1. Chapter 1 Characterization of Distributed Systems
Distributed Systems: Concepts and Design
Edition 5, © Addison-Wesley 2012

2. DISTRIBUTED SYSTEMS (Essential Issue)
A distributed system is: A collection of independent computers that appears to its users as a single coherent system. Another Definition A distributed system is a collection of autonomous computers linked by a computer network that appear to the users of the system as a single computer.

3. Distributed Systems Multiple independent computers that appear as one
Introduction
Distributed Systems
Multiple independent computers that appear as one
Lamport’s Definition
“ You know you have one when the crash of a computer you have never heard of stops you from getting any work done.”
“A number of interconnected autonomous computers that provide services to meet the information processing needs of modern enterprises.”
Distributed Systems

4. Key Issue
Concurrency: In a network of computers, concurrent program execution is the norm. No global clock: When programs need to cooperate they coordinate their actions by exchanging messages. (a direct consequence of the fact that the only communication is by sending messages through a network.) Independent failures: Faults in the network result in the isolation of the computers that are connected to it, but that doesn’t mean that they stop running.

5. Characteristics of DS
How it works is hidden from user. Interaction is consistent & uniform Scalability Continuously available, even if some parts are out of order

6. DS Layered Architecture
Commonly implemented through layers & middleware

7. Make Resources Available Hide the fact that resources are distributed
DS Goals
Make Resources Available
Hide the fact that resources are distributed
Distribution Transparency
Be Open
Be Scalable
- coordinate their activities
- share resources: hardware, software, data.

8. Make Resources Available
E.g. Printers, storage facilities, data, files, webpages, networks etc.
For economic reasons
For collaboration reasons
To create virtual organisations
This produces challenges
Security
Privacy

9. Examples of Distributed Systems
Transactional applications - Banking systems
Manufacturing and process control
Inventory systems
General purpose (university, office automation)
Communication – , IM, VoIP, social networks
Distributed information systems
WWW
Cloud Computing Infrastructures
Federated and Distributed Databases
Distributed Systems

10. Selected application domains and associated networked applications
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
Instructor’s Guide for Coulouris, Dollimore, Kindberg and Blair, Distributed Systems: Concepts and Design Edn © Pearson Education 2012

11. An example financial trading system
Instructor’s Guide for Coulouris, Dollimore, Kindberg and Blair, Distributed Systems: Concepts and Design Edn © Pearson Education 2012

12. A typical portion of the Internet
intranet
ISP
desktop computer:
backbone
satellite link
server: ☎
network link:
Instructor’s Guide for Coulouris, Dollimore, Kindberg and Blair, Distributed Systems: Concepts and Design Edn © Pearson Education 2012

13. Portable and handheld devices in a distributed system
Instructor’s Guide for Coulouris, Dollimore, Kindberg and Blair, Distributed Systems: Concepts and Design Edn © Pearson Education 2012

14. Cloud computing
Instructor’s Guide for Coulouris, Dollimore, Kindberg and Blair, Distributed Systems: Concepts and Design Edn © Pearson Education 2012

15. Growth of the Internet (computers and web servers)
Date
Computers
Web servers
Percentage
1,776,000
130
0.008
1993, July
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
2003, July
~200,000,000
42,298,371 21
2005, July
353,284,187
67,571,581 19
Instructor’s Guide for Coulouris, Dollimore, Kindberg and Blair, Distributed Systems: Concepts and Design Edn © Pearson Education 2012

16. Advantages of Distributed Systems
Performance: very often a collection of processors can provide higher performance (and better price/performance ratio) than a centralized computer. Distribution: many applications involve, by their nature, spatially separated machines (banking, commercial, automotive system). Reliability (fault tolerance): if some of the machines crash, the system can survive. Incremental growth: as requirements on processing power grow, new machines can be added incrementally. Sharing of data/resources: shared data is essential to many applications (banking, computer supported cooperative work, reservation systems); other resources can be also shared (e.g. expensive printers).

17. Disadvantages of Distributed Systems
Difficulties of developing distributed software: how should operating systems, programming languages and applications look like? Networking problems: several problems are created by the network infrastructure, which have to be dealt with: loss of messages, overloading, ... Security problems: sharing generates the problem of data security.

18. Distributed Systems Challenges
Design issues that arise specifically from the distributed nature of the application: • Transparency • Communication • Performance & scalability • Heterogeneity • Openness • Reliability & fault tolerance • Security

19. Transparency in a Distributed System
An important goal of distributed systems is to hide the fact that processes / resources are physically distributed Enabling users to use the system without worrying about where the resources are.
Transparency
Description
Access
Hide differences in data representation and how a resource is accessed
Location
Hide where a resource is located
Migration
Hide that a resource may move to another location
Relocation
Hide that a resource may be moved to another location while in use
Replication
Hide that a resource may be shared by several competitive users
Concurrency
Failure
Hide the failure and recovery of a resource
Persistence
Hide whether a (software) resource is in memory or on disk

