2.DS Concepts and Architecture

7/30/2019 2.DS Concepts and Architecture

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Distributed system concept and architecture

Goals of Distributed OS:1. Efficiency:

Due to communication delay in distributed

environment.(milliseconds to microseconds)

Delay is caused by

-Data propagation

-Load distribution

Need effective communication primitives at

the language or O.S level and good

communication protocols at the network

levels 1


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2. Flexibility:

Users view Related to friendliness and freedom of

using system.

Provide consistency and reliability Systems view

Systems ability to evolve & migrate

Key properties are

Modularity

Scalability2

Portability

Interoperability


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3. Consistency:

System behavior should be predictable. Uniformity in using the system

Inconsistency causes because of

Replication and partitioning of data Component failures

Complexity of interaction among

modules

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4. Robustness :

Ability to handle exceptional situationsand errors such as

Changes to the system topology

Long message delays

Inability to locate a server

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Other major responsibilities of DOSare

Reliability

Protection

Access control

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Transparency

Hide all system dependent details from user.

Different types:

1. Access transparency-Ability to access both local and

remote system objects in a uniform way

2. Location transparency- Users have no awareness ofobject locations

3. Migration transparency- Object is not only referred to

by its logical name but can also be moved to a

different physical location without changing the name

4. Concurrency transparency- Sharing of objects without

interference

5. Replication transparency-Provides consistency of

multiple instances of files and data6


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6. Parallelism transparency-Permits parallel

activities without users knowing how, where andwhen these activities are carried out by the systems

7.Failure transparency-Provide fault tolerance such

that failures in the system can be transformed into

graceful system performance degradation

8.Performance transparency-Achieve a consistent

and predictable performance level even with

changes of the system structure or load distribution7


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9.Size transparency-It allows incremental

growth of a system without the users awareness

10.Revision transparency- Refers to the

vertical growth of systems as opposed to the

horizontal growth in system size

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Categorization of transparencies based on

system goals

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System Goals Transparencies

Efficiency

Concurrency

Parallelism

Performance

Flexibility

AccessLocation

Migration

Size

Revision

Consistency AccessReplication

Performance

Robustness

Failure

Replication

SizeRevision


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Distributed system issues and

transparencies

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Major Issues TransparenciesCommunication

Synchronization

Distributed algorithms

Interaction and control

transparency

Process schedulingDeadlock handling

Load balancing

Performance transparency

Resource scheduling

File sharingConcurrency control

Resource transparency

Failure handling

Configuration

Redundancy

Failure transparency


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Services

1. Primitive services:

-Implemented inside kernel.

-They are basic services for process communication.- Process communication is by message passing.

-Synchronization

-Processor multiplexing

2. Services by System servers:Implemented any where in system

1) Name server:

-Find the location of file.

2) Network server:-Name and location from name server is system

dependent.

-Translation of address and routing information are

provided by network server. 11


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3) Time server:-Used for Synchronization and scheduling of hardware

and software.

- 2 types of timing information are:

Physical clock

provide real-time.

Logical clockprovide artificial time to preserve event ordering

4) Servers for managing shared system resources are:

File server

Print server

Authentication server

Migration server

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3) Value added services:

- They are special purpose services.

- Group server:Manage the creation and termination of group

activities.

Responsible for group addressing and

communication. contain information like :

-Membership

-Admission policy

-Members privileges

Ex: Distributed conference group, Network news

group

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A. Distributed System architecture

1. Workstation Model

2. Workstation- Server Model

3. Processor-pool model

4. Hybrid model

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1. Workstation Model

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Workstation

Communicati

on network

Workstation

Workstation

Workstation

Workstation


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Consists of several workstations

interconnected by communication network

Eg. A companys office have several

workstations scattered throughout a

building

Each workstation equipped with its own

disk and serving as a single user computer

Eg. Sprite system[1988]

Xerox PARC[1982]

