distributed systems cs 3850 soufiane noureddine lectures mwf 14:00 – 14:50 (pe207d) office hours...

Distributed SystemsCS 3850

Soufiane Noureddine

LecturesMWF 14:00 – 14:50 (PE207D)

Office HoursMW 11:00 – 12:00 (C520)

Introduction

Chapter 1

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

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

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

Organization of a Distributed System

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

1.1

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

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

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)

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

Clients Software Developers

e.g. Company

Network

(e.g. Telecom)

Network Provider


e.g. Company

Network

(e.g. Telecom)

Network Provider

Company A

Company B


e.g. Company

Network

(e.g. Telecom)

Network Provider

Applications

Operating System & Hardware


e.g. Company

Network

(e.g. Telecom)

Network Provider

Applications

Operating System & Hardware

AEM: Availability Enhancing Middleware

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.

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

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)


1.4

The difference between letting:

a) a server or

b) a client check forms as they are being filled


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


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

Hardware Concepts

1.6

Different basic organizations and memories in distributed computer

systems

Multiprocessors (1)

A bus-based multiprocessor.Issues:

Scalability

Cache consistency

1.7

Multiprocessors (2)

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

1.8

Homogeneous Multicomputer Systems

a) Gridb) Hypercube

1-9

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

Uniprocessor Operating Systems

Separating applications from operating system code through a microkernel.

1.11

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;}

}

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


Alternatives for blocking and buffering in message passing.

1.15


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

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)

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

Network Operating System - NOS (1)

General structure of a network operating system.

Main features: Independent operating systems

Services for accessing remote resources

1-19

Network Operating System (2)

Two clients and a server in a network operating system.

1-20

Network Operating System (3)

Different clients may mount the servers in different places.

1.21

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

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

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

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

Clients and Servers

General interaction between a client and a server.

1.25

An Example Client and Server (1)

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


A sample server.


A client using the server to copy a file.

1-27 b

Processing Level

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

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 Architectures

An example of horizontal distribution of a Web service.

1-31

distributed systems cs 3850 soufiane noureddine lectures mwf 14:00 – 14:50 (pe207d) office hours...

Documents

distributed system differences

single system image

single coherent system

slides slide

cellular phones system

disk slide

gigabitssec slide

c520 slide