Distributed Systems

Definition

Definition of a Distributed System

A distributed system is a collection of independent computers that appears to its users as a single system.

A definition (Coulouris, et al)

System of networked computers that

communicate and coordinate their actions only by passing messages

concurrent execution of programs components fail independently of

one another

A definition (Lamport)

“You know you have a distributed system when the crash of a computer you’ve never heard of stops you from getting any work done.” inter-dependencies shared state

Distributed System

Operating System types Centralized Systems

Process management Network System

Share resources Remote access Telnet / FTP No direct control from machine to another

Distributed system Global view of files system Global name Global time….

Distributed system properties

Connect users and resources

Transparency

Scalability

Openness

Transparency

To hide the fact that machines are

physically distributed and how much

the distributed system appears as

single system

Transparency in a Distributed System

Different forms of transparency in a distributed system.

TransparencyDescription

AccessHide differences in data representation and how a resource is accessed

LocationHide 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

FailureHide the failure and recovery of a resource

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

Types of transparency– Location Transparency: users cannot tell where hardware and

software resources such as CPUs, printers, files, data bases are located.

– Migration Transparency: resources must be free to move from one location to another without their names changed.E.g., /usr/lee, /central/usr/lee

– Replication Transparency: OS can make additional copies of files and resources without users noticing.

– Concurrency Transparency: The users are not aware of the existence of other users. Need to allow multiple users to concurrently access the same resource. Lock and unlock for mutual exclusion.

– Parallelism Transparency: Automatic use of parallelism without having to program explicitly. The holy grail for distributed and parallel system designers.

Users do not always want complete transparency: a fancy printer 1000 miles away

Scalability

It is to able to send anything to

anyone anywhere

System scalability to size means

easily add more users & resources

Scalability of geographically

Scalability A distributed system is scalable if it

remains effective as the number of users and/or resources increase

Challenges: Controlling resource costs Controlling performance loss Preventing resources from running out Avoiding performance bottlenecks

Scalability Problems• No machine has complete information

about the system state.• Machines make decisions based only on

local information.• Failure of one machine does not ruin the

algorithm.• There is no implicit assumption that a

global clock exists.

Openness / Flexibility

The distributed system has a clear

rules controls its services

Open distributed system is flexible

means easy to configure system for

different developers

Openness When protocols are known to developers

extensibility and maintainability becomes possible

Openness allows re-implementation of different components of the system

Important factors: Specification Documentation Published interfaces (often bypassing

standards organizations)

Reliability

• Distributed system should be more reliable than single system. Example: 3 machines with .95 probability of being up. 1-.05**3 probability of being up.– Availability: fraction of time the system is usable.

Redundancy improves it.– Need to maintain consistency– Need to be secure– Fault tolerance: need to mask failures, recover

from errors.

Security Three components:

Confidentiality (protection against disclosure to unauthorized individuals)

Integrity (protection against alteration or corruption)

Availability (protection against interference with the means of accessing the resources)

The challenge: sending sensitive information in a network message in a secure manner efficiently

Security Scenario 1: Accessing exam information

via a network file system Authentication: how do we know for sure

that the user is a teacher who should have access to the data?

Scenario 2: Sending a credit card number over the Internet Confidentiality: no other than the recipient

should be able to read the data

Hardware concept of DS

Distributed system may be

multi-processor or multi-computers

Hardware Concepts

Tightly Coupled versus Loosely Coupled Tightly coupled systems (multiprocessors)

o shared memoryo intermachine delay short, data rate high

Loosely coupled systems (multicomputers)o private memoryo intermachine delay long, data rate low

11/14/00

Multicomputers

Bus-Based Multicomputers easy to build communication volume much smaller relatively slow speed LAN (10-100 MIPS,

compared to 300 MIPS and up for a backplane bus)

Switched Multicomputers interconnection networks: E.g., grid,

hypercube hypercube: n-dimensional cube

Hardware Concepts

1.6

Multi-processor Multi-computers

Multiprocessors

1.7

11/14/00

Switched Multiprocessors

for connecting large number (over 64) of processors

crossbar switch: n**2 switch points omega network: 2x2 switches for n CPUs

and n memories, log n switching stages, each with n/2 switches,

total (n log n)/2 switches building a large, tightly-coupled, shared

memory multiprocessor is possible, but is difficult and expensive

Multiprocessors (2)

1.8

Multicomputer Systems

Grid Hypercube

1-9

Software Concepts

DOS (Distributed Operating Systems) NOS (Network Operating Systems) Middleware

SystemDescriptionMain Goal

DOSTightly-coupled operating system for multi-processors and multicomputers

Tries to maintain global view of resources

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

Manages collection of machines with local services and OS

MiddlewareAdditional layer atop of NOS implementing general-purpose services

Provide distribution transparency

11/14/00

Software Concepts• Software more important for users• Three types:

1. Network Operating Systems

2. (True) Distributed Systems

3. Multiprocessor Time Sharing

Uniprocessor Operating Systems

Separating applications from operating system code through a microkernel.

1.11

Network Operating Systems

loosely-coupled software on loosely-coupled hardware

A network of workstations connected by LAN each machine has a high degree of autonomy Files servers: client and server model Clients mount directories on file servers Best known network OS:

o Sun’s NFS (network file servers) for shared file systems

Multicomputer Operating Systems (1) General structure of a multicomputer operating

system

1.14

Positioning Middleware General structure of a distributed system as

middleware.

1-22

(True) Distributed Systems

tightly-coupled software on loosely-coupled hardware

provide a single-system image or a virtual uniprocessor

a single, global interprocess communication mechanism, process management, file system; the same system call interface everywhere

Ideal definition: “ A distributed system runs on a collection of computers that

do not have shared memory, yet looks like a single computer to its users.”

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 OS Multiproc

.Multicomp.

Degree of transparency

Very HighHighLowHigh

Same OS on all nodesYesYesNoNo

Number of copies of OS

1NNN

Basis for communication

Shared memory

MessagesFilesModel specific

Resource management

Global, central

Global, distributed

Per nodePer node

ScalabilityNoModeratelyYesVaries

OpennessClosedClosedOpenOpen

Distributed object architecture

Software bus

o1 o2 o3 o4

o5 o6

S (o1) S (o2) S (o3) S (o4)

S (o5) S (o6)

A data mining system

Database 1

Database 2

Database 3

Integrator 1

Integrator 2

Visualiser

Display

Report gen.

Data mining system

The logical model of the system is not one of service provision where there are distinguished data management services

It allows the number of databases that are accessed to be increased without disrupting the system

It allows new types of relationship to be mined by adding new integrator objects