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Distributed Systems CS 3850 Soufiane Noureddine Lectures MWF 14:00 – 14:50 (PE207D) Office Hours MW 11:00 – 12:00 (C520). Introduction. Chapter 1. Computer Networks. - PowerPoint PPT Presentation
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Distributed Systems CS 3850 Soufiane Noureddine Lectures MWF 14:00 – 14:50 (PE207D) Office Hours MW 11:00 – 12:00 (C520)
Transcript
Page 1: Introduction

Distributed SystemsCS 3850

Soufiane Noureddine

LecturesMWF 14:00 – 14:50 (PE207D)

Office HoursMW 11:00 – 12:00 (C520)

Page 2: Introduction

Introduction

Chapter 1

Page 3: Introduction

Computer Networks

A collection of computers communicating through an

underlying network is called a computer network (in contrast to

a single-processor system)

Local Area Network: LAN10 to 1000 Mb/sec

Wide Area Network: WAN64 kbps to gigabits/sec

Page 4: Introduction

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.

A. Tanenbaum

Page 5: Introduction

Definition of a Distributed System (2)

“You know you have one when the crash of a computer you never heard of stops you from getting

any work done.”

L. Lamport

Page 6: Introduction

Organization of a Distributed System

A distributed system organized as middleware.Note that the middleware layer extends over multiple machines.

1.1

Page 7: Introduction

Characteristics of a distributed System

Differences between computers are hidden

Communication between computers is hidden

Internal organization of the system is hidden

Single system image: interaction with the system is independent from location (and time)

Ease of extension

High availability

Page 8: Introduction

Examples of distributed Systems1. Network of workstations + Pool of processors:

- Single file system (uniform naming scheme)

- Processors are allocated dynamically when needed (load sharing)

- System acts like a single-processor system

2. Workflow systems

- Orders arrive dynamically (e.g. via laptops, cellular phones)

- System assigns orders to the corresponding departments and initiates the needed business processes and users are unaware of the internal flow of orders

- System acts like a centralized database

3. WWW

- No need to know where documents are stored (at least in theory)

- Accessing remote documents is like accessing local ones

Page 9: Introduction

Goals of distributed Systems1. Connecting users and resources:

consequence: communication and collaboration (e.g. joint editing)

issue: security

2. Openness

- Services should obey to standard rules specifying syntax and semantics

- Services are specified in general as interfaces in an interface definition language

- IDL includes rather syntax and no semantics

- Specification of interface should be completed and neutral (w.r.t. implementation)

- Openness promotes interoperability and portability

3. Transparency (see next slides)

4. Scalability (see next slides)

Page 10: Introduction

Transparency in a Distributed System

Different forms of transparency in a distributed system.

Degree of transparency: More transparency means less performance

In reality: Systems are transparent only to certain degree

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 is replicated

ConcurrencyHide that a resource may be shared by several competitive users

FailureHide the failure and recovery of a resource

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

Page 11: Introduction

Clients Software Developers

e.g. Company

Network

(e.g. Telecom)

Network Provider

Page 12: Introduction

Clients Software Developers

e.g. Company

Network

(e.g. Telecom)

Network Provider

Company A

Company B

Page 13: Introduction

Clients Software Developers

e.g. Company

Network

(e.g. Telecom)

Network Provider

Company A

Company B

Page 14: Introduction

Clients Software Developers

e.g. Company

Network

(e.g. Telecom)

Network Provider

Company A

Company B

Page 15: Introduction

Clients Software Developers

e.g. Company

Network

(e.g. Telecom)

Network Provider

Company A

Company B

Page 16: Introduction

Clients Software Developers

e.g. Company

Network

(e.g. Telecom)

Network Provider

Applications

Operating System & Hardware

Page 17: Introduction

Clients Software Developers

e.g. Company

Network

(e.g. Telecom)

Network Provider

Applications

Operating System & Hardware

AEM: Availability Enhancing Middleware

Page 18: Introduction

Scalability Problems

Examples of scalability limitations.

Centralized solutions tend to be non-scalable

Decentralized solutions promote scalability

Concept Example

Centralized servicesA single server for all users (e.g. a single DNS server)

Centralized dataA single on-line telephone book (e.g. single table in a frequently used database)

Centralized algorithmsDoing routing based on complete information

A system is scalable when it is easy to extend without loss of performance.

Page 19: Introduction

Decentralized Algorithms

1. No machine has complete information about the system state

2. Machines make decisions based only on local information

3. Failures of one machine does not ruin the whole algorithm

4. No assumption on the existence of a global time

Page 20: Introduction

Scaling Techniques (1)

Ways to solve scalability problems:

a) Hide communication latencies (geographical scalability)

b) Use of distribution (in order to avoid bottlenecks)

c) Use of replication

Ad a)

Batch processing: Asynchronous communication instead of synchronous communication

Interactive applications: Reduce overall communication

Example: Move part of computation from server to client (e.g. Java applet)

Page 21: Introduction

Scaling Techniques (2)

1.4

The difference between letting:

a) a server or

b) a client check forms as they are being filled

Page 22: Introduction

Scaling Techniques (3)

1.5

An example of dividing the DNS name space into zones.

