--- title: "Need of multicast and consensus" subject: "Distributed System" chapter: 6 key: "semester_7/distributed_system:multicast-consensus-need" source: https://www.bctengineer.com/q/semester_7/distributed_system/multicast-consensus-need site: BCT Engineering --- # Need of multicast and consensus Distributed System · Chapter 6: Coordination and Agreement - Asked in: Model question - Video: [Distributed Consensus: Definition & Properties of Consensus, Steps & Fault-Tolerance in Consen. ALG.](https://www.youtube.com/watch?v=wMrLPtyU_dE) — Dr Nitin Naik - Aston University, UK - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/distributed_system/multicast-consensus-need - Whole chapter: https://www.bctengineer.com/md/semester_7/distributed_system/chapter-6 ## Question What is the need of multicast communication and consensus in distributed system, explain in brief. ## Short answer Multicast provides efficient group communication for replicated services, discovery, updates, and events. Consensus is required for agreeing on a single value despite failures, used in mutual exclusion, leader election, atomic commit, and total order multicast. ## Answer #### Need of multicast communication Multicast sends a message from one process to a group of processes in a single operation. It is needed for: - **Fault tolerance based on replicated services**: a request is sent to all replicas, which all execute it. - **Finding discovery services** in spontaneous networking, e.g. a client multicasts to find printers. - **Better performance through replicated data**: updates are sent to all copies, for example cached or replicated database copies. - **Propagation of event notifications**: news, stock prices, publish-subscribe systems. - **Efficiency**: one send instead of N separate messages saves bandwidth and time. Reliable and ordered (FIFO, causal, total) multicast is needed so that all members see consistent updates. #### Need of consensus Consensus lets processes agree on a single value even if some fail. It is needed for: - **Mutual exclusion**: agreeing who enters the critical section. - **Leader election**: agreeing on the new coordinator or primary. - **Atomic commit of transactions**: all participants commit or all abort. - **Replicated state machines and total order multicast**: agreeing on order of operations so that all replicas remain identical. - **Group membership** and failure decisions, e.g. who has crashed. - **Agreement in fault-tolerant control systems**, e.g. several sensors agreeing on a reading. Without consensus, replicas diverge and the system behaves inconsistently. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How is multicast used for fault tolerance? A. A message is sent to one server B. A request is sent to all replicas, which all execute it C. The clock is multicast D. Sessions are encrypted Answer: B. Replicated services receive the same request via multicast. (Key point: Replication use) 2. What is the efficiency advantage of multicast? A. It guarantees ordering automatically B. It removes failures C. One send instead of N separate messages saves bandwidth and time D. It avoids replication Answer: C. A single operation reaches the whole group. (Key point: Efficiency of multicast) 3. Why is consensus needed for atomic commit? A. To elect a printer B. To order disk blocks C. To compress logs D. All participants must agree to commit or all abort Answer: D. Without agreement some would commit while others abort. (Key point: Consensus need) 4. What happens without consensus in replicated systems? A. Replicas diverge and behave inconsistently B. Replicas run faster C. Messages double D. Clocks drift Answer: A. Agreement on operation order keeps replicas identical. (Key point: Without consensus) ## Related in Distributed System - [Need of consensus; Bully algorithm](https://www.bctengineer.com/question.md?key=semester_7%2Fdistributed_system%3Aconsensus-bully) — Distributed System, chapter 6 - [Distributed objects and communication](https://www.bctengineer.com/question.md?key=semester_7%2Fdistributed_system%3Adistributed-objects-communication) — Distributed System, chapter 2, asked 2× - [Reliable multicast properties, algorithm](https://www.bctengineer.com/question.md?key=semester_7%2Fdistributed_system%3Areliable-multicast) — Distributed System, chapter 6 - [Central coordinator election; consensus](https://www.bctengineer.com/question.md?key=semester_7%2Fdistributed_system%3Aelection-central-consensus) — Distributed System, chapter 6 - [One election technique; consensus steps](https://www.bctengineer.com/question.md?key=semester_7%2Fdistributed_system%3Aelection-technique-consensus) — Distributed System, chapter 6 - [Consensus; highly available service](https://www.bctengineer.com/question.md?key=semester_7%2Fdistributed_system%3Aconsensus-high-availability) — Distributed System, chapter 6 ## Related in other subjects - [Communication system block diagram; need for modulation](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Acomm-block-diagram-needs-modulation) — Communication Systems, chapter 1 - [Elements of optical communication system](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Aoptical-communication-elements) — Propagation and Antenna, chapter 6, asked 3× - [Digital communication system elements; Shannon-Fano coding](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Adcs-block-shannon-fano) — Communication Systems, chapter 6 - [Distributed system in big data example](https://www.bctengineer.com/question.md?key=semester_8%2Fbig_data_technologies%3Adistributed-system-example) — Big Data Technologies, chapter 1