---
title: "Single coherent system justification"
subject: "Distributed System"
chapter: 1
key: "semester_7/distributed_system:single-coherent-features"
source: https://www.bctengineer.com/q/semester_7/distributed_system/single-coherent-features
site: BCT Engineering
---

# Single coherent system justification

Distributed System · Chapter 1: Introduction

- Asked in: Model question
- 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-features
- 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 examples.

## Short answer

The system hides independent computers and network behind middleware, giving a single view through transparency, resource sharing, consistency, fault tolerance, and scalability. Examples include the Web, Google Drive, NFS, and online banking, where users see one coherent service despite distributed storage and replication.

## 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).

This statement means the many independent computers, the network and the replication behind it are hidden so that users and programs see a single, consistent computer.

**Features that support this view**
1. **Transparency (access, location, migration, replication, concurrency, failure):** the user uses one name space and one interface.
2. **Middleware:** a common layer between applications and the heterogeneous OS and network gives a uniform programming model.
3. **Resource sharing:** all authorised users share files, printers and services as in one system.
4. **Consistency and concurrency control:** concurrent users see a consistent state.
5. **Fault tolerance:** replicas and recovery hide crashes, so the service seems always available.
6. **Scalability and openness:** machines can be added without a change in the user's view.

**Examples**
- **World Wide Web:** a user types a URL and sees a page without knowing the server, its location or caches/mirrors behind it.
- **Google Drive / Gmail:** data is spread and replicated over data centres, but looks like one account.
- **NFS:** remote directories appear as part of the local file tree.
- **Online banking:** an account can be used from any branch or ATM as if it were one bank computer.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What does 'single coherent system' mean to a user?
   A. The many computers, network and replication are hidden, so one consistent computer is seen
   B. The user must select a server manually
   C. The user sees every replica
   D. The system has only one computer
   Answer: A. Hiding the distribution gives the single-system appearance. (Key point: Single view)

2. What role does middleware play in giving a single view?
   A. It removes the need for applications
   B. A common layer gives a uniform programming model over heterogeneous OS and networks
   C. It makes hardware identical
   D. It stores user passwords only
   Answer: B. Middleware sits between applications and heterogeneous platforms. (Key point: Middleware)

3. How does fault tolerance support the single-system view?
   A. Crashes are shown to the user
   B. Crashes stop the service
   C. Replicas and recovery hide crashes, so the service seems always available
   D. Replicas are disabled
   Answer: C. Masking failures makes the service look continuous. (Key point: Fault tolerance)

4. Which example shows a single coherent view?
   A. Google Drive files appear in one account though replicated over many data centres
   B. A laptop running one offline program
   C. A user choosing among data centres by hand
   D. A printer with no network
   Answer: A. The user sees one account while data is stored and replicated across data centres. (Key point: Example)

## Close variants set in other exams

- [DS as a single coherent system](https://www.bctengineer.com/question.md?key=semester_7%2Fdistributed_system%3Asingle-coherent-system) — Distributed System, chapter 1, asked 2×

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- [System administration; special vs general user](https://www.bctengineer.com/question.md?key=semester_6%2Foperating_system%3Asysadmin-special-vs-general-user) — Operating System, chapter 9
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