1. Characterization of Distributed Systems
Source:
George Colouris, Jean Dollimore, Tim Kinderberg & Gordon Blair (2012). Distributed Systems: Concepts & Design (5th Ed.). Essex: Addison-Wesley

2. Contents
2.1 Introduction 2.2 Examples of distributed systems 2.3 Trends in distributed systems 2.4 Focus on resource sharing 2.5 Challenges 2.6 Case Study: The World Wide Web 2.7 Summary

3. Contents
2.1 Introduction 2.2 Examples of distributed systems 2.3 Trends in distributed systems 2.4 Focus on resource sharing 2.5 Challenges 2.6 Case Study: The World Wide Web 2.7 Summary

4. 2.1 Introduction Networks? Everywhere
E.g. : Internet, mobile phone networks, corporate networks, factory networks, campus networks, home networks, in-car networks, etc.
What are the differences and similarities of these networks?

5. 2.1 Introduction What are distributed systems?
Systems that have hardware or software components located at networked computers
Communication and coordination of actions are done by passing messages
Networked computers may separated by any distance
Separate continents, buildings, rooms
OR may be in the same room

6. 2.1 Introduction Features of distributed systems: Concurrency
Concurrent program execution
No global clock
Coordination of actions
Independent failures
Failure in a machine would not known by other machines
Concurrent program execution: parallel execution of multiple programs.
Coordination of actions: exchange message between machines in order to access remote resources
Failure in a machine would not known by other machines: Terminated programs due to issues in other components of a machine is not immediately made known to the communicated components.

7. 2.1 Introduction Motivation for distributed systems: Resource sharing
E.g. hardware components, software entity

8. Contents
2.1 Introduction 2.2 Examples of distributed systems 2.3 Trends in distributed systems 2.4 Focus on resource sharing 2.5 Challenges 2.6 Case Study: The World Wide Web 2.7 Summary

9. 2.2 Examples of distributed systems
Finance and commerce
eCommerce e.g. Amazon and eBay, PayPal, online banking and trading
The information society
Web information and search engines, ebooks, Wikipedia; social networking: Facebook and MySpace.
Creative industries and entertainment
online gaming, music and film in the home, user-generated content, e.g. YouTube, Flickr
Healthcare
health informatics, on online patient records, monitoring patients
Education
e-learning, virtual learning environments; distance learning
Transport and logistics
GPS in route finding systems, map services: Google Maps, Google Earth
Science
The Grid as an enabling technology for collaboration between scientists
Environmental management
sensor technology to monitor earthquakes, floods or tsunamis

10. 2.2 Examples of distributed systems
Web search engine
An important tool for Internet users
Index the www contents (web pages, multimedia sources, e-books, etc.
How many pages of pages do the web contain?
Over 63 billion
1 trillion unique web addresses

11. 2.2 Examples of distributed systems
Massively multiplayer online games (MMOGs)
Large number of users interact through the Internet
E.g. Sony Ever Quest II, EVE online
Fast response time, real-time propagation of event

12. 2.2 Examples of distributed systems
Financial trading
Real-time access to a wide range of information access (e.g. current share prices and trends, economic and political development)
Example, Reuters & Financial Information eXchange (FIX) events

13. 2.2 Examples of distributed systems
An example of financial trading systems

14. Contents
2.1 Introduction 2.2 Examples of distributed systems 2.3 Trends in distributed systems 2.4 Focus on resource sharing 2.5 Challenges 2.6 Case Study: The World Wide Web 2.7 Summary

15. 2.3 Trends in distributed systems
Top Technology Trends for 2014

16. 2.3 Trends in distributed systems
The Fruits of Innovation: Top 10 IT Trends in 2014

17. 2.3 Trends in distributed systems
Significant changes in current distributed systems:
The emergence of pervasive technology
The emergence of ubiquitous & mobile computing
The increasing demand of multimedia technology
The view of distributed systems as a utility

18. 2.3 Trends in distributed systems
Pervasive technology
Modern Internet – Collection of internetworked devices- wired & wireless
Pervasive resources and devices can be connected at any time and in any place

19. 2.3 Trends in distributed systems
intranet
ISP
desktop computer:
backbone
satellite link
server: ☎
network link:
- This figure shows a typical portion of the Internet. Computers in the network interact by passing messages.
- The Internet Protocol (IP) enables programs running anywhere to address messages to programs anywhere.
The Internet is an example of a very large distributed system.
The figure also shows collections of Intranets ( a sub-network that operated by organizations)
A backbone is a network link with a high transmission capacity, utilizing satellite connections, fibre optics and other high-bandwidth circuits.
A typical portion of the Internet

