--- title: "DS as a single coherent system" subject: "Distributed System" chapter: 1 key: "semester_7/distributed_system:single-coherent-system" source: https://www.bctengineer.com/q/semester_7/distributed_system/single-coherent-system site: BCT Engineering --- # DS as a single coherent system Distributed System · Chapter 1: Introduction - Asked in: 2071 Chaitra, 6+2 marks - Video: [Distributed Systems 01: Definitions, Key Features, and the 8 Types of Transparency (Sinhala)](https://www.youtube.com/watch?v=-k7u0WNA6Jg) — Annotations & Discussions - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/distributed_system/single-coherent-system - Whole chapter: https://www.bctengineer.com/md/semester_7/distributed_system/chapter-1 ## Question "Distributed system acts as a single coherent system to its end user." Justify the statement with its features and challenges. What is fundamental model? ## Short answer A distributed system hides its independent nodes behind middleware, giving one interface through transparency, resource sharing, consistency, fault tolerance, and scalability. Challenges include latency, partial failures, replica consistency, heterogeneity, security, and no global clock. Fundamental models are interaction, failure, and security models. ## Answer #### Justification A **distributed system** is a collection of independent computers, connected by a network and running distribution middleware, that appears to its users as a single coherent system. Its components communicate and coordinate their actions only by passing messages (Tanenbaum; Coulouris). The statement means that, although the system is built from many independent machines, the user and the application see one computer. This is achieved through the following features. **Features that give a single view** - **Transparency:** the user does not see where files are, which server answers, how many replicas exist, or when a node fails. - **Common middleware and uniform interface:** the same API or protocol is used for all resources (for example a web browser). - **Resource sharing** through a single name space (one file tree, one login). - **Concurrency control and consistency:** many users work at once without seeing interference. - **Fault tolerance:** failures are masked, so the service seems continuous. - **Scalability and openness:** the system can grow while the view stays the same. Example: when a user uses Google Drive, the files appear on one drive though they are stored and replicated over many data centres. **Challenges** - Network latency and partial failures cannot be fully hidden. - Keeping replicas consistent and controlling concurrent access. - Heterogeneity of hardware, OS and data formats. - Security and trust over an open network. - Scalability without bottlenecks. - No global clock for ordering events. #### Fundamental model A fundamental model is an abstract description of the essential properties that all distributed systems share. It is used to reason about the design, to state what is and is not possible, and to prove correctness without implementation details. There are three: - **Interaction model:** processes interact by messages, with communication delay, no global clock and clock drift. It is *synchronous* (known bounds on execution time, message delay, drift) or *asynchronous* (no bounds). - **Failure model:** classifies faults as *omission* (process or channel fails to act), *arbitrary/Byzantine* (any behaviour) and *timing* (response outside the time interval). - **Security model:** assumes an adversary who can send or read messages and so threatens processes and channels; defends with authentication, encryption and access control. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What makes many independent computers appear as one system to users? A. Transparency and a common middleware with uniform interface B. Exposing every server address to the user C. Using different interfaces for each resource D. Removing the network Answer: A. Transparency, middleware and single name space give the single-system view. (Key point: Single view) 2. What is a fundamental model? A. A hardware wiring diagram B. An abstract description of the essential properties common to all distributed systems, used to reason and prove correctness C. A specific implementation of one product D. A list of all vendors Answer: B. Fundamental models abstract essential properties without implementation details. (Key point: Fundamental model) 3. What is the difference between synchronous and asynchronous interaction models? A. Synchronous has no bounds; asynchronous has known bounds B. They differ only in the programming language C. Synchronous has known bounds on execution time, message delay and clock drift; asynchronous has none D. Both have known bounds Answer: C. Synchronous systems assume bounds; asynchronous (like the Internet) do not. (Key point: Interaction model) 4. Which failure classes make up the failure model? A. Hardware, software and human failures B. Read, write and execute failures C. Security, interaction and omission failures D. Omission, arbitrary (Byzantine) and timing failures Answer: D. The failure model classifies faults as omission, arbitrary/Byzantine and timing. (Key point: Failure model) ## Close variants set in other exams - [Single coherent system justification](https://www.bctengineer.com/question.md?key=semester_7%2Fdistributed_system%3Asingle-coherent-features) — Distributed System, chapter 1, asked 2× ## Related in Distributed System - [Goals and design challenges of DS](https://www.bctengineer.com/question.md?key=semester_7%2Fdistributed_system%3Ads-goals-challenges) — Distributed System, chapter 1, asked 3× - [Challenges of DS with example](https://www.bctengineer.com/question.md?key=semester_7%2Fdistributed_system%3Ads-challenges-example) — Distributed System, chapter 1 - [DS properties and interaction model](https://www.bctengineer.com/question.md?key=semester_7%2Fdistributed_system%3Ads-properties-interaction-model) — Distributed System, chapter 1 - [Transparency: importance and challenges](https://www.bctengineer.com/question.md?key=semester_7%2Fdistributed_system%3Atransparency-importance-challenges) — Distributed System, chapter 1 ## Related in other subjects - [Features of end-fire array](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Aendfire-array-features) — Propagation and Antenna, chapter 2 - [Single-phase diode bridge, average and fundamental](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_electronics%3Adiode-bridge-220v-fundamental) — Power Electronics, chapter 2 - [Single-phase inverter fundamental output voltage](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_electronics%3Asingle-phase-inverter-r-fundamental) — Power Electronics, chapter 5 - [Single-phase square-wave inverter rms and fundamental](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_electronics%3Asquare-wave-inverter-rms-fundamental) — Power Electronics, chapter 5