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Introduction to DISTRIBUTED COMPUTING

Tran, Van HoaiDepartment of Systems & Networking

Faculty of Computer Science & EngineeringHCMC University of Technology

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Outline

• Why distributed computing needed ?– performed by distributed systems

• Examples• Definitions• Goals to build distributed systems

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Why distributed systems needed ? (1)

• Functional distribution: computers have different functional capabilities– Client/server– Host/terminal– Data gathering/data processing

sharing of resources with specific functionalities• Inherent distribution: stemming from application

domain, e.g.,– cash register and inventory systems for supermarket chains– computer supported collaborative work

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Why distributed systems needed ? (2)

• Load distribution/balancing: assign tasks to computers such that overall performance is optimized

• Replication of processing power: independent computers working on the same task– collection of microcomputers may have processing

power that no supercomputer will ever achieve

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Why distributed systems needed ? (3)

• Physical separation: relying on the fact that computers are physically separated (e.g., to satisfy reliability requirements)

• Economics: collections of microprocessors offer a better price/performance ratio than large mainframes– mainframes: 10 times faster, 1000 times as

expensive

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Examples (1)

• Network of workstations– all files accessible from all machines in the same way and

using the same path name– system looks for the best place to execute a command

distributed system• Workflow information system: automatic order

processing– people from several departments at different locations– users unaware how an order to be processed

distributed system

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Examples (2)

• World Wide Web: offering uniform model of distributed documents– in theory, no need to know where the document is

fetched– in practice, the location should be awared

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Examples (3)

• Internet

intranet

ISP

desktop computer:

backbone

satellite link

server:

%

network link:

%

%

%

• interconnected collection of computer networks of many different types• computer interacts by passing messages using a common means of communication

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Examples (4)

• Intranet

the rest of

email server

Web server

Desktopcomputers

File server

router/firewall

print and other servers

other servers

print

Local areanetwork

email server

the Internet • resources shared to different computers

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Definitions (1)

“A system in which hardware or software located at networked computers communicate and coordinate their actions only by message passing”.[Coulouris]

“A system that consists of a collection of two or more independent computers which coordinate their processing through exchange of synchronous or asynchronous message passing”.

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Definitions (2)

“A distributed system is a collection of independent computers that appear to the users of the system as a single computer”.[Tanenbaum]

“A distributed system is a collection of autonomous computers linked by a network with software designed to produce an integrated computing facility”.

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Definitions (3)

• There are several autonomous computational entities, each of which has its own local memory.[Andrews et al]

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Computer networks vs. Distributed systems

• Computer network: autonomous computers are explicitly visible (have to be explicitly addressed)

• Distributed system: existence of multiple computers is transparent

• However,– many problems in common– in some sense networks (or parts of them, e.g. name

services) are also distributed systems– normally, every distributed system relies on services

provided by a computer network

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Which examples are distributed systems ?

• Network of workstationsdistributed system

• Workflow information system: automatic order processing

distributed system• World Wide Web

not fully qualified as a distributed system (Tanenbaum)distributed system (Coulouris)

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Machine A

Local OS

Machine B

Local OS

Machine C

Local OS

Distributed applications

Middleware service

Middleware service

• To guarantee– supporting heterogeneous computers– providing single view to users

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Goals to build a distributed systems (1)

• Connecting users and resources– sharing resource– easier to collaborate and exchange information

disadvantage: security (intrusion), privacy violation (communication tracking)

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Goals to build a distributed systems (2)

• TransparencyTransparency 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 have many copies

Concurrency Hide that a resource may be shared by several competitive users

Failure Hide the failure and recovery of a resource

Persistence Hide whether a (software) resource is in memory or on disk

tradeoff between a high degree of transparency and the performance of system

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Goals to build a distributed systems (3)

• Openness– Offering services according to standard rules that

describe syntax and semantics of those services• syntax specification: in interface definition language• semantic specification: in natural language

– Interoperability and portability– Flexibility: using different components from

different developers

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Goals to build a distributed systems (4)

• Scalability– Measured in three dimensions

• size: more users, resources can be added easily• geographics: users, resources may lie far apart• administration: still easy to manage even spanning

many independent administrative organizations

– Some problems must be solved• size: centralization

– centralized service: single server for all users– centralized data: single online telephone book– centralized algorithm: routing based on complete information

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Goals to build a distributed systems (5)

• size: centralization– centralized service: single server for all users– centralized data: single online telephone book– centralized algorithm: routing based on complete information

• geographics: synchronous & unreliable communication, – some system only designed for LAN (blocking communication

depends strongly on quick response)

• administration: conflicting policies w.r.t. resource usage, management, security

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Scaling techniques

• Asynchronous communication• Distribution• Replication, caching

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Typical properties

• tolerate failures in individual computers• The structure of the system (network

topology, network latency, number of computers) is not known in advance

• Each computer has only a limited, incomplete view of the system

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Architectures

• Client-server: – permanent data on server

• 3-tier architecture:– stateless client, – N-tier: web applications

• Tightly-coupled (clustered): – NOW, cluster of machines

• Peer-to-peer– Grid computing (VO level)

• Space-based– virtualization as one single address-space

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source:wikipedia.org

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Some numbers (1)

• Computers in the Internet

Date Computers Web servers

1979, Dec. 188 0

1989, July 130,000 01999, July 56,218,000 5,560,8662003, Jan. 171,638,297 35,424,956

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Some numbers (2)

• Computers vs. Web servers in the Internet

Date Computers Web servers Percentage

1993, July 1,776,000 130 0.0081995, July 6,642,000 23,500 0.41997, July 19,540,000 1,203,096 61999, July 56,218,000 6,598,697 122001, July 125,888,197 31,299,592 25

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Text books & materials

• Andrew S. Tanenbaum, Maaten Van Steen, Distributed Systems: Principles and Paradigms, Prentice Hall, Second Edition, 2007

• George Coulouris, Jean Dollimore, Tim Kindberg, Distributed Systems: Concepts and Design, Addison Wesley, Fourth Edition, 2005

• Google