--- title: "Why NOS preferred over DOS" subject: "Distributed System" chapter: 3 key: "semester_7/distributed_system:nos-vs-dos-realization" source: https://www.bctengineer.com/q/semester_7/distributed_system/nos-vs-dos-realization site: BCT Engineering --- # Why NOS preferred over DOS Distributed System · Chapter 3: Operating System Support - Asked in: 2069 Chaitra, 4+4 marks; Model question (asked 2 times) - Video: [Distributed Operating System | Goals | Features](https://www.youtube.com/watch?v=NYBKXzl5bWU) — making IT simple - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/distributed_system/nos-vs-dos-realization - Whole chapter: https://www.bctengineer.com/md/semester_7/distributed_system/chapter-3 ## Question Why is network operating system (NOS) widely preferred over distributed operating system (DOS) in practical distributed systems? Explain DOS as a middleware. (Even though a distributed operating system is needed in a distributed system, it is not widely popular in use, why? What and how a DOS is realized in a practical distributed system?) ## Short answer Network OS is preferred because it works on heterogeneous, autonomous machines with existing operating systems, offering simplicity and scalability. A distributed OS is impractical; instead a middleware layer (e.g., CORBA, Java RMI) runs on top of network OS, providing distribution transparency, communication, naming, and other services, thus realizing a single system image. ## Answer #### Network OS versus distributed OS - A **network operating system (NOS)** gives each machine its own OS, with added network services (remote login, file transfer, remote file access). Users know that there are several machines and where resources are. Examples: Windows or UNIX with NFS. - A **distributed operating system (DOS)** manages all machines as one and gives a single system image. Examples: Amoeba, Mach. | Point | NOS | DOS | |---|---|---| | Transparency | Low; user knows the machines | High; single system view | | Autonomy | High, each node independent | Low; global control | | Scalability, openness | Good | Poor | | Fault tolerance | Weak | Better in design | | OS | Different OS allowed (heterogeneous) | Same OS on all nodes | | Complexity | Low | High | #### Why NOS is preferred over DOS in practice 1. Users and organisations already have computers with their own OS (Windows, Linux), and do not want to replace them. A DOS needs the same OS on all nodes (homogeneous). 2. A NOS is simple, cheap and works on existing hardware, software and applications. 3. Nodes want to be autonomous (own users, own policy), which a DOS does not give. 4. Scalability of a DOS to large, wide-area, heterogeneous networks (the Internet) is poor. 5. Developers lack applications, tools and support for DOS, and global resource management is difficult. 6. Administrative boundaries and security make a single global OS unrealistic. #### DOS as middleware In practice the desired single-system view is realised not as a new OS but as a **middleware layer** over network operating systems on every node. The middleware (CORBA, Java RMI, web services, cloud frameworks) runs on top of the local OS and gives distribution transparency, communication (RPC/RMI), naming, security, replication and transactions. ```text App A App B --------------------------- Middleware (distributed OS services) --------------------------- Local OS 1 | Local OS 2 | Local OS 3 (NOS) --------------------------- Network ``` The local kernels manage local resources (processes, memory, devices), and the middleware coordinates global matters (naming, RPC, distributed file system, scheduling). So a DOS is realised as network OS plus middleware: heterogeneous and autonomous, but offering a uniform view. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which reason best explains why a DOS is rarely used in practice? A. It needs the same OS on all nodes and gives poor scalability and autonomy B. It cannot hide the location of resources C. It is cheaper than a NOS D. It lets every node keep its own OS Answer: A. A DOS needs homogeneous nodes, removes node autonomy and scales poorly on wide-area heterogeneous networks. (Key point: Why NOS preferred) 2. How is the single-system view realised in practice? A. By replacing every local OS with one new kernel B. By a middleware layer on top of the network OS on each node C. By removing the network D. By running all applications on one machine Answer: B. Middleware (CORBA, RMI, web services) runs over local OSs and provides the transparency a DOS would. (Key point: DOS as middleware) 3. Which statement about NOS versus DOS is correct? A. NOS has low autonomy and DOS has high autonomy B. NOS requires the same OS on all nodes C. DOS has low autonomy and high transparency; NOS allows different OSs D. DOS is simpler to build than NOS Answer: C. NOS nodes are autonomous and heterogeneous with low transparency; DOS offers high transparency but low autonomy. (Key point: NOS vs DOS table) 4. In the middleware realisation, what do the local kernels manage? A. Naming and RPC for the whole system B. Global scheduling C. Local resources such as processes, memory and devices D. Distributed file system Answer: C. Local kernels handle local resources; middleware coordinates global matters like naming, RPC and distributed files. (Key point: Division of work) ## Related in Distributed System - [True distributed system; middleware organisation](https://www.bctengineer.com/question.md?key=semester_7%2Fdistributed_system%3Atrue-ds-middleware) — Distributed System, chapter 3 - [Heterogeneous model; DOS realization](https://www.bctengineer.com/question.md?key=semester_7%2Fdistributed_system%3Ahetero-model-dos-realized) — Distributed System, chapter 3 - [Network OS vs distributed OS](https://www.bctengineer.com/question.md?key=semester_7%2Fdistributed_system%3Anos-vs-dos-diff) — Distributed System, chapter 3 - [DOS; monolithic and microkernel](https://www.bctengineer.com/question.md?key=semester_7%2Fdistributed_system%3Ados-monolithic-microkernel) — Distributed System, chapter 3 - [Context switching; distributed vs network OS](https://www.bctengineer.com/question.md?key=semester_7%2Fdistributed_system%3Acontext-switch-dos) — Distributed System, chapter 3 - [Operating system layers](https://www.bctengineer.com/question.md?key=semester_7%2Fdistributed_system%3Aos-layers) — Distributed System, chapter 3 ## Related in other subjects - [(4,3) even parity code is linear, odd parity is not](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Aeven-odd-parity-linear-block) — Communication Systems, chapter 8 - [pipe(); semaphore in distributed systems](https://www.bctengineer.com/question.md?key=semester_6%2Foperating_system%3Apipe-semaphore-distributed) — Operating System, chapter 3 - [ISI in baseband systems; ideal and practical solutions](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Aisi-ideal-practical-solutions) — Communication Systems, chapter 6, asked 2× - [Why serial transfer is preferred over parallel](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Aserial-vs-parallel-preferred) — Instrumentation II, chapter 3