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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
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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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Netprog: OSIReference Model
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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Netprog: OSIReference Model
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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Netprog: OSIReference Model
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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Netprog: OSIReference Model
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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Netprog: OSIReference Model
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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Netprog: OSIReference Model
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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Netprog: OSIReference Model
The Presentation Layer
Responsibilities:
data encryption
data compression
data conversion
Many protocol suites do not include a
Presentation Layer.
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Netprog: OSIReference Model
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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