CS 7210 Distributed Computing Exam
Practice Questions And Correct Answers
(Verified Answers) Plus Rationale 2027
Q&A| Instant Download Pdf.
1. In a distributed system consisting of multiple autonomous processes
that communicate through a network and do not share a single global
clock or shared memory, which characteristic most fundamentally
distinguishes the system from a centralized system?
A. All processes execute identical instructions at exactly the same
physical time.
B. All processes access a single globally consistent memory space.
C. The system must coordinate computation despite independent
execution, communication delays, and the possibility of partial
failures.
D. Every process must have complete knowledge of the state of every
other process.
Rationale: A distributed system is characterized by independent processes
whose coordination occurs through communication. Network delays,
concurrency, lack of a universal clock, and partial failures make
coordination fundamentally different from centralized computation.
2. Which statement most accurately describes the significance of the
absence of a global clock in a distributed system?
A. Processes cannot perform any meaningful ordering of events.
B. Processes cannot directly determine the exact global temporal
, order of all events using local physical clocks alone.
C. Every message must contain an absolute timestamp generated by
the operating system.
D. Logical clocks completely eliminate communication delays.
Rationale: Distributed processes possess local clocks that may differ in
offset and drift, while messages experience unpredictable delays. Logical-
time mechanisms therefore provide ordering information without requiring
perfectly synchronized physical clocks.
3. Lamport's happened-before relation is used primarily to capture which
property of distributed executions?
A. Exact physical execution time of every event.
B. Network bandwidth consumed by each process.
C. Causal ordering between events in a distributed computation.
D. The probability that a process will fail.
Rationale: Lamport's happened-before relation establishes a partial
ordering based on local execution order and message transmission and
reception. It captures causality rather than exact physical time.
4. Suppose event a occurs before event b in the same process, and event
b occurs before event c in another process because b sends a message
that c receives. What relationship exists between a and c under
Lamport's happened-before relation?
A. They are concurrent because they occur on different processes.
B. c happened before a.
C. No ordering can be inferred.
D. a happened before c.
Rationale: The happened-before relation is transitive. Since a precedes b
locally and b precedes c through message communication, a must precede
c in the causal ordering.
5. Two events occur on different processes, and neither event causally
precedes the other according to the happened-before relation. What
, is their relationship?
A. Sequential.
B. Synchronized.
C. Concurrent.
D. Mutually exclusive.
Rationale: Events are concurrent when neither is in the happened-before
relation with the other. Concurrency does not mean they occurred at
exactly the same physical instant; it means that no causal ordering can be
established between them.
6. A Lamport logical clock is incremented by a process before each local
event, while a message carries the sender's current timestamp. When
the receiver obtains the message, what operation is required to
preserve Lamport clock ordering?
A. Set the receiver's clock equal to zero.
B. Decrease the receiver's clock to the message timestamp.
C. Advance the receiver's clock to greater than both its current value
and the received timestamp.
D. Ignore the message timestamp because physical clocks are
sufficient.
Rationale: On receiving a message with timestamp T, a Lamport clock is
updated to at least max(local clock, T) + 1. This ensures that the send event
has a smaller logical timestamp than the corresponding receive event.
7. What is the principal limitation of Lamport logical clocks?
A. They cannot assign timestamps to local events.
B. They require synchronized physical clocks.
C. If L(a) < L(b), the timestamps do not necessarily imply that a
happened before b.
D. They cannot represent message transmission.
Rationale: Lamport clocks guarantee that a happened-before b implies L(a)
< L(b), but the converse is not guaranteed. Two concurrent events may
receive timestamps that appear ordered numerically.
, 8. Which mechanism extends logical timestamps so that they can
distinguish causal relationships more precisely than scalar Lamport
clocks?
A. Physical clock synchronization.
B. Vector clocks.
C. Round-trip-time measurement.
D. Network packet sequencing.
Rationale: Vector clocks maintain a component corresponding to each
participating process and can determine whether one event causally
precedes another or whether the events are concurrent.
9. Consider a vector-clock system with processes P1, P2, and P3. If event
a has vector [2,1,0] and event b has vector [2,2,0], what can be
concluded?
A. a and b are concurrent.
B. b happened before a.
C. a happened before b.
D. The vectors are invalid because their first components are equal.
Rationale: Vector a is component-wise less than vector b, with at least one
strict inequality. Therefore, a causally precedes b according to vector-clock
semantics.
10. What is the primary purpose of a consistent cut in a distributed
computation?
A. To identify the fastest process in the system.
B. To eliminate all network delays.
C. To represent a possible global state that does not contain a
message receive without its corresponding send.
D. To force every process to stop simultaneously.
Rationale: A consistent cut respects causality. If a receive event is included
in the cut, the corresponding send must also be included; otherwise the
represented global state would be impossible.
