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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 enforcelocal security policies

The main issues arising in the design of

components for use in intranets are: fileservices, 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 convenientlyto networks in different places, makes mobile

computing possible.

Ubiquitous computing is the harnessing of many

small cheap computational devices that are presentin 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 mobilecompuing and outlines the issues that arise from it,including how to support the discovery ofresources in a host environment; eliminating theneed for users to reconfigure their mobile devicesas they move around; helping users to cope with

limited connectivity as they travel; and providingprivacy and other security guarantees to users andthe 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 anetworked computer system. It extends

from hardware components such as disks

and printers to software-defined entitiessuch 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 acceptsrequests 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 forpublishing and accessing resources and

services across the Internet. The Web is an open system:

Its oeration is based on communicationstandards and document standards tht are freely

published and widely implemented.

The Web is one with respect to the types ofresource 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 inhyperspace.

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 degreeto 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 keyinterfaces are published.

Open DS are based on the provision of a uniformcommunication mechanism and published interfaces foraccess to shared resources.

Open DS can be constrcted from heterogeneous hardwareand software.


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Security

Security for information resources has threecomponents: Confidentiality: protection against disclosure to

unauthorized individuals.

Integrity: protection against alteration or corruption.

Availability: protection against interference with themeans 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 theymay stop before they have completed the intendedcomputation.

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 aconcurrent environment.


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Transparency

Transparency is defined as the concealment from theuser and the application programmer of the separation ofcomponents in a distributed system, so that the system is

perceived as a whole rather than as a collection ofindependent 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 commonproperties and design issues for distributedsystems in the form of descriptive modes.

An architectural model defines the way in whichthe components of systems interact with oneanother and the way in which they are maped ontoan underlying network of computers

In section2.3, we introduce three fundamentalmodels that help to reveal key problems for thedesigners 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 asystem.


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Architectural models

An architectural model of a distributed

system first simplifies and abstracts the

functions of the individual components of aDS and then it considers:

The placement of the components across a

network of computersThe 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 distributedarchitectures


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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 spontaneousnetworking

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 hasbeen 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 throughredundancy.

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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Computer clock and timing event

It is impossible to maintain a single global

notion of time.

There are several approaches to correctingthe 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 anduper bounds.

Each message transmitted over a channel is received within aknown bounded time

Each process has a local clock whose drift rate from real time has aknown bound.

Asynchronous distributed system

No bound on process executiong speedsNo 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 willlead the charge against the Blue Meanies, and when the chargewill take place.

In asynchronous pepperland, the messengers are very variablein their speed.

The divisions know some useful constraints: every messagetakes 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 orconcurrently 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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