20. 2.3 Trends in distributed systems
Mobile & ubiquitous computing
Small and portable devices are possible to be used within distributed systems
E.g. laptop computers, handheld devices, wearable devices, devices embedded in appliances
Mobile computing: portability of the devices and the ability to connect to networks in different places
Ubiquitous computing: small computing devices that available everywhere and are easily attached to networks

21. 2.3 Trends in distributed systems
This figure shows how a user can access resources at office network via his/her home network
Portable & handheld devices in a distributed system

22. 2.3 Trends in distributed systems
Distributed multimedia systems
The use of multimedia contents in distributed systems
Multimedia support
Major benefits of multimedia support
Distributed multimedia computing can be accessed through desktop or mobile devices. E.g. live tv broadcast, video-on-demand, IP telephony, webcasting, etc.

23. 2.3 Trends in distributed systems
Distributed computing as a utility
distributed resources as commodity or utility in similar as water and power.
Physical and logical service resources are rented rather than owned by the end users.
Physical resources: e.g. : storage and processing
Logical services: e.g. , calendars
Cloud computing: distributed computing utility. A cloud is a set of internet-based application, storage and computing services sufficient to support most users’ needs

24. 2.3 Trends in distributed systems
The aim of cloud computing is to reduce requirement of end-user devices, allowing them to has all the required resources through simple desktop or handheld devices.
Cloud computing

25. 2.3 Trends in distributed systems
Cloud are implemented on cluster computers to provide the appropriate scale and performance required by such services
A cluster computer: a set of interconnected computers that cooperate closely to provide a single integrated high-performance computing capability
A blade server: a computer server that has been designed to minimize the use of physical space and energy

26. 2.3 Trends in distributed systems
Grid Computing
Is a form of cloud computing
Authorized users share processing power, memory and data storage
Use to support scientific applications

27. Contents
2.1 Introduction 2.2 Examples of distributed systems 2.3 Trends in distributed systems 2.4 Focus on resource sharing 2.5 Challenges 2.6 Case Study: The World Wide Web 2.7 Summary

28. 2.4 Focus on resource sharing
Sharing of high-level resources
Disks and processors
Sharing of search engines, tools –which servers host these applications?
Computer-supported cooperative working (CSCW)
A group of users who cooperate directly share resources such as documents in a small closed group.

29. 2.4 Focus on resource sharing
Service: a distinct part of a computer system that manages a collection of related resources and presents their functionality to users and applications.
E.g. File service, printing service, payment service
Service restricts resource access. A program is required to handle resource access for effective sharing .

30. 2.4 Focus on resource sharing
Client-server computing:
Server: A running program (a process) that accepts requests from programs running on other computers
Client: The requesting process
Client invokes an operation – client send a request for an operation to be performed
Remote invocation – complete interaction between a client and a server

31. Contents
2.1 Introduction 2.2 Examples of distributed systems 2.3 Trends in distributed systems 2.4 Focus on resource sharing 2.5 Challenges 2.6 Case Study: The World Wide Web 2.7 Summary

32. 2.5 Challenges
As distributed systems are getting complex, developers face a number of challenges:
Heterogeneity
Openness
Security
Scalability
Failure handling
Concurrency
Transparency
Quality of service

33. 2.5 Challenges Heterogeneity
Users access services and run applications on a heterogeneous collection of computers and networks in the Internet
Heterogeneity in the context of – networks; computer hardware, OS, programming languages, implementations by different developers
Internet protocols are used by these different entities to communicate to each other

34. 2.5 Challenges Heterogeneity
A standard that allows different entities to communicate efficiently
Middleware : a software layer that provides a programming abstraction and masking the heterogeneity of the underlying networks, hardware, OS and programming languages. E.g.– CORBA, RMI
Mobile code : a program code that can be transferred from one computer to another and run at the destination. E.g. Java Applet, Javascript

35. 2.5 Challenges
Openness
Openness- can the system be extended and re-implemented in different ways?
Key to openness  availability of software specification and documentation
Examples of specifications – Request for Comment (RFC)  provides technical specification and documentation about the systems
Open distributed systems – distributed systems that support resource sharing and extensible

