ECI-M-811 Distributed Systems and Internetworking

Coordinator: Dr. Zhanfang Zhao

Office: T409

Tel: 020 7815 6340

Fax: 020 7815 7051

Email: zhaoza@lsbu.ac.uk

URL: http://ecce3.sbu.ac.uk/staff/zhaoza/dsi/

Blackboard: http://blackboard.lsbu.ac.uk/

Textbooks

Core reading:– ‘Distributed Systems: Principles and Paradigms

Tanenbaum AS. Prentice Hall 2002.

Background reading:– ‘Distributed Systems: Concept and Design’

Coulouris G etc, Addison Wesley, 2001.

Topics

• Introduction to DS

• Communication in DS

• Process in DS

• Naming System in DS

• Synchronization in DS

• New technologies

Outcomes (Distributed System Part)At the end of this unit the student will be able to:

• Familiar with the concepts of distributed systems hardware and distributed system facilities.

• Able to appreciate the capabilities and limitations of applications software running over local and wide area networks.

• Able to understand, design and implement simple client-server systems.

• Able to know how the World Wide Web system works

Introduction to Distributed System

Contents of this lesson:

• Definition of the Distributed System

• The Goals when Building a Distributed System

• Basic Software Concepts

• The Middleware

• System Architecture & Client-Server Model

Definition of a Distributed System (1)A distributed system is:A collection of independent computers that appears

to its users as a single coherent system. --Tanenbaum

Two aspects of this definition:• Hardware: The machines are autonomous.• Software: The user think they are dealing with a single system.Important characteristics of DS:• Differences between various computers and the ways in which they

communicate are hidden from users• Users and applications can interact with a DS in a consistent and uniform way• Ds should also be easy to extend or scale• To support heterogeneous computers and networks while offering a single

system view, DS is often organized in certain layers called middleware

Definition of a Distributed System (2)

A distributed system is a collection of autonomous hosts that that are connected through a computer network. Each host executes components and operates a distribution middleware, which enables the components to coordinate their activities in such a way that users perceive the system as a single, integrated computing facility.

-----------Wolfgang Emmerich, 2000

Hostn-1

Hostn

Host2

Host1

Definition of a Distributed System (3)

MiddlewareMiddleware

MiddlewareMiddleware

Network Operating SystemNetwork Operating System

Network Operating SystemNetwork Operating System

HardwareHardware

HardwareHardware

Component1 Componentn

Component1 Componentn

Component1 Componentn

Component1 Componentn

Network

Middleware ExamplesTransaction-oriented

– IBM CICS

– BEA Tuxedo

– IBM Encina

– Microsoft Transaction Server

Message-oriented– Microsoft Message

Queue

– Sun Tooltalk

Procedural– Sun ONC

– Linux RPCs

– OSF DCE

Object-oriented– OMG CORBA

– Sun Java/RMI

– Microsoft COM

.net Remote

– Sun Enterprise Java Beans

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

Goals

• Four important goals should be met to make it worth to build a distributed systems:

1. Connecting users and resources

2. Transparency

3. Openness

4. Scalability

Transparency in a Distributed System

Different forms of transparency in a distributed system.

Definition: A DS that is able to present itself to users and applications as if it were only a single computer system is said to be transparent

Transparency Description

AccessHide 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

RelocationHide that a resource may be moved to another location while in use

Replication Hide that a resource may be replicated

ConcurrencyHide that a resource may be shared by several competitive users

Failure Hide the failure and recovery of a resource

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

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

• These represent various properties that distributed systems should have.

Access Transparency

Access Transparency

LocationTransparency

LocationTransparency

ConcurrencyTransparencyConcurrencyTransparency

Migration Transparency

Migration Transparency

Performance TransparencyPerformance Transparency

Scalability Transparency

Scalability Transparency

Replication TransparencyReplication

Transparency

FailureTransparency

FailureTransparency

Distribution 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: 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

Openness• An open distributed system is a system that offers

services according to standard rules that describe the syntax and semantics of those services.

• Openness means that the system can easily be extended and modified.

• To facilitate the openness, the system should have a well defined and well-documented interfaces. These interface must declare the services that a component offers. And these interfaces are often described in an Interface Define Language (IDL)

• A service is an operation that a component performs on behalf of a user or another component.

• The interface definition should be complete and neutral.– Complete means that everything that is necessary to make an

implementation has indeed been specified.

