Distributed Systems: Concepts and Design

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Distributed Systems Concepts and Design

• Jinghai Rao
• 13,9,2000

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Topics

• Chapter 1 Characterization of Distributed
  Systems
• Chapter 2 System Models

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What is Distributed Systems

• A distributed system is one in which components
  located at networked computers communicate and
  coordinate their actions only by passing
  messages.
• Three Examples
• The internet
• An intranet which is a portion of the internet
  managed by an organization
• Mobile and ubiquitous computing

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The Internet

• The Internet is a very large distributed system.
• The implementation of the internet and the
  services that it suports has entailed the
  development of practical solutions to many
  distributed system issues.

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Intranets

• An intranet is a portion of the Internet that is
  separately administered and has a boundary that
  can be configured to enforce local security
  policies
• The main issues arising in the design of
  components for use in intranets are file
  services, firewalls, cost.

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Mobil and ubiquitous computing

• The portability of the devices, such as laptop
  computers, PDA, mobil phone, refrigerators,
  togather with their ability to connect
  conveniently to networks in different places,
  makes mobile computing possible.
• Ubiquitous computing is the harnessing of many
  small cheap computational devices that are
  present in users physical environments,
  including the home, office and elsewhere.

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Mobil and ubiquitous computing (continue)

• Mobile and ubiquitous computing raise significant
  system issues presents an architecture for mobile
  compuing and outlines the issues that arise from
  it, including how to support the discovery of
  resources in a host environment eliminating the
  need for users to reconfigure their mobile
  devices as they move around helping users to
  cope with limited connectivity as they travel
  and providing privacy and other security
  guarantees to users and the environments that the
  visit.

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Significant Consequences of DS

• Concurrency
• The capacity of the system to handle shared
  resources can be increased by adding more
  resources to the network.
• No global clock
• The only communication is by sending messages
  through a network.
• Independent failures
• The programs may not be able to detect whether
  the network has failed or has become unusually
  slow.

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Resource

• The term resource is a rather abstract one,
  but it best characterizes the range of things
  that can usefully be shared in a networked
  computer system. It extends from hardware
  components such as disks and printers to
  software-defined entities such as files,
  databases and data objects of all kinds.

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Important Terms of Web

• Services
• A distinct part of a computer system that manages
  a collection of related resources and presents
  their functionality to users and applications.
• http, telnet, pop3...
• Server
• A running program (a process) on a networked
  computer that accepts requests from programs
  running on other computers to perform a service,
  and responds apppropriately.
• IIS, Apache...
• Client
• The requesting processes.

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The World Wide Web

• The WWW is an evolving system for publishing and
  accessing resources and services across the
  Internet.
• The Web is an open system
• Its oeration is based on communication standards
  and document standards tht are freely published
  and widely implemented.
• The Web is one with respect to the types of
  resource that can be published and shared on.

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The main standard components of Web

• HyperText Markup Language (HTML)
• Uniform Resource Laocators (URLs)
• HyperText Transfer Protocol (HTTP)
• HTTP is a request-reply protocol.

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More Discussion of Web

• Dynamic pages
• CGI
• Javascript
• ASP, PHP...
• Discussion
• Hypertext model is lacking in some respects, such
  as lost in hyperspace.
• HTML is limited in exchanging structured data,
  one solution is XML.
• The problems of scale.
• A Web page is not always a satisfactory user
  interface.

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Challenges

• Heterogeneity
• Openness
• Security
• Scalability
• Failure handling
• Concurrency
• Transparency

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Heterogeneity

• Different networks, hardware, operating systems,
  programming languages, developers.
• We set up protocols to solve these
  heterogeneities.
• Middleware a software layer that provides a
  programming abstraction as well as masking the
  heterogeneity.
• Mobile code code that can be sent from one
  computer to another and run at the destination.

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Openness

• The openness of DS is determined primarily by the
  degree to which new resource-sharing services can
  be added and be made available for use by a
  variety of client programs.
• Open systems are characterized by the fact that
  their key interfaces are published.
• Open DS are based on the provision of a uniform
  communication mechanism and published interfaces
  for access to shared resources.
• Open DS can be constrcted from heterogeneous
  hardware and software.

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Security

• Security for information resources has three
  components
• Confidentiality protection against disclosure to
  unauthorized individuals.
• Integrity protection against alteration or
  corruption.
• Availability protection against interference
  with the means to access the resources.
• Two new security challenges
• Denial of service attacks (DoS).
• Security of mobile code.

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Scalability

• A system is described as scalable if it remains
  effective when there is a significant increase in
  the number of resources and the number of users.
• Challenges
• Controlling the cost of resources or money.
• Controlling the performance loss.
• Preventing software resources from running out
• Avoiding preformance bottlenecks.

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Failure handling

• When faults occur in hardware or software,
  programs may produce incorrect results or they
  may stop before they have completed the intended
  computation.
• Techniques for dealing with failures
• Detecting failures
• Masking failures
• Tolerating failures
• Recovering form failures
• Redundancy

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Concurrency

• There is a possibility that several clients will
  attempt to access a shared resource at the same
  time.
• Any object that represents a shared resource in a
  distributed system must be responsible for
  ensuring that operates correctly in a concurrent
  environment.

