Distributed Operating System

DISTRIBUTED OPERATING
SYSTEM
BY
Ajitha G
P19CS001

Distributed computing system is a collection of
processors interconnected by a communication network in
which each processor has its own local memory and other
peripherals and the communication between any two
processors of the system takes place by message passing over
the communication network.
Distributed Computing system models can be broadly
classified into five categories. They are
 Minicomputer model
 Workstation model
 Workstation – server model
 Processor – pool model
 Hybrid model

Minicomputer Model:
The minicomputer model is a simple extension of the
centralized time-sharing system.
A distributed computing system based on this model consists
of a few minicomputers interconnected by a communication
network were each minicomputer usually has multiple users
simultaneously logged on to it.
Several interactive terminals are connected to each
minicomputer. Each user logged on to one specific minicomputer
has remote access to other minicomputers.

 The network allows a user to access remote resources
that are available on some machine other than the one on to
which the user is currently logged.
The minicomputer model may be used when resource
sharing with remote users is desired.
The early ARPA net is an example of a distributed
computing system based on the minicomputer model.

WORK STATION MODEL:
A distributed computing system based on the
workstation model consists of several workstations
interconnected by a communication network.
 An organization may have several workstations located
throughout an infrastructure were each workstation is
equipped with its own disk & serves as a single-user
computer.
In such an environment, at any one time a significant
proportion of the workstations are idle which results in the
waste of large amounts of CPU time.

Therefore, the idea of the workstation model is to
interconnect all these workstations by a high-speed LAN.
So that idle workstations may be used to process jobs
of users who are logged onto other workstations & do
not have sufficient processing power at their own
workstations to get their jobs processed efficiently.
 Example: Sprite system & Xerox PARC.

Problems:
1. How does the system find an idle
workstation?
2. How is a process transferred from one
workstation to get it executed on another
workstation?
3. What happens to a remote process if a user
logs onto a workstation that was idle
until now and was being used to execute a
process of another workstation?

WORKSTATION SERVER MODEL:
The workstation model is a network of personal
workstations having its own disk & a local file system.
A workstation with its own local disk is usually called a
diskful workstation & a workstation without a local disk is
called a diskless workstation.
Diskless workstations have become more popular in network
environments than diskful workstations, making the
workstation-server model more popular than the workstation
model for building distributed computing systems.

A distributed computing system based on the workstation-server
model consists of a few minicomputers & several workstations
interconnected by a communication network.
In this model, a user logs onto a workstation called his or her
home workstation.
Normal computation activities required by the user’s
processes are performed at the user’s home workstation, but
requests for services provided by special servers are sent to a
server providing that type of service that performs the user’s
requested activity & returns the result of request processing to
the user’s workstation.
Therefore, in this model, the user’s processes need not
migrated to the server machines for getting the work done by
those machine

PROCESSOR POOL MODEL:
The processor-pool model is based on the observation that
most of the time a user does not need any computing power but
once in a while the user may need a very large amount of
computing power for a short time.
Therefore, unlike the workstation-server model in which a
processor is allocated to each user, in processor-pool model the
processors are pooled together to be shared by the users as
needed.
The pool of processors consists of a large number of
microcomputers & minicomputers attached to the network.

Each processor in the pool has its own memory to load &
run a system program or an application program of the
distributed computing system
In this model no home machine is present & the user does
not log onto any machine.
This model has better utilization of processing power &
greater flexibility.
Example: WEB SEARCH ENGINE.

HYBRID MODEL:
The workstation-server model has a large number of computer
users only performing simple interactive tasks & executing small
programs.
In a working environment that has groups of users who often
perform jobs needing massive computation, the processor-pool
model is more attractive & suitable.
To combine Advantages of workstation-server & processor-pool
models, a hybrid model can be used to build a distributed system.
The processors in the pool can be allocated dynamically for
computations that are too large or require several computers for
execution.
The hybrid model gives guaranteed response to interactive jobs
allowing them to be more processed in local workstations of the users

TRANSPARENCY:
Transparency “is the concealment from the user of the
separation of components of a distributed system so that the system
is perceived as a whole”.
Transparency in distributed systems is applied at several
aspects such as:
Access Transparency – Local and Remote access to the resources
should be done with same efforts and operations. It enables local and
remote objects to be accessed using identical operations.
Location transparency – User should not be aware of the location
of resources. Wherever is the location of resource it should be made
available to him as and when required.
Migration transparency – It is the ability to move resources
without changing their names.

Replication Transparency – In distributed systems to achieve fault
tolerance, replicas of resources are maintained. The Replication
transparency ensures that users cannot tell how many copies exist.
Concurrency Transparency – As in distributed system multiple
users work concurrently, the resource sharing should happen
automatically without the awareness of concurrent execution by
multiple users.
Failure Transparency – Users should be concealed from partial
failures. The system should cope up with partial failures without the
users awareness.
Performance Transparency: This transparency allows distributed
system to be reconfigured to improve the performance as the
load varies. The load variation should not lead to performance
degradation and this is difficult to achieve.

