The software concept of the distributed system is referred to as which platform we select as an operating system.

As we know that an operating system is nothing but the interface between the user and the hardware.

There are three largely used operating system types:

1) Distributed operating system

2) Network operating system

3) Middleware operating system

1. Distributed operating system:

It is different from multiprocessor and multi-computer hardware.

Multiprocessors work by using different operating system services. This manages resources connected in a system and use system calls to communicate with the processor.

In case of multi-computer, the distributed Operating system uses a separate uni-processor OS on each computer for communicating between different computers e.g. window.

In distributed OS, a common set of services is shared among multiple processors in such a way that they execute a distributed application effectively and also provide services to separate independent computers connected in a network as shown in fig below.

The communication with all computer is done using a message passing interface (MPI). It is based on the tightly coupled architecture.

It uses Data structure like a queue to handle the messages and avoid message loss between the sender and receiver computer.

E.g. Automated banking system, railway reservation system, etc.

Advantages:

1. High performance than a single system.
2. If one computer fails, the remaining system continues to work.
3. External resources can be added easily.
4. Sharing is easy and economically affordable on multiple PC's. e.g. files sharing, printers sharing, etc.

Disadvantages:

1. The problem of scalability - This system supports only a limited number of independent computers with shared resources.
2. Message passing semantics are needed prior to the execution of messages.
3. Security problems may occur due to the sharing of resources.
4. Data administration is difficult.
5. Messages can be lost.

2. Network operating system:

It is specially designed for the heterogeneous multi-computer system, where multiple hardware and network platforms are supported. It has multiple operating systems running on different hardware platforms connected in a network. In this system, each computer is connected to a network. It is on the basis of the loosely coupled architecture style which allows a user to use services provided by the local machine itself as shown in fig below.

E.g. In remote login, where user computer is used to log in to the remoter server and execute remote commands over the network.

Advantage:

This system provides scalability feature, where a large number of resources and users are supported.

Disadvantage:

It doesn’t provide a single flow view.

3. Middleware operating system:

There are few drawbacks in a distributed operating system such as it has lack of scalability. There are few drawbacks in network operating system like it fails to provide a single coherent view. To overcome these problems, a new layer is formed between the distributed and network operating system is called the middleware operating system.

All local and remote services are provided by this system. The services which are common are provided for the local applications and independent set of services for the remote applications.

It supports heterogeneity that means it supports multiple languages and operating system where the user gets the freedom to write the application using any of the supported languages under any platform.

It allows locating the objects or interfaces by their names, finding the location of objects, maintaining the quality of services, handling the protocol information, synchronization, concurrency and security of the objects and much more. The figure below shows the middleware operating system which has distributed applications and middleware services common for all machines. And each machine has it’s own network operating system service.