2900649
Question 1
Question
Which of the following is false about the Repulsion forces approach in large scale coordinator election
Answer
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each token is “repulsed” by nearby tokens (token holder sends token to another peer if too many tokens nearby)
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token must be held by a node for a set time period before node can become superpeer
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each node holding a token learns about other tokenholders
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a superpeer which receives another token sends it to the lowest prime numbered neighbour
Question 2
Question
How can a coordinator failure be addressed in a centralised mutual exclusion system?
Answer
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By using election algorithms to change the coordinator
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By using triple modular redundancy to compare the results of two coordinators
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By emitting a heartbeat that informs other nodes when the coordinator cannot provide a lock
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By moving the coordinator periodically to the next node in the token ring
Question 3
Question
Which of these is not a drawback of a typical distributed mutual exclusion algorithm?
Answer
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large number of messages required
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one slow process slows down others
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more points of failure
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locks may be granted unfairly
Question 4
Question
Which of the following is true about Lamport's algorithm
Answer
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when a message is received, its time is compared against the local clock. If the local clock is less than C(b), it is set to C(b) + 1
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messages carry their sending time according to the receiver's clock e.g., C(b)
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between any two events, the clock must tick at least twice (i.e. one on the transmitting side, one on the receiving side)
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Events occurring in processes that do not interact (even indirectly through third parties) are said to be concurrent
Question 5
Question
In the bully algorithm with nodes 1,2,3,4,5,6,7 if 7 is the coordinator and 4 notices the coordinator has crashed, who will be the new coordinator?
Answer
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6
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4
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1
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7 after it reboots
Question 6
Question
Which of the following is false about wireless network clock synchronisation algorithms
Answer
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Clock synchronisation is difficult because nodes cannot always contact one another
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Nodes are resource-constrained
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Receivers record difference between reference message timestamp and their own clock
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The coordinating node must transmit at a higher power level to ensure all nodes can receive clock broadcasts
Question 7
Question
Which of the following is false about Lamport's algorithm?
Answer
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if a and b are events in the same process, and a occurs before b, then a - > b is true
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if a is the event of a message being sent by one process, and b is the receipt of that message in another process, then a ->b is also true
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a -> b and b -> c, then a -> c
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if a is the event of a message being sent by one process, and b is the receipt of that message in another process, then b -> a is also true
Question 8
Question
Which of these are clock synchronisation protocols
Answer
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Network Time Protocol, The Berkeley Algorithm
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Network Time Protocol, Town Clock Algorithm, The Berkeley Algorithm
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UTC lock algorithm, NTP
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Network Time Protocol, Town Clock Algorithm
Question 9
Question
Which is true about NTP?
Answer
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Stands for Numerical Time Protocol
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Daemon calculates new time based on average of reported values
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In general, synchronising with stratum-k server makes you stratum-k+1
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Stratum-1 servers typically have direct access to a reference clock
Question 10
Question
What issue is addressed by a vector clock?
Answer
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Lamport’s algorithm does not capture causality
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Sometimes messages are received out of order
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Sometimes messages are created at precisely the same time
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Sometimes events must be recorded before an event which occurred after it
Question 11
Question
Which is true about the Berkeley algorithm?
Answer
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Uses averaging approach to correct clocks
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Daemon tells each node how to adjust its clock
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Servers are divided into strata reflecting the accuracy of their clocks
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Lower stratum servers are assumed to be more accurate => A will try to synchronise with B’s clock
Question 12
Question
Which of the following is false about superpeers
Answer
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Large-scale systems may require many local coordinators, creating the need for superpeers
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Should offer low-latency access to other superpeers
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Serve no more than a set number of regular nodes
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Can be selected by reserving a fraction of the identifier space in a distributed hash table containing all nodes
Question 13
Question
In the ring election algorithm with nodes 1,2,3,4,5,6,7 if 7 is the coordinator and 5 and 2 both notice which node becomes the new coordinator?
Answer
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6
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5
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5 or 2
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2
Question 14
Question
In a token ring mutual exclusion algorithm, how does a node respond if it currently holds the resource?
Answer
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The token is passed on to the next node in the ring when it is finished accessing the shared resource
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It sends a message in the opposite direction back to the requester stating that it holds the lock
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It sends a message in the forward direction to the requester stating that it holds the lock
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The previous node sends a message back to the requester when it doesn't get a response from the node holding the resource
Question 15
Question
In a regular distributed mutual exclusion system, how does a node reply if it currently holds the resource?
Answer
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It ignores the message
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It sends a NACK response
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It queues the message until it is finished with the resource then replies OK
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It replies to the next node in the token ring
Question 16
Question
Which of these is not an advantage of centralised mutual exclusion?
Answer
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easy to implement
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low message overhead
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fair (access requests are processed in order)
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improves performance with high message volumes
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