Clientserver Z1: nl.vu.cs.fluit

Server Z1 Client: address of Z2

Clientserver Z2: vu.cs.fluit

…

Ad b) Use of distribution

Page 23: Introduction

Scaling Techniques (4)

Ad c) Use of replication

Replication raises:

Availability: in general the primary goal

Performance:

By balancing the load among replicas

By locating replicas close to users/clients

Example: Caching (e.g. browser cache for used WWW pages)

Main issue in connection with replication: consistency

Page 24: Introduction

Hardware Concepts

1.6

Different basic organizations and memories in distributed computer

systems

Page 25: Introduction

Multiprocessors (1)

A bus-based multiprocessor.Issues:

Scalability

Cache consistency

1.7

Page 26: Introduction

Multiprocessors (2)

a) A crossbar switch: n2 switches!b) An omega switching network: less switches

1.8

Page 27: Introduction

Homogeneous Multicomputer Systems

a) Gridb) Hypercube

1-9

Page 28: Introduction

Software Concepts

An overview of • 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

Page 29: Introduction

Uniprocessor Operating Systems

Separating applications from operating system code through a microkernel.

1.11

Page 30: Introduction

Multiprocessor Operating Systems (1)

A monitor to protect an integer against concurrent access.

monitor Counter {

private:

int count = 0;

public:

int value() { return count;}

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

void decr() { count = count – 1;}

}

Page 31: Introduction

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;

}

}

Page 32: Introduction

Multicomputer Operating Systems (1)

General structure of a multicomputer operating system

1.14

Page 33: Introduction

Multicomputer Operating Systems (2)

Alternatives for blocking and buffering in message passing.

1.15

Page 34: Introduction

Multicomputer Operating Systems (3)

Relation between blocking, buffering, and reliable communications.

Synchronization point Send bufferReliable comm.

guaranteed?

Block sender until buffer not full Yes Not necessary

Block sender until message sent No Not necessary

Block sender until message received No Necessary

Block sender until message delivered No Necessary

Page 35: Introduction

Distributed Shared Memory Systems (1)

a) Pages of address space distributed among four machines

b) Situation after CPU 1 references page 10

c) Situation if page 10 is read only and replication is used

Replicating all pages:

Coherence protocols:

a) strong (transparent)

b) weak (not transparent)

Page 36: Introduction

Distributed Shared Memory Systems (2)

False sharing of a page between two independent processes.

Problem with DSM: not as efficient as expected

1.18

Page size: small more communication overhead

large less communication, but false sharing may occur

Page 37: Introduction

Network Operating System - NOS (1)

General structure of a network operating system.

Main features: Independent operating systems

Services for accessing remote resources

1-19

Page 38: Introduction

Network Operating System (2)

Two clients and a server in a network operating system.

1-20

Page 39: Introduction

Network Operating System (3)

Different clients may mount the servers in different places.

1.21

Page 40: Introduction

NOS vs DOS (3)

Distributed operating system:

Fully transparent

For homogeneous systems computers are not independentMore secure, but less scalable and less open

Network operating system:

Not transparent

For heterogeneous system computers are independent

Easier to extend (e.g. adding a new node in the Internet)

Both do not really qualify as a distributed system!!!

Solution: Middleware atop of a NOS hiding heterogeneity and improving transparency

Page 41: Introduction

Positioning Middleware

General structure of a distributed system as middleware.

Middleware Services: rather a functionally complete set of services in order to hide heterogeneity, direct access to NOS is discouraged.

1-22

Page 42: Introduction

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. implementations of the middleware should not use the NOS-provided protocols

1.23

Page 43: Introduction

Comparison between Systems

A comparison between multiprocessor operating systems, multicomputer operating systems, network operating systems, and middleware-based distributed systems.

ItemDistributed OS

Network OS

Middleware-based OSMultiproc

.Multicomp.

Degree of transparency

Very High High Low High

Same OS on all nodes Yes Yes No No

Number of copies of OS

1 N N N

Basis for communication

Shared memory

Messages FilesModel

specific

Resource management

Global, central

Global, distributed

Per node Per node

Scalability No Moderately Yes Varies

Openness Closed Closed Open Open

Page 44: Introduction

Clients and Servers

General interaction between a client and a server.

1.25

Page 45: Introduction

An Example Client and Server (1)

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

Page 46: Introduction

An Example Client and Server (2)

A sample server.

Page 47: Introduction

An Example Client and Server (3)

A client using the server to copy a file.

1-27 b

Page 48: Introduction

Processing Level

The general organization of an Internet search engine into three different layers

1-28

Page 49: Introduction

Multitiered Architectures (1)

Alternative client-server organizations (a) – (e).

1-29

Page 50: Introduction

Multitiered Architectures (2)

An example of a server acting as a client.

1-30

Page 51: Introduction

Modern Architectures

An example of horizontal distribution of a Web service.

1-31


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