Distributed Systems
CS3.401Prof. Kishore Kothapalli•Monsoon 2025-26•4 credits
Mock Paper A — Foundations & Synchronisation
Duration: 180 min • Max marks: 100
Section A — Short Answers (4 × 5 = 20)
20 marks- 1.State Tanenbaum's definition of a distributed system. List five required features.5 m
- 2.State CAP and explain why CA is impossible in a real distributed system.5 m
- 3.Why is the edge perpendicular to the gradient — wait, scratch that. Why does Chandy-Lamport require FIFO channels?5 m
- 4.Distinguish consistent vs strongly consistent logical clocks.5 m
Section B — Algorithm Traces (3 × 10 = 30)
30 marks- 1.Trace vector clocks for the following execution: P1 sends m1 to P2 at e1, P2 sends m2 to P3 at e2 (after receiving m1), P3 receives m2. List timestamps at every event.10 m
- 2.Trace Chandy-Lamport on 3 processes P1, P2, P3 with FIFO bidirectional channels. Assume P1 initiates and one message ($50) is in transit C₂₃.10 m
- 3.Run Lamport's DME for 2 sites with simultaneous requests at timestamps (2,1) and (2,2). Show the queue at each site and explain who enters CS first.10 m
Section C — Long Answers (2 × 15 = 30)
30 marks- 1.Compare the five DME algorithms (Lamport, Ricart-Agrawala, Maekawa, Suzuki-Kasami, Raymond) on message complexity, sync delay, and assumptions.15 m
- 2.Explain how 3PC avoids the blocking problem of 2PC. State the assumptions and limitations of 3PC.15 m
Section D — Numerical / Application (2 × 10 = 20)
20 marks- 1.Crash-consensus runs (f+1)·n² messages. For n = 8 and f = 3 crashes, give the message count and round count.10 m
- 2.Byzantine OM(1) with N=4. Show one scenario where general is faulty (sends 0 to L1, L2 and 1 to L3) and verify loyal lieutenants agree.10 m
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