This document discusses distributed systems and their key characteristics. It defines a distributed system as a collection of independent computers that appear as a single computer to users. Examples provided include web search engines, massively multiplayer online games, financial trading systems, and social networks. Advantages over personal and centralized systems are described, such as increased speed, reliability, flexibility, and economics. Design issues and communication methods like message passing and remote procedure calls are also outlined.

1. Distributed Systems
By- Sumita Das

2. Topics Covered
 Introduction to Distributed systems
 Examples
 Advantages over Personal Systems
 Advantages over Centralized Systems
 Disadvantages
 Design Issues
 Communication in Distributed Systems
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3. Distributed System
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4. Definition
 Collection of independent computers that
appears to users of the system as a single
computer.
 Each computer consists:
•Memory
•CPU
 Allow many user to work together.
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5. Examples of distributed systems
 Web search
–Index the entire contents of the Web.
 Massively multiplayer online games
–Very large number of users sharing a virtual world.
 Financial trading
–Real time access and processing of a wide range of
information sources
 Social Networking (Facebook)
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6. Advantages over Personal Systems
1. Data sharing : Many users access same
database.
2. Device sharing: Allow many users to share
peripherals.
3. Communication: Make human-to-human
communication easier.
4. Flexibility: Spread workload over available
machines. 6Created by Sumita Das

7. Advantages over Centralized
systems
1. Speed: Has more total computing power
than mainframe.
2. Reliability: If one machine crashes, the
system as a whole can still survive.
3. Incremental growth: Computing power
can be added
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8. Advantages over Centralized
systems
4. Economics: Offer better price and
performance than mainframes.
5. Inherent distribution: Some machines
involve spatially separated machines
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9. Disadvantages of Distributed
System
1. Software: Little software exists at present
2. Networking: Network can saturate or cause
other problems
3. Security: Easy access also applies to secret
data
4. Privacy: Data may be accessed securely but
without the owner’s consent 9Created by Sumita Das

10. Design Issues
 Making Resources Accessible
 Transparency
 Flexibility
 Reliability
 Performance
 Scalability
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11. Communication in Distributed
Systems
Uniprocessor
Systems
Distributed
Systems
Inter Process
Communication
Shared
Memory
Message
Passing
Remote
Procedure Call

12. Message Passing Model
Communication Primitives :
•SEND
•Message
•Destination
•RECEIVE
•Source of message
•Buffer for storing message

13. Message Passing Model
Blocking
Primitives
Non-Blocking
Primitives
•The user buffer can
be reused as soon as
the control is
returned to the user
program.
•Returns control to the
caller immediately
before message is sent
to the user.

14.  Disadvantage: Sender can’t modify the message
buffer until the message has been sent.
Non-Blocking Primitives
Non-Blocking Primitives
Buffered Option Unbuffered Option
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15. Buffered Option
Messages are copied three times
1. User buffer to the kernel buffer
2. Kernel buffer on the sending computer to the
kernel buffer on the receiving computer
3. Buffer on the receiving computer to a user
buffer.
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16.  Data is copied from one user buffer to
another user buffer directly.
 SEND : It should avoid reusing the user
buffer until the message has been
transmitted.
 RECEIVE : If a message is present, the
consumer process reads it, otherwise it
performs some other computation.
Unbuffered Option
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17. Message Passing Model
Reliable
Primitives
Unreliable
Primitives
Unreliable SEND : It does not return control to the user
program until the message has been sent.
Reliable SEND : It does not return control to the user
program until an acknowledgment has been received.
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18. RECEIVE : does not return control until a
message is copied to the user buffer.
 Reliable RECEIVE automatically sends an
acknowledgment.
 Unreliable RECEIVE does not send an
acknowledgment.
Message Passing Model
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19. References
• Andrew S.Tanenbaum, Maarten Van Steen,
“DISTRIBUTED SYSTEMS: Principles and
Paradigms, Second Edition
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