A distributed system is one in which components located at networked computers communicate and coordinate their actions only by passing messages. The document discusses the definition and types of distributed systems, including distributed computing systems, distributed information systems, and distributed pervasive systems. It also covers common characteristics like heterogeneity, openness, security, scalability, and transparency. Basic design issues for distributed systems such as naming, communication, software structure, and system architectures are also explained.

1. DISTRIBUTED SYSTEM
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2. CONTENT
 What is aDistributed System
 Types of Distributed Systems
 Examples of Distributed Systems
 Common Characteristics
 Basic Design Issues
 Advantages
 Disadvantages
 Conclusion
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3. 1. WHAT IS A DISTRIBUTED SYSTEM?
Definition: A distributed system is one in which components
located at networked computers communicate and coordinate
their actions only by passing messages. This definition leads
to the following characteristics of distributed systems:
 Concurrency of components
 Lack of a global ‘clock’
 Independent failures of components
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4. 2. TYPES OF DISTRIBUTED SYSTEMS
 Distributed Computing Systems.
 Distributed Information Systems.
 Distributed Pervasive Systems.
Distributed Computing Systems: The distributed
computing systems include the following:
 Cluster computing systems
 Grid computing systems
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5. Distributed Informative Systems: In the distributed
systems, the following forms are concentrated:
 Transaction processing systems
 Enterprise application integration
Distributed Pervasive Systems: Few examples of
distributed pervasive systems are as below:
 Home systems
 Electronic health care systems
 Sensor networks
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6. 3. EXAMPLES OF DISTRIBUTED SYSTEMS
 Local Area Network and Intranet
 Database Management System
 Automatic Teller Machine Network
 Internet/World-Wide Web
 Mobile and Ubiquitous Computing
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7. 3.1 LOCAL AREA NETWORK
the rest of
email server
Web server
Desktop
computers
File server
router/firew all
print and other servers
other servers
print
Local area
netw ork
email server
the Internet
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8. 3.2 DATABASE MANAGEMENT SYSTEM
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9. 3.3 AUTOMATIC TELLER MACHINE NETWORK
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10. 3.4 INTERNET
intranet
ISP
desktop computer:
backbone
satellite link
server:
%
network link:
%
%
%
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11. 3.4.1 WORLD-WIDE-WEB
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12. 3.4.2 WEB SERVERS AND WEB BROWSERS
Internet
Browsers
Web servers
www.google.com
www.uu.se
www.w3c.org
Protocols
Activity.html
http://www.w3c.org/Protocols/Activity.html
http://www.google.comlsearch?q=lyu
http://www.uu.se/
File system of
www.w3c.org
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13. 3.5 MOBILE AND UBIQUITOUS COMPUTING
Laptop
Mobile
Printer
Camera
Internet
Host intranet Home intranet
GSM/GPRS
Wireless LAN
phone
gateway
Host site
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14. 4. COMMON CHARACTERISTICS
 What are we trying to achieve when we construct a distributed
system?
 Certain common characteristics can be used to assess
distributed systems
 Heterogeneity
 Openness
 Security
 Scalability
 Failure Handling
 Concurrency
 Transparency
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15. 4.1 HETEROGENEITY
 Variety and differences in
 Networks
 Computer hardware
 Operating systems
 Programming languages
 Implementations by different developers
 Middleware as software layers to provide a programming abstraction
as well as masking the heterogeneity of the underlying networks,
hardware, OS, and programming languages (e.g., CORBA).
 Mobile Code to refer to code that can be sent from one computer to
another and run at the destination (e.g., Java applets and Java
virtual machine).
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16. 4.2 OPENNESS
 Openness is concerned with extensions and
improvements of distributed systems.
 Detailed interfaces of components need to be published.
 New components have to be integrated with existing
components.
 Differences in data representation of interface types on
different processors (of different vendors) have to be
resolved.
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17. 4.3 SECURITY
 In a distributed system, clients send requests to access
data managed by servers, resources in the networks:
 Doctors requesting records from hospitals
 Users purchase products through electronic commerce
 Security is required for:
 Concealing the contents of messages: security and privacy
 Identifying a remote user or other agent correctly (authentication)
 New challenges:
 Denial of service attack
 Security of mobile code
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18. 4.4 SCALABILITY
 Adaptation of distributed systems to
 accommodate more users
 respond faster (this is the hard one)
 Usually done by adding more and/or faster processors.