Scalability

Adaption of distributed systems to– accomodate 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!

Scalability Problems

Examples of scalability limitations.

Concept Example

Centralized services A single server for all users

Centralized data A single on-line telephone book

Centralized algorithmsDoing routing based on complete information

Software Concepts

An overview between • DOS (Distributed Operating Systems)• NOS (Network Operating Systems)• Middleware

System Description Main Goal

DOSTightly-coupled operating system for multi-processors and homogeneous multicomputers

Hide and manage hardware resources

NOSLoosely-coupled operating system for heterogeneous multicomputers (LAN and WAN)

Offer local services to remote clients

MiddlewareAdditional layer atop of NOS implementing general-purpose services

Provide distribution transparency

Uniprocessor Operating SystemsSeparating applications from operating

system code through a microkernel.

1.11

Multiprocessor Operating Systems (1)

A monitor to protect an integer against concurrent access.

•An important extension of MOS is that it supports multiple processors having access to a shared memory. These data in the shared memory must be protected against the concurrent access to guarantee consistency.•Two synchronized primitives are used, one is semaphore, another is monitor

monitor Counter {

private:

int count = 0;

public:

int value() { return count;}

void incr () { count = count + 1;}

void decr() { count = count – 1;}

}

Multiprocessor Operating Systems (2)

A monitor to protect an integer against concurrent access, but blocking a process.

monitor Counter {

private:

int count = 0;

int blocked_procs = 0;

condition unblocked;

public:

int value () { return count;}

void incr () {

if (blocked_procs == 0)

count = count + 1;

else

signal (unblocked);

}

void decr() {

if (count ==0) {

blocked_procs = blocked_procs + 1;

wait (unblocked);

blocked_procs = blocked_procs – 1;

}

else

count = count – 1;

}

}

Multicomputer Operating Systems (1)

General structure of a multicomputer operating system

1.14

Multicomputer Operating Systems (2)

Alternatives for blocking and buffering in message passing.

1.15

Network Operating System (1)General structure of a network operating system.

1-19

Network Operating System (2)Two clients and a server in a network operating system.

File systems is supported by one or more machines called file servers

1-20

Positioning MiddlewareGeneral structure of a distributed system as middleware.

1-22

Middleware and Openness

In an open middleware-based distributed system, the protocols used by each middleware layer should be the same, as well as the interfaces they offer to applications.

1.23

System Architecture & Client Server Model

Important styles of architecture for distributed systems

• Layered architectures• Object-based architectures• Data-centered architectures• Event-based architectures

Architectural Styles (1)

The layered architectural style and …

Architectural Styles (2)

The object-based architectural style.

Architectural Styles (3)

The event-based architectural style ( Publish/Subscribe System)

Architectural Styles (4)

The shared data-space architectural style.

Clients and Servers

General interaction between a client and a server.

• In the basic client server model, the process in a computing system are divided into two groups. A server is a process implementing a specific service, for example database service. A client is a process that requests a service from a server by sending it a request and subsequently waiting or the server’s reply

1.25

An Example Client and Server (1)

The header.h file used by the client and server.

Used for parameters

An Example Client and Server (2)

A sample server.

An Example Client and Server (3)

A client using the server to copy a file.

1-27 b

Application LayeringFor the web applications, the CS applications generally been organized into three levels:

1-28

Multitiered Architectures (1)Alternative client-server organizations (a) – (e).

1-29

Multitiered Architectures (2)An example of a server acting as a client.

1-30

Modern ArchitecturesAn example of horizontal distribution of a Web service.

1-31

Distributed System Paradigms

World Wide Web Architecture Model:

• Traditional Web-based Systems– Overall organization– Web Documents– Programming Languages: HTML, XML &

MIME (Multipurpose Internet Mail Exchange)

– Multi-tiered Architectures

• Web Services

Traditional Web-Based SystemsFigure 12-1. The overall organization

of a traditional Web site.

Web Documents

Six top-level MIME types and some common subtypes.

Multitiered Architectures

The principle of using server-side CGI programs.

Web Services Fundamentals

The principle of a Web service:• WSDL (Web Services Definition Language) is a formal language very much

the same as the interface definition languages of RPC

• SOAP ( Simple Object Access Protocol) is essentially a framework in which much of the communication between two process can be standardized

• UDDI ( Universal Description, Discovery and Integration)