CSSE4004 - Lecture 5

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CSSE4004 - Lecture 5

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Question 1

Question
Which of the following is false about the Repulsion forces approach in large scale coordinator election
Answer
  • each token is “repulsed” by nearby tokens (token holder sends token to another peer if too many tokens nearby)
  • token must be held by a node for a set time period before node can become superpeer
  • each node holding a token learns about other tokenholders
  • 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
  • By using election algorithms to change the coordinator
  • By using triple modular redundancy to compare the results of two coordinators
  • By emitting a heartbeat that informs other nodes when the coordinator cannot provide a lock
  • 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
  • large number of messages required
  • one slow process slows down others
  • more points of failure
  • locks may be granted unfairly

Question 4

Question
Which of the following is true about Lamport's algorithm
Answer
  • 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
  • messages carry their sending time according to the receiver's clock e.g., C(b)
  • between any two events, the clock must tick at least twice (i.e. one on the transmitting side, one on the receiving side)
  • 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
  • 6
  • 4
  • 1
  • 7 after it reboots

Question 6

Question
Which of the following is false about wireless network clock synchronisation algorithms
Answer
  • Clock synchronisation is difficult because nodes cannot always contact one another
  • Nodes are resource-constrained
  • Receivers record difference between reference message timestamp and their own clock
  • 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
  • if a and b are events in the same process, and a occurs before b, then a - > b is true
  • 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
  • a -> b and b -> c, then a -> c
  • 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
  • Network Time Protocol, The Berkeley Algorithm
  • Network Time Protocol, Town Clock Algorithm, The Berkeley Algorithm
  • UTC lock algorithm, NTP
  • Network Time Protocol, Town Clock Algorithm

Question 9

Question
Which is true about NTP?
Answer
  • Stands for Numerical Time Protocol
  • Daemon calculates new time based on average of reported values
  • In general, synchronising with stratum-k server makes you stratum-k+1
  • Stratum-1 servers typically have direct access to a reference clock

Question 10

Question
What issue is addressed by a vector clock?
Answer
  • Lamport’s algorithm does not capture causality
  • Sometimes messages are received out of order
  • Sometimes messages are created at precisely the same time
  • Sometimes events must be recorded before an event which occurred after it

Question 11

Question
Which is true about the Berkeley algorithm?
Answer
  • Uses averaging approach to correct clocks
  • Daemon tells each node how to adjust its clock
  • Servers are divided into strata reflecting the accuracy of their clocks
  • 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
  • Large-scale systems may require many local coordinators, creating the need for superpeers
  • Should offer low-latency access to other superpeers
  • Serve no more than a set number of regular nodes
  • 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
  • 6
  • 5
  • 5 or 2
  • 2

Question 14

Question
In a token ring mutual exclusion algorithm, how does a node respond if it currently holds the resource?
Answer
  • The token is passed on to the next node in the ring when it is finished accessing the shared resource
  • It sends a message in the opposite direction back to the requester stating that it holds the lock
  • It sends a message in the forward direction to the requester stating that it holds the lock
  • 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
  • It ignores the message
  • It sends a NACK response
  • It queues the message until it is finished with the resource then replies OK
  • 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
  • easy to implement
  • low message overhead
  • fair (access requests are processed in order)
  • improves performance with high message volumes
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