 Components should not need to be changed when scale
of a system increases.
 Design components to be scalable!
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19. 4.5 FAILURE HANDLING (FAULT TOLERANCE)
 Hardware, software and networks fail!
 Distributed systems must maintain availability even at
low levels of hardware/software/network reliability.
 Fault tolerance is achieved by
 recovery
 redundancy
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20. 4.6 CONCURRENCY
 Components in distributed systems are executed in
concurrent processes.
 Components access and update shared resources (e.g.
variables, databases, device drivers).
 Integrity of the system may be violated if concurrent
updates are not coordinated.
 Lost updates
 Inconsistent analysis
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21. 4.7 TRANSPARENCY
 Distributed systems should be perceived by users and
application programmers as a whole rather than as a
collection of cooperating components.
 Transparency has different aspects.
 These represent various properties that distributed
systems should have.
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22. 5. BASIC DESIGN ISSUES
 General software engineering principles include
rigor and formality, separation of concerns,
modularity, abstraction, anticipation of change, …
 Specific issues for distributed systems:
 Naming
 Communication
 Software structure
 System architecture
 Workload allocation
 Consistency maintenance
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23. 5.1 NAMING
 A name is resolved when translated into an interpretable form
for resource/object reference.
 Communication identifier (IP address + port number)
 Name resolution involves several translation steps
 Design considerations
 Choice of name space for each resource type
 Name service to resolve resource names to comm. id.
 Name services include naming context resolution, hierarchical
structure, resource protection
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24. 5.2 COMMUNICATION
 Separated components communicate with sending processes
and receiving processes for data transfer and synchronization.
 Message passing: send and receive primitives
 synchronous or blocking
 asynchronous or non-blocking
 Abstractions defined: channels, sockets, ports.
 Communication patterns: client-server communication (e.g.,
RPC, function shipping) and group multicast
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25. 5.3 SOFTWARE STRUCTURE
 Layers in centralized computer systems:
Applications
Middleware
Operating system
Computer and Network Hardware
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26. 5.3 SOFTWARE STRUCTURE
 Layers and dependencies in distributed systems:
Applications
Distributed programming
support
Open
services
Open system kernel services
Computer and network hardware
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27. 5.4 SYSTEM ARCHITECTURES
 Client-Server
 Peer-to-Peer
 Services provided by multiple servers
 Proxy servers and caches
 Mobile code and mobile agents
 Network computers
 Thin clients and mobile devices
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28. 5.4.1 CLIENTS INVOKE INDIVIDUAL SERVERS
Server
Client
Client
invocation
result
Server
invocation
result
Process:
Key:
Computer:
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29. 5.4.2 PEER-TO-PEER SYSTEMS
Application
Application
Application
Peer 1
Peer 2
Peer 3
Peers 5 .... N
Sharable
objects
Application
Peer 4
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30. 5.4.3 A SERVICE BY MULTIPLE SERVERS
Server
Server
Server
Service
Client
Client
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31. 5.4.4 WEB PROXY SERVER
Client
Proxy
Web
server
Web
server
server
Client
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32. 5.4.5 WEB APPLETS
a) client request results in the dow nloading of applet code
Web
server
Client
Web
server
Applet
Applet code
Client
b) client interacts w ith the applet
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33. 5.4.6 THIN CLIENTS AND COMPUTE SERVERS
Thin
Client
Application
Process
Network computer or PC
Compute server
network
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34. 6.ADVANTAGES
 Sharing Data : There is a provision in the environment where
user at one site may be able to access the data residing at
other sites.
 Autonomy : Because of sharing data by means of data
distribution each site is able to retain a degree of control over
data that are stored locally.
 Availability : If one site fails in a distributed system, the
remaining sites may be able to continue operating. Thus a
failure of a site doesn't necessarily imply the shutdown of the
System.
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35. 7. DISADVANTAGES
 Software Development Cost
 Greater Potential for Bugs
 increased Processing Overhead
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36. REFERENCES
 www.google.com
 www.wikipedia.com
 www.studymafia.org

37. THANKS