Introduction

Chapter 1

Definition of a Distributed System

A distributed system [Tannenbaum & Steen] can be defined as a collection of independent computers that appears to its users as a single coherent system.

Building a Distributed System

– Development of powerful microprocessors – High speed computer networks– Cheap off the shelf components

– The above three has resulted in feasible computer systems consisting of large number of computers connected by a high speed network

Organization of a Distributed System

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

1.1

Goals in Building a Distributed System

– Connect users and resources• Access and share resources in a controlled way

– Transparent• A system that presents itself as a single system

– Open• offers services according to commonly accepted standards

– Scale• Performance proportional to resources and system complexity

Connect Users and Resources

Goals:

Provide easy access to resources

share costly resources in a controlled manner

Challenges:

security versus privacy violation

Transparency in a Distributed System

Different forms of transparency in a distributed system.

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

ReplicationHide that a resource may be shared by several competitive users

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

Openness

Goals:Interoperability: extent to which two different system implementations can work together by relying on each other’s services as specified by a common standardPortability: extent to which an application developed for system A can be executed on a system B that implements the same interfacesExtensible: easy to configure and add/replace components

- separate mechanism from policy ( i.e. memory management)

Achieving Openness

Services are generally specified through interfaces which are described in Interface Definition Language (IDL).

For a distributed system to be inter-operable and portable the specifications should be complete and neutral

ScalabiltyScalability measured in terms of

– Size: how the system scales by adding users and resources

– Geography: the extent to which the users and resources are apart

– Administration: the number of administrative domains

Centralized versus distributed services/algorithms

Distributed services/algorithms

- Machines make decision based on local information

- no machine maintains complete system state

- no single point of failure

- no global clock

Tradeoffs between centralized versus decentralized algorithms/services

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

Problems in Scalability

Local area networks are designed for synchronous communication

Communication in WANs are unreliable and virtually point to point

Many systems have centralized components

Different administrative domains have conflicting policies for resource usage, management and security

Scaling Techniques

– Use asynchronous communication• Applications do not block• Special handler needs to be set to handle interrupts on service

completion• Not a solution for interactive applications

– Distribution• splitting components and spreading them across (eg. DNS is

split into domains and zones, distributing load in web servers)

– Replication • replicate components increases availability and balances load

(Web servers, caching)• Issues with consistency

Scaling Techniques (1)

1.4

The difference between letting:

a) a server or

b) a client check forms as they are being filled

Scaling Techniques (2)

1.5

An example of dividing the DNS name space into zones.

Hardware Concepts

1.6

Different basic organizations and memories in distributed computer

systems

Multiprocessors (1)

A bus-based multiprocessor.

1.7

Multiprocessors (2)

a) A crossbar switchb) An omega switching network

1.8

Homogeneous Multicomputer Systems

a) Gridb) Hypercube

1-9

Heterogeneous Multi-computer systems

– Most existing distributed systems built on top of heterogeneous multi-computers

– Inter-connection networks may also be heterogeneous

– Sophisticated software is needed since these systems lack a global system view

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

Multicomputer Operating Systems (2)

Alternatives for blocking and buffering in message passing.

1.15

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

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

Distributed Shared Memory Systems (2)

False sharing of a page between two independent processes.

1.18

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.

1-20

Network Operating System (3)

Different clients may mount the servers in different places.

1.21

Positioning Middleware

General 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

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

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

References

Lecture slides of Distributed Systems principles and paradigms by Andrew Tannenbaum and Van Steen, Prentice Hall India, 2002.