1. Distributed Systems
Lecture#25

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.
Figure 1-1. A distributed system organized as middleware. The middleware layer extends over multiple machines, and offers each application the same interface. 7

8. Distributed computing
Distributed computing deals with software systems containing more than one processing element or storage element, concurrent processes, or multiple programs, running under a loosely or tightly controlled regime.
In distributed computing a program is split up into parts that run simultaneously on multiple computers communicating over a network.

9. Distributed computing
Distributed computing is a form of parallel computing

10. 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

11. PROBELEM INSTRUCTION SET
Distributed computing
In distributed computing a program is split up into parts that run simultaneously on multiple computers communicating over a network
PROBELEM INSTRUCTION SET
TASK 2 T A S K 5
TASK 4
TASK 5
TASK 1
T 2 T4 T3 T5 T1

12. Distributed computing
Consider If There Are n Systems Connected In A Network, Then We Can Split One Program Instruction Into n Different Tasks And Compute Them Concurrently.

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

14. 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.

15. 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.

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

17. 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.

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

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

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

21. 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.

22. 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.

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

24. TRANSPARENCY (Contd.)
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 24

25. 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.

26. 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.

27. TYPES OF D.S. : Distributed Computing Systems.
Cluster Computing Systems.
Grid Computing Systems.
Distributed Information Systems.
Distributed Pervasive Systems.

28. 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

29. (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: WWW…

30. 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)

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

32. 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

33. (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.

34. DCS Models Minicomputer Model Workstation Model
Workstation Server Model
Processor Pool Model
Hybrid Model

35. Minicomputer Model (cont. …)
Extension of the centralized time sharing system.
few minicomputers
Remote access to other minicomputers.
resource sharing with remote users is desired.

36. Minicomputer Model (Cont. …)
The distributed system based on minicomputer model

37. Workstation Model
Consists of several workstations.
The workstations are independent computers with memory, hard disks, keyboard and console.
A company's office or a university department may have several workstations scattered throughout a building or compass.
User logs onto one of the workstations called his or her home workstation

38. Workstation Model (Cont. …)
A distributed system based on the workstation model

39. Workstation Model (Cont. …)
This model is not so simple to implement because several issues must be resolved. These issues are as follows:
Idle workstation
How is a process transferred from one workstation to get it executed on another workstation
a workstation that was idle until now and was being used

40. Workstation-Server Model
few minicomputers and several workstations interconnected
Diskfull Workstation
Diskless Workstation
types of services, such as database service and print service

41. Workstation-Server Model (Cont. …)
Fig 3: A Distributed System based on the workstation-server model

42. Advantages Workstation Server Model
Workstation-Server Model (Cont. …)
Advantages Workstation Server Model
Cheaper
Diskless workstations are also preferred to dishful workstations from a system maintenance point of view.
Users have the flexibility to use any workstation
The request-response protocol
Guaranteed response time .

43. Processor Pool Model large number of microcomputer and minicomputers
based on the observation that most of the time a user does not need any computing power
A user submits a Job for computations are temporarily assigned to the job by the run server
the entire processing power is available for use by the currently logged-users

44. Processor Pool Model (Cont. …)
Figure 4: A distributed computing system based on processor-poor model

45. Hybrid Model
Based on the workstation-server model but with the addition of a pool of processors.
Requires several computers concurrently for efficient execution.
Gives guaranteed response to interactive jobs by allowing them to be processed on local workstations of the users.
Process allocated dynamically for the computations.
More expensive.

46. 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.

47. SUMMARY
The concept of distributed computing is the most efficient way to achieve the optimization.
It deals with systems(hardware and software) , that contain more than one processing / storage and run in concurrently.