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When the system finds that the users

workstation does not have sufficient

processing power for executing theprocesses of the submitted jobs efficiently,

it transfers one or more of the processes

from the users workstation to some otherworkstation that is currently idle and get

the process executed there, and finally the

results of execution is returned to theusers workstation

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2. Workstation-Server Model

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Workstation Workstation

WorkstationCommunication

network

Mini-Computer

File server

Mini-Computer

Data baseserver

Mini-Computer

Print server

Workstation


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Diskful workstationsworkstation with its

own disk

Diskless workstationsworkstation without

its own disk

This model consists of a few

minicomputers and several

workstations(most of which are diskless,

but a few of which may be diskful )

interconnected by a communicationnetwork

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When diskless workstations are used on a

network, the file system to be used by

these workstations must be implementedeither by a diskful workstation or by a

minicomputer equipped with a disk for file

storage Eg. V-system [1998]

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Client workstations

Diskless Graphic/interactive applications

processed in local

All file, print, database computationrequests are sent to servers.

Server minicomputers

Each minicomputer is dedicated to oneor more different types of services.

Client-Server model of communication

RPC (Remote Procedure Call) 21


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Clients:

They log in one of terminals (disklessworkstations)

Servers: Necessary number of processors are

allocated to each user from the pool.

Better utilization but less interactivity

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3. Processor-Pool Model

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Communication

network

Data

baseserver

Fileserver

Terminals

Pool of processor


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When the user needs a large amount of

computing power for a short time, theprocessors are pooled together to be

shared by the users as needed

Each processor in the pool has its ownmemory to load and run a system program

or an application program

Eg. Amoeba[1990], Plan 9[1990]

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4. Hybrid Model

Combine the advantages of Workstation-

server model and processor-pool model

The processors in the pool can be

allocated dynamically for computations

that are too large for workstations or that

require several computers concurrently for

efficient execution

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B. Communication NetworkArchitecture

point-to-point connection

multipoint connection

IEEE 802 LAN standards Ethernet

Token Bus

Token Ring FDDI

SONET26


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Communication Network Protocols

Set of rules , that regulate the exchange ofmessages to provide a reliable and orderly

flow of information

Two categories of communication services

1. Connection oriented

2. Connectionless

Circuit switching Packet switching

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Cont..

Popular network protocol suite:

1. OSI protocol suite

- 7 layers

-data transparency is achieved by an

encapsulation process

- protocol data unit

2. TCP/IP protocol suite

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Netprog: OSIReference Model

OSI Reference Model

Layered model:

7. Application

6. Presentation5. Session

4. Transport

3. Network2. Data Link

1. Physical


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The Physical Layer

Responsibility:

transmission of raw bits over a

communication channel.

Issues:

mechanical and electrical interfaces

time per bit

distances


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The Data Link Layer -

Data Link Control Responsibility: provide an error-free communication link

Issues: framing(dividing data into chunks) header & trailer bits

addressing

10110110101 01100010011 10110000001


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The Data Link Layer -

The MAC sublayer Medium Access Control - needed by

mutiaccess networks.

MAC provides DLC with virtual wires

on multiaccess networks.


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The Network Layer

Responsibilities:

path selection between end-systems (routing).

subnet flow control.

fragmentation & reassembly

translation between different network types.

Issues:

packetheaders

virtual circuits


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The Transport Layer

Responsibilities: provides virtual end-to-end links between

peer processes.

end-to-end flow control Issues:

headers

error detection reliable communication


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The Session Layer

Responsibilities:

establishes, manages, and terminates

sessions between applications.

service location lookup

Many protocol suites do not include a

session layer.


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The Presentation Layer

Responsibilities:

data encryption

data compression

data conversion

Many protocol suites do not include a

Presentation Layer.


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The Application Layer

Responsibilities:

anything not provided by any of the other

layers

Issues:

application level protocols

appropriate selection of type of service


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Cont..

Two major types of system interactions:

1.Interprocess communication

-uses process id

2.Internode communication

TCP is a transport layer protocolequivalent to TP4 in OSI

IP

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Cont..

Socket abstraction of network I/O

Allows standard Read and Write operation

created using socketsystem call

Given by a socket descriptor System calls

connect

bind sendto

recvfrom

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2.DS Concepts and Architecture

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