36. 2.5 Challenges
Security
The security components of information resources
Confidentiality – protection against disclosure to unauthorized individuals
Integrity – protection against alteration or corruption
Availability - protection against interference with the means to access the resources
Examples of security challenges:
Denial of service attacks
Security of mobile code

37. 2.5 Challenges Scalability
How effective the system works when there is an increase number of users or increase number of resource access
The challenges of designing scalable distributed systems:
Controlling the cost of physical resources
Controlling the performance loss
Preventing software resources running out
Avoiding performance bottleneck

38. Growth of the Internet (computers and web servers)
2.5 Challenges
Date
Computers
Web servers
Percentage
1993, July
1,776,000
130
0.008
1995, July
6,642,000
23,500
0.4
1997, July
19,540,000
1,203,096 6
1999, July
56,218,000
6,598,697 12
2001, July
125,888,197
31,299,592 25
42,298,371
2003, July
2005, July
~200,000,000
353,284,187
67,571,581 21 19
Growth of the Internet (computers and web servers)

39. 2.5 Challenges
Number of Websites:
630 million web sites in January 2013
861 million in January 2014 (+37%)

40. 2.5 Challenges Failure handling
Distributed system failures are partial- some parts fail while others not
Dealing with failures in distributed systems:
Detecting failures – known/unknown failures
Masking failures – hide the failure from become severe. E.g. retransmit messages, backup of file data
Tolerating failures – clients can be designed to tolerate failures – e.g. inform users of failure and ask them to try later
Recovery from failures - recover and rollback data after a server has crashed
Redundancy- the way to tolerate failures – replication of services and data in multiple servers

41. 2.5 Challenges Concurrency
Access to a single service simultaneously/concurrently by multiple clients
services and applications must be designed to support concurrent access and work consistently

42. 2.5 Challenges Transparency
Concealment of services and applications, so that they can be seen as a whole system rather than a collection of independent objects or components
8 forms of transparency:
Access transparency – access to local an remote resources using identical operations
Location transparency – access to resources without knowing the physical location of the machine

43. 2.5 Challenges 8 forms of transparency:
Concurrency transparency – several processes operate concurrently without interfering each other
Replication transparency – replication of resources in multiple servers. Users are not aware of the replication
Failure transparency – concealment of faults, allows users to complete their tasks without knowing of the failures
Mobility transparency – movement of resources and clients within a system without affecting users operations
Performance transparency – systems can be reconfigured to improve performance by considering their loads
Scaling transparency – systems and applications can be expanded without changing the structure or the application algorithms

44. 2.5 Challenges
Access and location transparency are the two most important – also known as network transparency
Web resource names or URLs are location-transparent. The names refer to a computer name (web server) in a domain rather than to Internet addresses. However, they are not mobility transparent. Web pages can be moved to other server in the domain. Links are points to the original page.

45. 2.5 Challenges Quality of service
The main nonfunctional properties of distributed systems that affect the quality of service experienced by users or clients are: reliability, security, performance, adaptability.
Reliability?
Security?
Performance ?
Adaptability?

46. Contents
2.1 Introduction 2.2 Examples of distributed systems 2.3 Trends in distributed systems 2.4 Focus on resource sharing 2.5 Challenges 2.6 Case Study: The World Wide Web 2.7 Summary

47. 2.6 Case Study: The World Wide Web
History & background
www (web) & the Internet
CERN (European Center for Nuclear Research), Switzerland
Document exchange
Technological components of web:
HTML
URL
HTTP
Advances in web – dynamic pages, web services

48. 2.6 Case Study: The World Wide Web
Internet
Browsers
Web servers
standards
faq.html
File system of

49. Contents
2.1 Introduction 2.2 Examples of distributed systems 2.3 Trends in distributed systems 2.4 Focus on resource sharing 2.5 Challenges 2.6 Case Study: The World Wide Web 2.7 Summary

50. 2.7 Summary Important points we have discussed: Resource sharing
Challenges in developing distributed systems
www

51. Group discussion Select TWO(2) popular web search engines
Compare how many web pages and documents are indexed by the selected search engines.
How long it takes to search for the following phrases and how many related web documents are found?
Distributed systems
Distributed systems projects
distributed systems projects and assignments
What is your conclusion?

52. Further information Top 15 Most Popular Search Engines in March 2014
Google Platform & how it works

53. Thank you…