Practice Questions And Correct Answers
(Verified Answers) Plus Rationale 2027
Q&A| Instant Download Pdf.
1. In a distributed system consisting of multiple autonomous processes
that communicate through a network and do not share a single global
clock or shared memory, which characteristic most fundamentally
distinguishes the system from a centralized system?
A. All processes execute identical instructions at exactly the same
physical time.
B. All processes access a single globally consistent memory space.
C. The system must coordinate computation despite independent
execution, communication delays, and the possibility of partial
failures.
D. Every process must have complete knowledge of the state of every
other process.
Rationale: A distributed system is characterized by independent processes
whose coordination occurs through communication. Network delays,
concurrency, lack of a universal clock, and partial failures make
coordination fundamentally different from centralized computation.
2. Which statement most accurately describes the significance of the
absence of a global clock in a distributed system?
A. Processes cannot perform any meaningful ordering of events.
B. Processes cannot directly determine the exact global temporal
, order of all events using local physical clocks alone.
C. Every message must contain an absolute timestamp generated by
the operating system.
D. Logical clocks completely eliminate communication delays.
Rationale: Distributed processes possess local clocks that may differ in
offset and drift, while messages experience unpredictable delays. Logical-
time mechanisms therefore provide ordering information without requiring
perfectly synchronized physical clocks.
3. Lamport's happened-before relation is used primarily to capture which
property of distributed executions?
A. Exact physical execution time of every event.
B. Network bandwidth consumed by each process.
C. Causal ordering between events in a distributed computation.
D. The probability that a process will fail.
Rationale: Lamport's happened-before relation establishes a partial
ordering based on local execution order and message transmission and
reception. It captures causality rather than exact physical time.
4. Suppose event a occurs before event b in the same process, and event
b occurs before event c in another process because b sends a message
that c receives. What relationship exists between a and c under
Lamport's happened-before relation?
A. They are concurrent because they occur on different processes.
B. c happened before a.
C. No ordering can be inferred.
D. a happened before c.
Rationale: The happened-before relation is transitive. Since a precedes b
locally and b precedes c through message communication, a must precede
c in the causal ordering.
5. Two events occur on different processes, and neither event causally
precedes the other according to the happened-before relation. What
, is their relationship?
A. Sequential.
B. Synchronized.
C. Concurrent.
D. Mutually exclusive.
Rationale: Events are concurrent when neither is in the happened-before
relation with the other. Concurrency does not mean they occurred at
exactly the same physical instant; it means that no causal ordering can be
established between them.
6. A Lamport logical clock is incremented by a process before each local
event, while a message carries the sender's current timestamp. When
the receiver obtains the message, what operation is required to
preserve Lamport clock ordering?
A. Set the receiver's clock equal to zero.
B. Decrease the receiver's clock to the message timestamp.
C. Advance the receiver's clock to greater than both its current value
and the received timestamp.
D. Ignore the message timestamp because physical clocks are
sufficient.
Rationale: On receiving a message with timestamp T, a Lamport clock is
updated to at least max(local clock, T) + 1. This ensures that the send event
has a smaller logical timestamp than the corresponding receive event.
7. What is the principal limitation of Lamport logical clocks?
A. They cannot assign timestamps to local events.
B. They require synchronized physical clocks.
C. If L(a) < L(b), the timestamps do not necessarily imply that a
happened before b.
D. They cannot represent message transmission.
Rationale: Lamport clocks guarantee that a happened-before b implies L(a)
< L(b), but the converse is not guaranteed. Two concurrent events may
receive timestamps that appear ordered numerically.
, 8. Which mechanism extends logical timestamps so that they can
distinguish causal relationships more precisely than scalar Lamport
clocks?
A. Physical clock synchronization.
B. Vector clocks.
C. Round-trip-time measurement.
D. Network packet sequencing.
Rationale: Vector clocks maintain a component corresponding to each
participating process and can determine whether one event causally
precedes another or whether the events are concurrent.
9. Consider a vector-clock system with processes P1, P2, and P3. If event
a has vector [2,1,0] and event b has vector [2,2,0], what can be
concluded?
A. a and b are concurrent.
B. b happened before a.
C. a happened before b.
D. The vectors are invalid because their first components are equal.
Rationale: Vector a is component-wise less than vector b, with at least one
strict inequality. Therefore, a causally precedes b according to vector-clock
semantics.
10. What is the primary purpose of a consistent cut in a distributed
computation?
A. To identify the fastest process in the system.
B. To eliminate all network delays.
C. To represent a possible global state that does not contain a
message receive without its corresponding send.
D. To force every process to stop simultaneously.
Rationale: A consistent cut respects causality. If a receive event is included
in the cut, the corresponding send must also be included; otherwise the
represented global state would be impossible.
