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Introduction To Distributed Systems

Introduction to Distributed Systems provides students of computer science and engineering with the skills they will need to design and maintain software for distributed applications. It will also be invaluable to software engineers and systems designers wishing to understand new and future developments in the field.

From mobile phones to the Internet, our lives depend increasingly on distributed systems linking computers and other devices together in a seamless and transparent way.

Introduction To Distributed Systems

A distributed system is a software system in which components located on networked computers communicate and coordinate their actions by passing messages. The components interact with each other in order to achieve a common goal.

Distributed systems Principles
A distributed system consists of a collection of autonomous computers, connected through a network and distribution middleware, which enables computers to coordinate their activities and to share the resources of the system, so that users perceive the system as a single, integrated computing facility.

Centralised System Characteristics

  • One component with non-autonomous parts
  • Component shared by users all the time
  • All resources accessible
  • Software runs in a single process
  • Single Point of control
  • Single Point of failure

Distributed System Characteristics

  • Multiple autonomous components
  • Components are not shared by all users
  • Resources may not be accessible
  • Software runs in concurrent processes on different processors
  • Multiple Points of control
  • Multiple Points of failure

Examples of distributed systems and applications

  • telecommunication networks:
  • telephone networks and cellular networks,
  • computer networks such as the Internet,
  • wireless sensor networks,
  • routing algorithms;
  • network applications:
  • World wide web and peer-to-peer networks,
  • massively multiplayer online games and virtual reality communities,
  • distributed databases and distributed database management systems,
  • network file systems,
  • distributed information processing systems such as banking systems and airline reservation systems;
  • real-time process control:aircraft control systems,
  • industrial control systems;
  • parallel computation: scientific computing, including cluster computing and grid computing ; distributed rendering in computer graphics.

Common Characteristics

Certain common characteristics can be used to assess distributed systems

  • Resource Sharing
  • Openness
  • Concurrency
  • Scalability
  • Fault Tolerance
  • Transparency

Resource Sharing

  • Ability to use any hardware, software or data anywhere in the system.
  • Resource manager controls access, provides naming scheme and controls concurrency.
  • Resource sharing model (e.g. client/server or object-based) describing how;
    • resources are provided,
    • they are used and
    • provider and user interact with each other.

Openness

  • Openness is concerned with extensions and improvements of distributed systems.
  • Detailed interfaces of components need to be published.
  • New components have to be integrated with existing components.
  • Differences in data representation of interface types on different processors (of different vendors) have to be resolved.

Concurrency
Components in distributed systems are executed in concurrent processes.

  • Components access and update shared resources (e.g. variables, databases, device drivers).
  • Integrity of the system may be violated if concurrent updates are not coordinated.
    • Lost updates
    • Inconsistent analysis

Scalability

  • Adaption of distributed systems to
    • accommodate more users
    • respond faster (this is the hard one)
  • Usually done by adding more and/or faster processors.
  • Components should not need to be changed when scale of a system increases.
  • Design components to be scalable

Fault Tolerance
Hardware, software and networks fail!

  • Distributed systems must maintain availability even at low levels of hardware/software/network reliability.
  • Fault tolerance is achieved by
    • recovery
    • redundancy

Transparency
Distributed systems should be perceived by users and application programmers as a whole rather than as a collection of cooperating components.

  • Transparency has different dimensions that were identified by ANSA.
  • These represent various properties that distributed systems should have.

transparency

Access Transparency
Enables local and remote information objects to be accessed using identical operations.

  • Example: File system operations in NFS.
  • Example: Navigation in the Web.
  • Example: SQL Queries

Location Transparency
Enables information objects to be accessed without knowledge of their location.

  • Example: File system operations in NFS
  • Example: Pages in the Web
  • Example: Tables in distributed databases

Concurrency Transparency
Enables several processes to operate concurrently using shared information objects without interference between them.

  • Example: NFS
  • Example: Automatic teller machine network
  • Example: Database management system

Replication Transparency
Enables multiple instances of information objects to be used to increase reliability and performance without knowledge of the replicas by users or application programs

  • Example: Distributed DBMS
  • Example: Mirroring Web Pages.

Failure Transparency

  • Enables the concealment of faults
  • Allows users and applications to complete their tasks despite the failure of other components.
    • Example: Database Management System

Migration Transparency
Allows the movement of information objects within a system without affecting the operations of users or application programs

  • Example: NFS
  • Example: Web Pages

Performance Transparency
Allows the system to be reconfigured to improve performance as loads vary.

  • Example: Distributed make.

Scaling Transparency
Allows the system and applications to expand in scale without change to the system structure or the application algorithms.

  • Example: World-Wide-Web
  • Example: Distributed Database

Introduction To Distributed Systems BookDistributed Systems, Concepts and Design, George Coulouris, J Dollimore and Tim Kindberg, Pearson Education, 5th Edition. The book aims to provide an understanding of the principles on which the Internet and other distributed systems are based; their architecture, algorithms and design; and how they meet the demands of contemporary distributed applications.

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