Transcript Distributed Systems

Distributed Computing
Chapter 01:
Character of Distributed Systems
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BOOKS
 TEXT BOOK
 Distributed Systems: Concepts and Design (Fourth/Third Edition)
By George Coulouris, Jean Dollimore and Tim Kindberg
Addison-Wesley, ©Pearson Education 2001
 REFERENCE BOOKS
 Distributed Systems: Principles and Paradigms, By Andrew S.
Tanenbaum and van Steen
 Distributed Operating System and Algorithms, By Randy Chow
& Theodore Johnson.
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Definition of Distributed System
 A distributed system is:
 Components located at networked computers communicate and
coordinate their actions by passing messages
 A distributed system is:
 Collection of independent computers that appear to the users of
the system as a single computer [Tanenbaum]
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Definition of Distributed System
Machine A
Machine B
Machine C
Distributed applications
Middleware service
Local OS
Local OS
Local OS
Network
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Definition of Distributed System
 "a collection of autonomous computers linked by a computer
network with distributed system software"
 "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"
 "one in which hardware or software components located at
networked computers communicate and coordinate their actions
only by passing messages"
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Definition of Distributed System
 In other words, to the user, a DS looks like a single system
 a virtual uniprocessor
 The user doesn't know (or care)
 - where (on what machine) files are located
 - where a job is executed
 A Distributed System is made of several computers which
 have no shared memory
 have no shared clock
 communicate with each other via messages
 have their own operating systems
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Advantages
 Why distributed systems
Resource sharing
Communication
Inherent distribution
Speed
 10000 CPU running at speed of 50 MIPS
 500000 MIPS
 Economy
 Incremental growth
 Reliability
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Characteristics of DS
 Concurrency
 Distributed systems are concurrent systems
 Every software or hardware components is autonomous
 Process
 Concurrent tasks
 A and B are concurrent if either A can happen before B or B can happen
before A
 Synchronization and coordination by message passing
 Lack of global clock
 Coordination and time
 Independent failure
 Failures of individual processes may remain undetected
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Examples of Distributed Systems
1. The internet
 Heterogeneous network of computers and applications
 Implemented through Internet protocol
intranet
%
ISP
%
%
%
backbone
satellite link
desktop computer:
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server:
network link:
Examples of Distributed Systems
2. Distributed multimedia
•Often use the Internet infrastructure
•Characteristics
•Heterogeneous data sources that need to be synchronized
in real time
•Video, Audio, Text
•Often: distribution services
•multicast
•Examples
•Tele teaching, Video conferencing
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Examples of Distributed Systems
3. Intranet
•Locally administered network
•Usually proprietary
•Interfaces with the Internet via router
•Firewall
email server
Desktop
computers
print and other servers
Web server
Issues
File services need to share
data
Firewall impede Legitimate
access
Local area
network
email server
File server
other servers
the rest of
the Internet
router/firewall
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print
Examples of Distributed Systems (1)
4. Mobile and Ubiquitous computing
•Resource Sharing while on the move
•Primary concerns:
•resource discovery
Internet
•efficient use of limited bandwidth
•security (privacy)
Host intranet
Mobile
phone
Laptop
Printer
Camera
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WAP
gatew ay
Wireless LAN
Host site
Home intranet
Resource sharing and the Web
Resource sharing pattern
•Search engine
•CSCW
•Computer-supported cooperative work
•Service
•Software that manages a collection of resources
and presents them to users and applications via
a well-defined interface
•Server
•Running program (a process that provide one or
more services)
•Remote invocation
•Client’s request to server.
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Challenges
 A number of challenges arises:
• Heterogeneity
• Openness
• Security
• Scalability
• Failure handling
• Concurrency of components
• Transparency
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Challenges
 Heterogeneity
 networks
 computer hardware
 Operating systems
 Programming languages
 Middleware
 To over come differences in network, OS and
languages
 CORBA
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Challenges
 Openness
 Ensures extensibility and maintainability of the system
 Standard interfaces & their publication
 Addition of new resources
 RFC (Request for comments) specification for internet
protocols. www. ietf.org
 Benefits of ODS
 key interfaces are published
 uniform communication mechanisms and access to shared
resources
 Construction
 Heterogeneous components
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Challenges
 Security
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confidentiality
integrity
availability
Example
 A doctor might request access to hospital patient data
 In electronic commerce and banking, users send their
credit card numbers across the internet
 Current security challenges
 Denial of service attack
 Security of mobile code
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Challenges
 Scalability
 Controlling the cost of the physical resources
 Does the system remain effective given the expected
growth
 Controlling the performance loss
 www.amzon.com is more than one computer
 Preventing the software resources running out
 IP addressees :32 bits to 128
 Avoiding performance bottleneck
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Decentralization of data/information
Challenges
 Computers vs. Web servers in the Internet
Date
1993, July
1995, July
1997, July
1999, July
Computers
1,776,000
6,642,000
19,540,000
56,218,000
Web servers
130
23,500
1,203,096
6,598,697
Mid. 2002 Internet :
150 million+ computers
667 million people
2.5 billion+ web pages
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Percentage
0.008
0.4
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Challenges
 Failure handling
 Detecting failure
 Checksum can be used to detect corrupted data
 System crash (impossible)
 Masking
 Retransmission
 Redundant storage
 Tolerating failure
 Exception handling (timeout when waiting for web source)
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Challenges
 Failure handling
 Recovery from failure
 Roll back
 Redundancy
 Redundant routes in network
 Replication of name tables in multiple domain name servers
 Database replication
 Availability
 Measure of the proportion of the time a server is available
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Challenges
 Concurrency
 Consistent scheduling of threads (so that
dependencies are preserved in concurrent
transactions)
 Avoidance of deadlock
 Transparency
 Concealing the heterogeneous and distributed nature
of the system so that it appears to the user like one
system
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Challenges :Transparency
 Access transparency: enables local and remote resources to be accessed using identical
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operations.
Location transparency: enables resources to be accessed without knowledge of their
location.
Concurrency transparency: enables several processes to operate concurrently using
shared resources without interference between them.
Replication transparency: enables multiple instances of resources to be used to
increase reliability and performance without knowledge of the replicas by users or
application programmers.
Failure transparency: enables the concealment of faults, allowing users and application
programs to complete their tasks despite the failure of hardware or software
components.
Mobility transparency: allows the movement of resources and clients within a system
without affecting the operation of users or programs.
Performance transparency: allows the system to be reconfigured to improve
performance as loads vary.
Scaling transparency: allows the system and applications to expand in scale without
change to the system structure or the application algorithms.
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