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Introduction
Chapter 1
Dr.Husam Osta (9th Floor)
www.husamo.wordpress.com
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Introduction
• Computer revolution started from the year 1945 (modern era)
• During the year 1985 large and expensive computers were available
• As time progressed new developments took place and two new technologies were
developed. They are
1. Powerful microprocessor (8 bit, 16 bit, 32 bit, 64 bit)
2. High speed networks
- LAN – Local area network,
- WAN– Wide area network
• The result of the above two technologies were;
1. Flexible
2. Easy to access and put together all the computing systems connected together
with high speed network.
• The above result leads to distributed systems…Than previous centralized systems.
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Definition of a Distributed System (1)
A distributed system is:
• A collection of independent computers that
appears to its users as a single coherent system.
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Definition of a Distributed System (2)
1.1
A distributed system organized as middleware.
Note that the middleware layer extends over multiple machines.
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Definition means..
1. Distributed systems consists of components (computer)
that are autonomous (independent, self-directing)
2. Users they are dealing with single system i.e. all the
autonomous computers need to collaborate
when autonomous computers are collaborated - we get distributed systems.
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Characteristic of Distributed systems
1. Resource sharing (h/w, s/w, data, files, should have resource manger)
2. Openness
3. Concurrency
4. Scalability
5. Fault Tolerance
6. Transparency
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Characteristic of Dis. Systems
• Access resources and share them in a controlled and efficient way.
– Printers, computers, storage facilities, data, files, Web pages,
and networks, …
• Connecting users and resources also makes it easier to collaborate
and exchange information.
– Internet for exchanging files, mail, documents, audio, and
video
• Security is becoming increasingly important
– Little protection against spying or intrusion on communication
– Tracking communication to build up a preference profile of a
specific user
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Openness
• Open distributed system – is a system that is able to interact with
services from other open systems, irrespective of the underlying
environment:
– System should conform to well-defined interfaces
– Systems should support portability of applications
– Systems should easily interoperate
• Achieving openness: The distributed system should be independent
from heterogeneity of the underlying environment:
– Hardware
– Platforms
– Languages
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Openness
• Implementing openness requires support
different polices:
for
– What level of consistency do we require for client-cached data?
– Which operations do we allow downloaded code to perform?
– What level of secrecy do we require for communication?
• In implementing openness, ideally, a distributed
system should provide mechanisms that:
– Allow (dynamic) setting of caching policies
– Support different levels of trust for mobile code
– Offer different encryption algorithms
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Scalability in Distributed Systems
Scalability of a system can be measured along at least three
different dimensions (Neuman, 1994):
• First, a system can be scalable with respect to its size, meaning
that we can easily add more users and resources to the system.
• Second, a geographically scalable system is one in which the
users and resources may lie far apart.
• Third, a system can be administratively scalable, among that it
can still be easy to manage even if it spans many independent
administrative organizations.
• Unfortunately, a system that is scalable in one or more of these
dimensions often exhibits some loss of performance as the
system scales up.
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Scalability in Distributed Systems
• Components of Scalability:
– Number of users and/or processes (size scalability)
– Maximum distance between nodes (geographical scalability)
– Number of administrative domains (administrative scalability)
• Observation: Most systems account only, to a certain extent, for
size scalability, such as powerful servers. However, the
challenge lies in geographical and administrative scalability.
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Scalability Problems @ size
• Decentralized algorithms:
–
–
–
–
No machine has complete information about the system state.
Machines make decisions based only on local information.
Failure of one machine does not ruin the algorithm
There is no implicit assumption that a global clock exists
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Scalability Problems @ geography
• Synchronous communication
– A party requesting service, generally referred to as a client, blocks until a
reply is sent back.
• WANs is unreliable and Point-to-point
• LANs provide reliable communication facilities based on
broadcasting.
• Geographical scalability is strongly related to problems of
centralized solutions that hinder size scalability.
– In addition, centralized components now lead to a waste of network
resources.
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Scalability Problems @ administration
• It’s difficult and in many cases open questions
– Conflicting polices with respect to resource usage, management,
and security.
– E.g.,
• A domain can often be trusted by users that operate within that same
domain.
• Downloading programs such as applets in Web browsers.
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Distributed Transparency
• An important goal of distributed system is to hide the
fact that it’s process and resources are physically
distributed across multiple computers.
• A distributed systems that is able to present itself to
users and applications as if it were only a single
computer systems is said to be Transparent.
Different forms of transparency in a distributed system.
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Distributed Transparency
Types of transparancy
Transparency
Access
Description
Hide differences in data representation and how a resource is
accessed
Location
Hide where a resource is located
Migration
Hide that a resource may move to another location
Relocation
Hide that a resource may be moved to another location while in
use
Replication
Hide that a resource may be shared by several competitive users
Concurrency
Hide that a resource may be shared by several competitive users
Failure
Persistence
Hide the failure and recovery of a resource
Hide whether a (software) resource is in memory or on disk
Different forms of transparency in a distributed system.
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Hardware Concepts
1.6
Different basic organizations and memories in distributed computer systems
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Multiprocessors (1)
1.7
A bus-based multiprocessor.
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Multiprocessors (2)
1.8
a) A crossbar switch
b) An omega switching network
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Homogeneous Multicomputer Systems
1-9
a) Grid
b) Hypercube
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Software Concepts
System
Description
Main Goal
DOS
Tightly-coupled operating system for multiprocessors and homogeneous multicomputer
Hide and manage
hardware resources
NOS
Loosely-coupled operating system for
heterogeneous multicomputer (LAN and WAN)
Offer local services
to remote clients
Additional layer atop of NOS implementing
general-purpose services
Provide distribution
transparency
Middleware
An overview of
• DOS (Distributed Operating Systems)
• NOS (Network Operating Systems)
• Middleware
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Multicomputer Operating Systems (1)
1.14
General structure of a multicomputer operating system.
(DOS)
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Network Operating System (1)
1-19
General structure of a network operating system. (NOS)
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Network Operating System (2)
1-20
Two clients and a server in a network operating system.
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Positioning Middleware
1-22
General structure of a distributed system as middleware.
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Middleware and Openness
In an open middleware-based distributed system, protocols used by each
middleware layer should be the same, as well as the interfaces they offer to
applications.
1.23
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Comparison between Systems
Item
Distributed OS
Network
OS
Middlewarebased OS
Multiproc.
Multicomp.
Degree of transparency
Very High
High
Low
High
Same OS on all nodes
Yes
Yes
No
No
Number of copies of OS
1
N
N
N
Basis for communication
Shared
memory
Messages
Files
Model specific
Resource management
Global,
central
Global,
distributed
Per node
Per node
Scalability
No
Moderately
Yes
Varies
Openness
Closed
Closed
Open
Open
•
A comparison between multiprocessor operating systems, multicomputer operating systems,
network operating systems, and middleware based distributed systems.
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Clients and Servers
1.25
General interaction between a client and a server.
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Multitiered Architectures (2)
1-30
An example of a server acting as a client.
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Modern Architectures
1-31
An example of horizontal distribution of a Web service.
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