20. Transparencies
Access transparency: enables local and remote resources to be accessed using identical operations.
Location transparency: enables resources to be accessed without knowledge of their physical or network location (for example, which building or IP address).
Concurrency transparency: enables several processes to operate concurrently using shared resources without interference between them.
Replication transparency: enables multiple instances of resources to be used to increase reliability and performance without knowledge of the replicas by users or application
programmers.
Failure transparency: enables the concealment of faults, allowing users and application programs to complete their tasks despite the failure of hardware or software components.
Mobility transparency: allows the movement of resources and clients within a system
without affecting the operation of users or programs.
Performance transparency: allows the system to be reconfigured to improve performance as loads vary.
Scaling transparency: allows the system and applications to expand in scale without change
to the system structure or the application algorithms.
Instructor’s Guide for Coulouris, Dollimore, Kindberg and Blair, Distributed Systems: Concepts and Design Edn © Pearson Education 2012

21. Access Transparency
Different Resources may represent data in different formats, but this shouldn’t be an issue for the user.
A user on an Intel workstation sending data to a Sun SPARC machine, shouldn’t be concerned that Intel orders its bytes by little endian format (high order bytes first) while SPARC uses big endian format (low order bytes first).
Different file naming formats should also not be of concern to the user. ‘/’ or ‘\’.

22. Location Transparency
Location Transparency refers to the physical position of a resource, which should be hidden from the user. This is normally achieved through naming, where normally only logical names are used;
Where is it (physically)?
Has it always been there?

23. Migration / Relocation Transparency
In the previous web address, you have no idea whether index.html has always been on the cis.payap.ac.th server, or when it might have moved there. If resources can be moved without affecting the way the resource is accessed then migration transparency is provided. If that movement occurs while the resource is being accessed, then relocation transparency is provided. Consider moving around using a wireless laptop.

24. Replication Transparency
The efficiency of distributed systems can be improved greatly by locating replicas (copies) of a resources physically closer to a user. Replication transparency enables the system to do this, without the user knowing they are using a replica.

25. Concurrency Transparency
A goal of distributed systems is often sharing of resources between users. These users may wish to access or even update the same data at the same time (concurrently). An important challenge when designing distributed systems is how to deal with concurrent accesses.
How to maintain consistency when different users use the same resource in different ways.

26. Failure Consistency tries to mask failures such as this.
“You know you have one when the crash of a computer you’ve never heard of stops you from getting any work done!”
Failure Consistency tries to mask failures such as this.
It is difficult to identify between a resource that has failed and a resource which is performing badly (slowly).
Consider opening a webpage - is it dead or painfully slow, how long should the browser wait?

27. Complete Transparency?
Complete Transparency isn’t always completely necessary.
E.g. daily newspaper arriving at 7am regardless of location in the world.
Nor is it always possible.
Physics behind signal transmission.

28. Scalability
A further goal of Distributed Systems is that they should be scalable - that is that they can grow;
Scalable by size; more users or resources can be added to the system.
Scalable by location; resources and users may be physically distant.
Scalable by administration; system can be easily manageable as it grows.

29. Scalability
One problem often encountered when dealing with scalability is dealing with centralisation.
Centralised services
Centralised data
Centralised algorithms
Imagine how the internet would work if there was only one single DNS table, and every address resolution request had to be directed through that computer.

30. Scalability
Another problem affecting scalability concerns whether synchronous communication is actually possible.
Many existing systems were designed for synchronous communication.
The laws of physics (including the speed of light), limits the speed of communication between physically distant resources.
Leaving a ‘client’ blocked until a reply is sent back.

31. Scalability & Administration
What happens when a system needs to scale across multiple, independent adminstrative domains?
Conflicting policies
Resource Usage
Management
Security

32. Solving Scalability (briefly & currently)
Hiding Communication Latencies
Essentially asynchronous communication. Not waiting for a reply, instead creating a special handler (thread) to complete previous requests.
Distribution
Splitting a component into smaller parts – e.g. DNS, splits .com, .th, .edu etc.
Replication
For example caching. A copy of the data closer to the request.

33. Replication & Scalability
Replication can have a downside effect on Scalability
Consistency Problems
How big a problem is this?

34. Examples of scalability limitations.
Scalability Problems
Concept
Example
Centralized services
A single server for all users
Centralized data
A single on-line telephone book
Centralized algorithms
Doing routing based on complete information
Examples of scalability limitations.

35. The difference between letting: a server or
Scaling Techniques (1)
1.4
The difference between letting:
a server or
a client check forms as they are being filled

36. An example of dividing the DNS name space into zones.
Scaling Techniques (2)
1.5
An example of dividing the DNS name space into zones.

37. Communication
Components of a distributed system have to communicate in order to interact. This implies
support at two levels:
1. Networking infrastructure (interconnections & network software).
2. Appropriate communication primitives and models and their implementation:
• communication primitives:
- send
- receive
- remote procedure call (RPC)
• communication models
- client-server communication: implies a message exchange between two processes: the process which requests a service and the one which provides it;
- group muticast: the target of a message is a set of processes, which are members of
a given group.

38. Case Study Web servers and web browsers
Internet
File system of
standards
faq.html
Instructor’s Guide for Coulouris, Dollimore, Kindberg and Blair, Distributed Systems: Concepts and Design Edn © Pearson Education 2012