Scaling Transparency: A system should be able to grow in the
condition of application algorithm is not be affected. Elegant
evolution and growth is very important for most enterprises.
A distributed should be able to scale down to small
environment where required, and be space and time efficient as
required. The example is World Wide Web.
Reliability
* One of the original goals of building distributed systems was
to make them more reliable than single-processor systems.
* The idea is that if a machine goes down, some other machine
takes over the job.
* A highly reliable system must be highly available, but that is
not enough.

system failures are of two types
Fail-stop failure
The system stops functioning after changing to a state in which
its failure can be detected.
Byzantine failure
The system continues to function but produces wrong
results.
Undetected software bugs often cause Byzantine failure of a
system.
Obviously, Byzantine failures are much more difficult to deal
with than fail-stop failures.
For higher reliability, the fault-handling mechanisms of a
distributed operating

For higher reliability, the fault-handling mechanisms of a distributed
operating system must be designed properly to avoid faults, to tolerate
faults, and to detect and recover from faults. Commonly used methods
for dealing with these issues are briefly described next.
Fault Avoidance : Fault avoidance deals with designing the
components of the system in such a way that the occurrence of faults
is minimized.
Fault Tolerance: Fault tolerance is the ability of a system to continue
functioning in the event of partial system failure
Fault Detection and Recovery: The fault detection and recovery
method of improving reliability deals with the use of hardware and
software mechanisms to determine the occurrence of a failure and
then to correct the system to a state acceptable for continued
operation

FLEXIBILITY:
Another important issue in the design of distributed
operating systems is flexibility. Flexibility is the most important
feature for open distributed systems.
The design of a distributed operating system should be
flexible due to the following reasons:
1. Ease of modification.
 From the experience of system designers, it has been found
that some parts of the design often need to be replaced/modified
either because some bug is detected in the design or because the
design is no longer suitable for the changed system environment
or new-user requirements.
 Therefore, it should be easy to incorporate changes in the
system in a user-transparent manner or with minimum
interruption caused to the users.

2. Ease of enhancement.
 In every system, new functionalities have to be added from
time to time to make it more powerful and easy to use.
 Therefore, it should be easy to add new services to the
system.
 Furthermore, if a group of users do not like the style in which
a particular service is provided by the operating system, they
should have the flexibility to add and use their own service that
works in the style with which the users of that group are more
familiar and feel more comfortable.

PERFORMANCE:
Always the hidden data in the background is the issue of
performance.
Building a transparent, flexible, reliable distributed
system, more important lies in its performance.
In particular, when running a particular application on a
distributed system, it should not be appreciably worse than
running the same application on a single processor.
Unfortunately, achieving this is easier said than done.

SCALABILITY:
•Distributed systems operate effectively and efficiently at many
different scales, ranging from a small intranet to the Internet.
•A system is described as scalable if it will remain effective when
there is a significant increase in the number of resources and the
number of users.
SECURITY:
Many of the information resources that are made available and
maintained in distributed systems have a high intrinsic value to
their users.
Their security is therefore of considerable importance. Security
for information resources has three components: confidentiality,
integrity, and availability.

HETEROGENEITY:
The Internet enables users to access services and run
applications over a heterogeneous collection of computers and
networks.
Internet consists of many different sorts of network their
differences are masked by the fact that all of the computers
attached to them use the Internet protocols to communicate
with one another.
For e.g.., a computer attached to an Ethernet has an
implementation of the Internet protocols over the Ethernet,
whereas a computer on a different sort of network will need an
implementation of the Internet protocols for that network.

COMPONENTS OF DCE:
DCE is blend of various technologies developed
independently and nicely integrated by OSF. Each of
technologies forms a components of DCE, the main components
of DCE are as follows:
Thread Package:
It provides a simple programming model for building
concurrent applications.
It includes operations to create and control multiple
threads of execution in a single process and to synchronize
access to global data within the application.

Remote Procedure Calls(RPC) Facility:
It provides programmers with a number of powerful tools
necessary to build client-server applications.
In DCE , RPC facility is the basis for all communication in
DCE because the programming model underlying all of DCE is
the client-server model.
It is easy to use, is network and protocol-independent,
provides secure communication between a client and a server.
Hides difference in data requirement by automatically
converting data to the appropriate forms needed by clients and
servers.

Distributed Time Service(DTS) :
It closely synchronizes the clocks of all computers in the
system.
It also permits the use of time values from external time
sources, such as those of the U.S National Institute for
Standards and Technology (NIST), to synchronize the clock of
the computers in the system with external time.
This facility can also be used to synchronize the clock of the
computers of one distributed environment with the clocks of
computers of another distributed environment.

Name Services :
 The name services of DCE include the Cell Directory
Services (CDS), the Global Directory Service (GDS), and
the Global Directory Agent (GDA).
 These services allow resources such as servers, files,
devices, and so onto be uniquely name and accessed in a
location-transparent manner.
Security Service:
It provides the tools needed for authentication and
authorization to protect system resources against illegitimate
access.

Distributed File Services(DFS):
It provides a system wide file system that has such
characteristics as location transparency, high performance,
and high availability.
 A unique feature of DCE & DFS is that it can also provide
file services to clients of other file systems.

Distributed Operating System

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