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Distributed Systems. Multiple software components on multiple computers running as a single system. Computers can be close or distant Homogeneous or heterogeneous Managed as a unit or disparate

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Distributed Systems

  • Multiple software components on multiple computers running as a single system.

    • Computers can be close or distant

    • Homogeneous or heterogeneous

    • Managed as a unit or disparate

  • Distribution will only grow as networks and computing devices become even more ubiquitous, wireless

Distributed Systems


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Distributed SystemsExamples

  • Domain Name service (DNS)

  • World Wide Web

  • Campus-wide File Systems

  • Email services: POP3, IMAP, MS Exchange

  • ATM networks

  • Electronic stock-trading “floors”

  • “Smart” house

Distributed Systems


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Distributed SystemsFeatures

  • Resource Sharing

    • Of disks, printers, software packages, etc.

  • Openness

    • Supports addition of new (non-proprietary) resources; heterogeneous components

  • Concurrency

    • Multiple communicating processes

Distributed Systems


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Distributed SystemsFeatures

  • Scalability

    • Capacity can be “easily” increased

  • Fault Tolerance

    • Continued operation in the face of limited (hw/sw) failure

  • Transparency

    • Distribution practically invisible to users

Distributed Systems


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Distributed SystemsDisadvantages

  • Complexity

    • Harder to test

    • System performance depends on network, relative speed of computers, resource location relative to computers

    • Additional design issues

Distributed Systems


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Client-server Architectures

  • Client processes make requests of server processes

  • Can often be structured in (three) layers

    • Presentation

    • Application processing

    • Data management

  • See Section 11.2 of Sommerville for details

Distributed Systems


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Client-server Architectures

  • Two-tier (c-s architecture)

    • Thin client

      • Presentation only

    • Fat client

      • Presentation and processing

Distributed Systems


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Two-tier ArchitecturesThin or Fat Clients?

  • Thin clients

    • Increases load on server, network

    • Doesn’t scale as well

  • Fat clients

    • Reduce processing load on server

    • Increases load on clients

    • Harder to manage; applications often evolve

Distributed Systems


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“Two tiers good, three tiers better”

  • Three-tier (c-s architecture)

    • Distinct processes for each layer (client-server-server)

    • Server processes easily split onto distinct processors, if/when necessary

    • More scalable (can replicate processing servers)

    • E.g. database-backed web service (flight search/reservation system, web banking)

Distributed Systems


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Web browser

Web browser

Web server

Web server

Web browser

Web browser

Database-based WWW service

Web browser

DBMS

Web server

Web browser

Web browser

Distributed Systems


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Peer-to-peer Architecture

  • Communication among the peers (“client” no longer appropriate) encouraged

  • Pure Peer-to-Peer: no dedicated server processes at all

  • Hybrid examples: Napster, ICQ, AIM

  • Pure examples: Gnutella, Morpheus

Distributed Systems


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Distributed Object Architectures

  • Object-oriented design

    • No client/server distinction

    • System as a collection of distributed objects

    • Objects map to processes, not (necessarily) processors

    • Communication via middleware (a “software bus”)

Distributed Systems


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Distributed Object ArchitecturesMiddleware

  • CORBA† (Common Object Request Broker Architecture)

  • Java RMI (Remote Method Invocation)

  • DCOM (Distributed Component Object Model)

  • .NET

    †See Section 11.4 of Sommerville for details

Distributed Systems


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