (15 questions)

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15 questions · 23 marks · 45 minutes. Negative marking on MCQ only: −1/3 mark for a wrong 1-mark MCQ, −2/3 for a wrong 2-mark MCQ. No negative marking for MSQ or NAT; MSQ has no partial credit.

Q1 · MCQ · 1 mark(s) — ACID

Which ACID property is mainly the programmer's responsibility?

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

Q2 · MCQ · 1 mark(s) — Anomaly

W1(A) R2(A) A1 shows a:

  1. (A) phantom
  2. (B) unrepeatable read
  3. (C) lost update
  4. (D) dirty read

Q3 · NAT · 2 mark(s) — Counting

T1 has 4 operations and T2 has 3. How many non-serial schedules?

Numerical answer: ________

Q4 · NAT · 1 mark(s) — Topological

Vertices T1, T2, T3, T4 with edges T1 → T2 and T3 → T4. How many topological orders?

Numerical answer: ________

Q5 · MCQ · 2 mark(s) — CS

S = R1(A) W2(A) R3(A) W1(B) R2(B) W3(B). Conflict serializable?

  1. (A) Yes, T1, T2, T3
  2. (B) Yes, T2, T1, T3
  3. (C) No
  4. (D) Yes, T1, T3, T2

Q6 · MCQ · 2 mark(s) — VS

S = R1(X) W2(X) W1(X) W3(X) R4(X). Which is true?

  1. (A) VS only as T4, T3, T2, T1
  2. (B) CS
  3. (C) VS, equivalent to T1, T2, T3, T4
  4. (D) Not VS

Q7 · MCQ · 1 mark(s) — Recoverability

R1(A) W1(A) R2(A) W2(A) C2 C1 is:

  1. (A) recoverable
  2. (B) not recoverable
  3. (C) cascadeless
  4. (D) strict

Q8 · MSQ · 2 mark(s) — Nesting

Which statements are true?

  1. (A) Strict ⊂ cascadeless.
  2. (B) Serial schedules are strict.
  3. (C) Recoverable ⊂ conflict serializable.
  4. (D) Cascadeless ⊂ recoverable.

Q9 · MCQ · 1 mark(s) — Locks

How many of the following lock requests are compatible with an S lock held by another transaction: S, X?

  1. (A) 2
  2. (B) depends on 2PL
  3. (C) 0
  4. (D) 1

Q10 · MCQ · 2 mark(s) — 2PL

Which transaction follows 2PL?

  1. (A) lock-X(A) unlock(A) lock-S(B) unlock(B)
  2. (B) lock-S(A) lock-X(B) unlock(A) unlock(B)
  3. (C) lock-X(B) unlock(B) lock-X(A) unlock(A)
  4. (D) lock-S(A) unlock(A) lock-S(A) unlock(A)

Q11 · MCQ · 1 mark(s) — Variants

Which protocol prevents cascading rollbacks?

  1. (A) Conservative 2PL alone
  2. (B) Basic timestamp ordering
  3. (C) Strict 2PL
  4. (D) Basic 2PL

Q12 · NAT · 2 mark(s) — Wait-die

Wait-die with TS(T1) = 3, TS(T2) = 6, TS(T3) = 9. Requests (each considered alone): T1 asks for an item held by T3; T3 asks for an item held by T2; T2 asks for an item held by T1. How many requests cause a rollback?

Numerical answer: ________

Q13 · NAT · 2 mark(s) — TO

TS(T1) = 1, TS(T2) = 2. Basic TO on R1(A) R2(A) W2(A) W1(A) R2(B). How many operations execute successfully?

Numerical answer: ________

Q14 · MCQ · 1 mark(s) — Granularity

Which lock lets a transaction read a whole table while updating some of its rows?

  1. (A) IS
  2. (B) SIX
  3. (C) IX
  4. (D) S

Q15 · MSQ · 2 mark(s) — Recovery

Immediate update. Log: ⟨T1 start⟩ ⟨T1, A, 4, 5⟩ ⟨T2 start⟩ ⟨T2, B, 1, 2⟩ ⟨checkpoint⟩ ⟨T2 commit⟩ ⟨T3 start⟩ ⟨T3, A, 5, 9⟩ — crash. Which are needed?

  1. (A) Redo T2
  2. (B) Undo T3
  3. (C) Undo T1
  4. (D) Redo T1

✓Answer Vault

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Q1 Answer: (D) · 1 mark(s)

Program logic.

  • (A) is wrong: Recovery manager.
  • (B) is wrong: Recovery manager.
  • (C) is wrong: Concurrency control.

