COMP212 Distributed Systems Exam 2025

Distributed System - Correct Ans-Collection of independent computers that appears to its users as a single coherent system Main goals of a distributed System - Correct Ans-1. Easy connection between users/resources 2. Transparency 3. Openness 4. Reliability 5. Performance 6. Scalability Transparent Distributed system - Correct Ans-A distributed system that appears to its users as if it were only a single computer system Scalable Distributed System - Correct Ans-One that is capable of growing in size, the maximum distance between its participants (geographical), and the number of administrative domains in it Processors - Correct Ans-Computing entities. Usually execute the same algorithm Networks - Correct Ans-Connects processors. EX. ring, tree, complete network Processors communicate by interchanging messages through the links of the network What should happen in a round? - Correct Ans-1. read incoming messages 2. Update states 3. Generate new messages, put them in transit 4. messages should be transmitted (Could also come first) What are some distributed tasks/problems? - Correct Ans-Message sending, leader election Distributed algorithm - Correct Ans-1. executed by all processors 2. May have some input (ids, other info)and have some initial state 3. Every processor reads incoming messages/updates states/generates out going messages 4. Network delivers the messages 5. Next round begins 6. Processors produce some output Informal description (of an algorithm) - Correct Ans-Gives the main idea in verbal form Must be clear and concise to provide an accurate highlight/sketch of how the algorithm works Psuedocode - Correct Ans-Informal code A piece of code to be re-executed by all processors in every round What should be taken into consideration when devising an algorithm? - Correct Ans-1. Convince that it is correct 2. Analyze its performance Correctness - Correct Ans-Proof that the algorithm does as expected Performance - Correct Ans-Measured by 3 things: 1. Time Complexity 2. Space complexity 3. Communication Complexity Time complexity - Correct Ans-measures the time it takes for an algorithm to execute (ex # rounds required) Space complexity - Correct Ans-A measurement of how much memory is used by the processors during the runtime of the algorithms Communication Complexity - Correct Ans-A measurement of message size and quantities during the execution of an algorithm How is a spanning tree of a network given? - Correct Ans-• Network G = (V, E) • Eʹ ⊆ E specifies a spanning tree T = (V, Eʹ) • Root: u0 (leader) • Processors know T in a distributed way • Each u "i" knows: a parent "i", a set children Time complexity of spanning tree broadcasting - Correct Ans-Depth d of T Communication complexity of spanning tree broadcasting - Correct Ans-n - 1 messages Time complexity of spanning tree construction - Correct Ans-O(D) where D is the maximum distance of a u "i" from u "0" in G Communication complexity of spanning tree construction - Correct Ans--size of messages: sends message M and an id -O(m) messages: where m denotes the # of edges of G Does leader election work on a directed ring with identical processors? - Correct Ans-No Does leader election work on a directed ring with unique id processes? - Correct Ans-Yes LCR Algorithm - Correct Ans-Used when we know the unique ids of a network. Uses only transmission and comparison of ids to select a leader Time complexity of LCR - Correct Ans-N rounds Communication complexity of LCR - Correct Ans-- size of messages: encoding in bits of the maximum id -O(n^2) messages in worst case Strongly connected graph - Correct Ans-A directed graph is strongly connected if there is a path from every node to every other node. Floodmax Algorithm - Correct Ans-Used when we know the unique ids and diameter of the network