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Transparency

• Transparency is defined as the concealment from
  the user and the application programmer of the
  separation of components in a distributed system,
  so that the system is perceived as a whole rather
  than as a collection of independent components.
• Eight forms of transparency
• Access transparency
• Location transparency
• Concurrency transparency
• Replication transparency
• Failure transparency
• Mobility transparency
• Performance transparency
• Scaling transparency

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Summary

• Distributed systems are everywhere
• Internet, intranet, wireless networks.
• Resource sharing is the main motivating factor
  for constructing distribute systems.
• The constrcution of distributed systems produces
  many challenges.

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Chapter 2 System Models

• Introduction
• Architectural models
• Fundamental models
• summary

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Introduction

• In this chapter we bring out the common
  properties and design issues for distributed
  systems in the form of descriptive modes.
• An architectural model defines the way in which
  the components of systems interact with one
  another and the way in which they are maped onto
  an underlying network of computers
• In section2.3, we introduce three fundamental
  models that help to reveal key problems for the
  designers of distributed system.

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Difficulties and threats for distributed systems

• Widely varying modes of use.
• Wide range of system environments
• Internal problems non-synchronized clocks,
  conflicting data updates, many modes of hardware
  and software failure involving the individual
  components of a system.

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Architectural models

• An architectural model of a distributed system
  first simplifies and abstracts the functions of
  the individual components of a DS and then it
  considers
• The placement of the components across a network
  of computers
• The interrelationships between the components.

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Architectural models (cont.)

• Software layers
• System architectures
• Variations on the client-server model
• Interfaces and objects
• Design requirements for distributed architectures

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Software layers

• Applications, services
• Middleware
• Operating system
• Computer and network hardware

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System architectures

• Client-server model
• Services provided by multiple servers
• Proxy srvers and caches
• Peer processes

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Variations on the client-server model

• Mobile code
• Mobile agents
• Network computers
• Thin client
• Mobile devices and spontaneous networking
• The X-11 window system

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Design requirements for distributed architectures

• Performance issues
• Use of caching and replication
• Dependability issues

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Performance issues

• Responsiveness
• Users of interactive aplication require a fast
  and consistent response to interaction.
• Throughput
• The rate at which computational work is done.
• Quality of services
• The ability to meet the deadlines of users need.
• Balancing computer loads
• In some case load balancing may involve moving
  partially-completed work as the loads on hosts
  changes.

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Use of caching and replication

• The performance issues often appear to be major
  obstacles to the successful deployment of DS, but
  much progress has been made in the design of
  systems that overcome them by the use of data
  replication and caching.

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Dependability issues

• The dependability of computer systems as
  correctness, security and fault tolerance.
• Fault tolerance reliability is achieved through
  redundancy.
• Security the architectural impact of the
  requirement for security concerns the need to
  locate sensitive data and other resources only in
  computers that can be effectively secured against
  attack.

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Fundamental Models

• Interaction model
• Failure model
• Security model

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Interaction model

• Performance of communication channels
• Computer clocks and timing events
• Two variants of the interaction model
• Agreement in pepperland
• Event ordering

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Performance of communication channels

• Communication performance is often a limiting
  characteristic.
• The delay between the sending of a message by one
  process and its receipt by another is referred to
  as latency.
• Bandwidth
• Jitter is the variation in the time taken to
  deliver a series of messages.

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Computer clock and timing event

• It is impossible to maintain a single global
  notion of time.
• There are several approaches to correcting the
  times on computer clocks. (from GPS)

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Two variants of the interaction model

• Synchronous distributed system
• The time to execute each step f a process has
  known lower and uper bounds.
• Each message transmitted over a channel is
  received within a known bounded time
• Each process has a local clock whose drift rate
  from real time has a known bound.
• Asynchronous distributed system
• No bound on process executiong speeds
• No bound on message transmisson delays
• No bound on clock drift rates.

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Agreement in pepperland

• The pepperland divisions need to agree on which
  of them will lead the charge against the Blue
  Meanies, and when the charge will take place.
• In asynchronous pepperland, the messengers are
  very variable in their speed.
• The divisions know some useful constraints every
  message takes at least min. Minutes and at most
  max minutes to arive.
• The leading division sends a message charge!,
  then waits for min minutes, then it charges.
• The other divisions charge is guaranteed to be
  after the leading divisions, but no more than
  (max-min) after it.

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Event ordering

• In many cases, we are interested in knowing
  whether an event (sending or receiving a message)
  at one process occurred before, after or
  concurrently with another event at another
  process. The execution of a system can be
  described in terms of events and their ordering
  despite the lack of accurate clocks.
• example(p. 58).

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Failure model

• Omission failures
• Arbitrary failures
• Failure detection
• Impossibility of reaching agreement in the
  presence of failure
• Masking failure
• Reliability of one to one communication

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Security model

• Protecting objects
• Securing processes and their interactions
• The enemy
• Defeating security threats
• Other possible threats from the enemy
• The uses of security models

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summary

• Most DS are arranged according to one of a
  variety of architectural models.
• The fundamental models interaction, failure,
  and security identify the common
  characteristics of the basic components from
  which distributed systems are constructed.