Re-read: Lesson 6.1.1 — Transaction basics: read/write, commit/abort, ACID, transaction states

Q2 Answer: (D) · 1 mark(s)

Read of uncommitted data that was rolled back.

  • (A) is wrong: No.
  • (B) is wrong: No.
  • (C) is wrong: No double write.

Re-read: Lesson 6.2.1 — Concurrency problems: lost update, dirty read, unrepeatable read, phantom

Q3 Answer: 33 · 2 mark(s)

7!/(4!3!) = 35; minus 2.

Re-read: Lesson 6.3.1 — Schedules: serial vs concurrent; counting possible schedules

Q4 Answer: 6 · 1 mark(s)

24/4.

Re-read: Lesson 6.4.1 — Directed graph primer: cycles, topological order

Q5 Answer: (A) · 2 mark(s)

R1(A)–W2(A): T1 → T2; W2(A)–R3(A): T2 → T3; W1(B)–R2(B): T1 → T2; W1(B)–W3(B): T1 → T3; R2(B)–W3(B): T2 → T3. Order T1, T2, T3.

  • (B) is wrong: T1 → T2 exists.
  • (C) is wrong: Acyclic.
  • (D) is wrong: T2 → T3 exists.

Re-read: Lesson 6.4.2 — Conflict serializability: conflicting ops, precedence graph, equivalent serial orders

Q6 Answer: (C) · 2 mark(s)

T1 reads initial X; T4 reads from T3; final writer T3. Serial T1, T2, T3, T4 matches. Not CS (T1 ⇄ T2 cycle).

  • (A) is wrong: T4 must read from T3.
  • (B) is wrong: Cycle between T1 and T2.
  • (D) is wrong: It is VS.

Re-read: Lesson 6.4.3 — View serializability; blind writes; relation to conflict serializability

Q7 Answer: (B) · 1 mark(s)

T2 reads from T1 and commits first.

  • (A) is wrong: No.
  • (C) is wrong: No.
  • (D) is wrong: No.

Re-read: Lesson 6.5.1 — Recoverable, cascadeless and strict schedules; how they nest

Q8 Answer: (A) (B) (D) · 2 mark(s)

Recoverability and serializability are independent.

  • (C) is wrong: Independent.

Re-read: Lesson 6.5.1 — Recoverable, cascadeless and strict schedules; how they nest

Q9 Answer: (D) · 1 mark(s)

Only S.

  • (A) is wrong: X is not.
  • (B) is wrong: Compatibility does not depend on 2PL.
  • (C) is wrong: S is compatible.

Re-read: Lesson 6.6.1 — Shared/exclusive locks; compatibility matrix

Q10 Answer: (B) · 2 mark(s)

All locks before the first unlock.

  • (A) is wrong: Locks after unlocking.
  • (C) is wrong: Locks after unlocking.
  • (D) is wrong: Locks after unlocking.

Re-read: Lesson 6.6.2 — Two-phase locking: growing/shrinking, lock point, guarantees

Q11 Answer: (C) · 1 mark(s)

X locks held until commit.

  • (A) is wrong: Can release early.
  • (B) is wrong: Not recoverable automatically.
  • (D) is wrong: Can cascade.

Re-read: Lesson 6.6.3 — Strict, rigorous and conservative 2PL

Q12 Answer: 2 · 2 mark(s)

T1 older than T3: waits. T3 younger than T2: dies. T2 younger than T1: dies.

Re-read: Lesson 6.6.4 — Deadlocks: wait-for graph, wait-die, wound-wait, starvation

Q13 Answer: 4 · 2 mark(s)

R1(A), R2(A), W2(A) ok; W1(A): 1 < RTS 2 → T1 aborts; R2(B) ok.

Re-read: Lesson 6.7.1 — Timestamp ordering protocol; Thomas write rule

Q14 Answer: (B) · 1 mark(s)

S on the table plus intention to write below.

  • (A) is wrong: Only reads below.
  • (C) is wrong: Does not read the whole table.
  • (D) is wrong: Cannot update rows.

Re-read: Lesson 6.8.1 — Multiple granularity, validation-based protocol, log-based recovery

Q15 Answer: (A) (B) (C) · 2 mark(s)

T1 never committed: undo. T2 committed after the checkpoint: redo. T3 never committed: undo.

  • (D) is wrong: T1 did not commit.

Re-read: Lesson 6.8.1 — Multiple granularity, validation-based protocol, log-based recovery

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