1. Distributed Computing
Chapter 01:
Character of Distributed Systems

2. BOOKS TEXT BOOK REFERENCE BOOKS
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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3. Definition of Distributed System
A distributed system is:
Components located at networked computers communicate and coordinate their actions by passing messages
Collection of independent computers that appear to the users of the system as a single computer [Tanenbaum]
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4. Definition of Distributed System
Machine A
Local OS
Machine B
Local OS
Machine C
Local OS
Distributed applications
Middleware service
Network
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5. 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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6. 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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7. Advantages Why distributed systems
Resource sharing
Communication
Inherent distribution
Speed
10000 CPU running at speed of 50 MIPS
MIPS
Economy
Incremental growth
Reliability
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8. Characteristics of DS Concurrency Lack of global clock
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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9. Examples of Distributed Systems 1. The internet
Heterogeneous network of computers and applications
Implemented through Internet protocol
intranet
ISP
desktop computer:
backbone
satellite link %
server:
network link:
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10. 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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11. Examples of Distributed Systems 3. Intranet
Locally administered network
Usually proprietary
Interfaces with the Internet via router
Firewall
the rest of
server
Web server
Desktop
computers
File server
router/firewall
print
and other servers
other servers
Local area
network
the Internet
Issues
File services need to share data
Firewall impede Legitimate access
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12. Examples of Distributed Systems (1) 4. Mobile and Ubiquitous computing
Resource Sharing while on the move
Primary concerns:
resource discovery
efficient use of limited bandwidth
security (privacy)
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13. 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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14. Challenges A number of challenges arises: Heterogeneity Openness
Security
Scalability
Failure handling
Concurrency of components
Transparency
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15. Challenges Heterogeneity Middleware networks computer hardware
Operating systems
Programming languages
Middleware
To over come differences in network, OS and languages
CORBA
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16. Challenges Openness Benefits of ODS
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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17. Challenges Security Current security challenges 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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18. Challenges Scalability Controlling the cost of the physical resources
Does the system remain effective given the expected growth
Controlling the performance loss
is more than one computer
Preventing the software resources running out
IP addressees :32 bits to 128
Avoiding performance bottleneck
Decentralization of data/information
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19. Challenges Computers vs. Web servers in the Internet
Date
Computers
Web servers
Percentage
1993, July
1,776,000
130
0.008
1995, July
6,642,000
23,500
0.4
1997, July
19,540,000
1,203,096 6
1999, July
56,218,000
6,598,697 12
Mid Internet :
150 million+ computers
667 million people
2.5 billion+ web pages
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20. Challenges Failure handling Detecting failure Masking
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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21. Challenges Failure handling Recovery from failure Redundancy
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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22. Challenges Concurrency Transparency
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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23. Challenges :Transparency
Access transparency: enables local and remote resources to be accessed using identical 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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