1. Distributed Systems
A Brief Introduction

2. OUTLINE
 BRIEF HISTORY.
 WHAT ARE DISTRIBUTED
SYSTEMS ? ?
 WHY DISTRIBUTED SYSTEMS ? ?
 ADVANTAGES OF D.S. OVER THE
CENTRALIZED SYSTEM.
 ADVANTAGES OF D.S. OVER
INDEPENDENT PC’S.

3. OUTLINE (Contd.)
 ORGANISATION OF DISTRIBUTED
SYSTEM.
 GOALS OF D.S.
− Resource Sharing.
− Openness.
− Transparency.
− Scalability.

4. OUTLINE (Contd.)
− Concurrency.
 TYPES OF DISTRIBUTED SYSTEMS.
− Distributed Computing Systems.
− Distributed Information Systems.
− Distributed Pervasive Systems.
 SUMMARY.

5. HISTORY
 1945~1985
− Computers were large and expensive.
− No way to connect them.
− All systems were Centralized Systems.
 Mid-1980s
− Powerful microprocessors.
− High Speed Computer Networks (LANs , WANs).

6. HISTORY (Contd.)
Then came the
DISTRIBUTED
SYSTEMS…

7. What are Distributed Systems ? ?
 A distributed system is a piece of software that
ensures that:
 a collection of independent computers appears
to its users as a single coherent system.
 Two aspects:
 (1) independent computers and (2) single
system => middleware.

8. EXAMPLES
 World Wide Web (WWW) is the biggest
example of distributed system.
 Others are
 The internet
 An intranet which is a portion of the internet
managed by an organization

9. WHY DISTRIBUTED
SYSTEMS ? ?
 availability of powerful yet cheap
microprocessors (PCs, workstations),
 continuing advances in communication
technology

10. ADVANTAGES OF D.S. OVER
CENTRALIZED SYSTEM:
 Economics:
A collection of microprocessors offer a better
price/performance than mainframes. Low
price/performance ratio: cost effective way to
increase computing power.
 Reliability:
• If one machine crashes, the system as a whole
can still survive. Higher availability and
improved reliability.

11. ADVANTAGES (Contd.)
Speed: a distributed system may have more
total computing power than a mainframe.
Ex.: 10,000 CPU chips, each running at 50
MIPS. Not possible to build 500,000 MIPS
single processor.
Enhanced performance through load
distributing.

12. ADVANTAGES (Contd.)
 Incremental growth: Computing power
can be added in small increments. This
leads to Modular expandability

13. ADVANTAGES OF D.S. OVER
INDEPENDENT PCs:
 Data sharing: allow many users to access
to a common data base.
 Resource Sharing: expensive peripherals
like color printers.

14. ADVANTAGES (Contd.)
 Communication: enhance human-to-
human communication. E.g.: email, chat.
 Flexibility: spread the workload over the
available machines

15. ORGANIZATION OF D.S.:
A distributed system organized as middleware.
−The middleware layer extends over multiple machines, and offers each
application the same interface.

16. GOALS OF D.S. :
− Resource Sharing.
− Openness.
− Transparency.
− Scalability.
− Concurrency.

17. RESOURCE SHARING:
 With Distributed Systems, it is easier for users to
access remote resources and to share resources
with other users.
− Examples: printers, files, Web pages, etc
 A distributed system should also make it easier for
users to exchange information.
 Easier resource and data exchange could cause
security problems – a distributed system should
deal with this problem.

18. OPENNESS:
 The openness of DS is determined primarily
by the degree to which new resource-
sharing services can be added and be made
available for use by a variety of client
programs.

19. TRANSPARENCY:
 It hides the fact that the processes and
resources are physically distributed across
multiple computers.
 Transparency is of various forms as
follows:

20. TRANSPARENCY (Contd.)

21. SCALABILITY:
 A system is described as scalable if it
remains effective when there is a significant
increase in the number of resources and the
number of users.
 Challenges:
 Controlling the cost of resources or money.
 Controlling the performance loss.

22. CONCURRENCY:
 There is a possibility that several clients
will attempt to access a shared resource at
the same time.
 Any object that represents a shared resource
in a distributed system must be responsible
for ensuring that operates correctly in a
concurrent environment.

23. TYPES OF D.S. :
 Distributed Computing Systems.
− Cluster Computing Systems.
− Grid Computing Systems.
 Distributed Information Systems.
 Distributed Pervasive Systems.

24. DISTRIBUTED COMPUTING
SYSTEMS:
 Goal: High performance computing tasks.
Cluster Computing Systems:
− A “supercomputer” built from “off the
shelf” computer in a high-speed network
(usually a LAN)
− Most common use: a single program is run
in parallel on multiple machines

25. (Contd.)
Grid Computing Systems:
− Contrary to clusters, grids are usually
composed of different types of computers
(hardware, OS, network, security, etc.)
− Resources from different organizations are
brought together to allow collaboration
− Examples: SETI@home, WWW…

26. DISTRIBUTED
INFORMATION SYSTEMS:
 Goal: Distribute information across several
Servers.
− Remote processes called Clients access the
servers to manipulate the information
− Different communication models are used.
The most usual are RPC (Remote Procedure
Calls) and the object oriented RMI (Remote
Method Invocations)

27. (Contd.)
− Often associated with Transaction systems
− Examples:
 Banks;
 Travel agencies;
 Rent-a-Cars’;
 Etc…

28. DISTRIBUTED PERVASIVE
SYSTEMS:
− These are the distributed systems involving mobile
and embedded computer devices like Small,
wireless, battery-powered devices (PDA’s, smart
phones, sensors, wireless surveillance cams,
portable ECG monitors, etc.)
− These systems characterized by their “instability”
when compared to more “traditional” distributed
systems

29. (Contd.)
− Pervasive Systems are all around us, and ideally
should be able to adapt to the lack of human
administrative control:
 Automatically connect to a different network;
 Discover services and react accordingly;
 Automatic self configuration (E.g.: UPnP –
Universal Plug and Play)…
− Examples: Home Systems, Electronic Health Care
Systems, Sensor Networks, etc.

30. SUMMARY
 Distributed systems are everywhere
 Internet, intranet, wireless networks.
 Resource sharing is the main motivating
factor for constructing distributed systems.
 The construction of distributed systems
produces many challenges like Secure
communication over public networks.