This document provides an outline on distributed systems. It discusses the brief history of distributed systems, defines what they are, and lists their advantages over centralized and independent systems. It also describes the goals of distributed systems including resource sharing, openness, transparency and scalability. Finally, it outlines different types of distributed systems such as computing systems, information systems, and pervasive systems.
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.
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).
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.
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.
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:
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.
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.
Editor's Notes
Figure 1-1. A distributed system organized as middleware. The middleware layer extends over multiple machines, and offers each application the same interface.
Middleware is responsible for hiding the underlying details of each computer while providing a unified view of services to the user. .distributed system is organized as middleware to hide the heterogeneity….
Figure 1-2. Different forms of transparency in a distributed system (ISO, 1995).
location = Hides where an object resides. Users cannot tell where hardware and software resources such as CPUs, printers, files, data bases are located.
Migration= Hides from an object the ability of a system to change that object’s location. Resources must be free to move from one location to another without their names changed. E.g., /usr/lee, /central/usr/lee
Relocation =Hides from a client the ability of a system to change the location of an object to which the client is bound
Replication =Hides the fact that an object or its state may be replicated and that replicas reside at different locations. OS can make additional copies of files and resources without users noticing.
Concurrency =Hides the coordination of activities between objects to achieve consistency at a higher level. The users are not aware of the existence of other users. Need to allow multiple users to concurrently access the same resource. Lock and unlock for mutual exclusion.
Failure =Hides failure and possible recovery of objects