DISTRIBUTED SYSTEMUNIT-1

Prepared By: G.S.Mishra

WHAT IS A DISTRIBUTED SYSTEM?

WHAT IS A DISTRIBUTED SYSTEM? CONTD….

• “ A distributed system in which hardware or, software components located at networked computers communicate & coordinate their actions only by passing messages.”

Consequences:

Concurrency No global clock Independent failure

EXAMPLES OF DISTRIBUTED SYSTEM

The Internet

The Intranet

Mobile & Ubiquitous Computing

MOBILE SYSTEMS Cellular Concept Frequency Reuse

CLUSTERS

ARCHITECTURE

TERMINOLOGIES USED

OTHER EXAMPLES

Web server ( Centralized )

MP3.com : a number of MP3 files stored in the Web

site.

Failure of this site cause unavailability of services

Napster: stores the MP3 files on the actual users’

machines

napster.com is used as a massive index (or meeting

place) for connecting users

EXAMPLES CONTD….

Users connect to Napster to search for the files

they desire

Thereafter connect to users directly to download

the file.

MP3 file is distributed across a number of servers

making it more reliable against failure.

But search is centralized.

Gnutella does not have a centralized search facility nor a central storage facility for the files.

EXAMPLE CONTD…

Each user in the network runs a servent (a client

and a server),

which allows him/her to act as both a provider and

consumer of information (as in Napster)

but acts as a search facility also.

if any of the servents are unavailable, users can

almost certainly still reach the file they require

DIFFERENCE BETWEEN CENTRALIZED & DECENTRAIZED SYSTEMS

Broad Areas:1. Resource Discovery2. Resource Availability3. Resource Communication

1-Resource Discovery:

A two-stage process The discovery service needs to be located The relevant information is retrieved. E.g. DNS

DIFFERENCE….. CONTD…

Resource Availability:

Web servers fall into the centralized category here because there is only one IP address that hosts a particular site.

If that machine goes down then the Web site is unavailable

machines could be made fault tolerant by replicating

the web site employ some internal switching mechanisms but

the availability of the IP address remains the same.

In a more decentralized approach; by offering many duplicate services that can perform the same functionality.

Resource availability is tied in closely to resource discovery.

3- Resource Communication

1. Brokered Communication: where the communication is always passed through

a central server and therefore a resource does not have to reference the other resource directly

2. Point-to-Point (or Peer-to-Peer) Communication:

this involves a direct connection (although this connection may be multi-hop) betweenthe sender and the receiver. In this case, the sender is aware of the receiver’s location.

GOALS OF DISTRIBUTED SYSTEM

Advantages of distributed system over centralized system

GOALS OF DISTRIBUTED SYSTEM CONTD…

Advantages of Distributed System over Independent PCs

GOALS OF DISTRIBUTED SYSTEM CONTD…

Disadvantages of Distributed System

HARDWARE CONCEPTS : TAXONOMY OF PARALLEL & DISTRIBUTED COMPUTER SYSTEMS

BUS BASED MULTIPROCESSOR

MULTICOMPUTERS WORKSTATION ON LAN

SWITCHED MULTIPROCESSORS

SWITCHED MULTICOMPUTERS

SOFTWARE CONCEPTS

SOFTWARE CONCEPTS

SOFTWARE CONCEPTS

SOFTWARE CONCEPTS

SOFTWARE CONCEPTS

SOFTWARE CONCEPTS

RESOURCE SHARING hardware resources like printers or disks to

reduce costs sharing databases, a set of web pages, search

engine

Service: file services - read, write, delete. We buy goods by electronic payment service

Server: a running program (process) on a networked computer that accepts requests

Remote invocation: The complete interaction between the client and the server from sending request to receiving server’s response

THE WORLD WIDE WEB

CHALLENGES

Heterogenity :

Networks, computer hardware, Operating systems, Programming languages

Openness

Security : Denial of Service attack

Scalability

CHALLENGES CONTD…

Failure Handling:

Detecting failures, Masking failures, tolerating failures, Recovery from failures, Redundancy

Concurrency

Transparency: Access transparency- Enables local & remote

resources to be accessed using identical operations.

CHALLENGES CONTD…

Location transparency- Enables resources to be accessed without knowledge of their location.

Concurrency transparency- Enables processes to operate concurrently using shared resources without interference between them.

Replication transparency- enables multiple instances of resources to be used to increase reliability and performance without knowledge of replicas by users.

CHALLENGES CONTD…Failure transparency- enables the concealment of

faults, allowing users and application programs to complete their tasks despite the failure of hardware or software components.

Mobility transparency- allow the movement of resources and clients within a system without affecting the operation of users or programs.

Performance transparency- allows the system to be reconfigured to improve performance as loads vary.

Scaling transparency- allows the system and applications to expand in scale without change to the system structure.

WEB SERVERS AND WEB BROWSERS

SHARED MEMORY MULTI-PROCESSOR

SYSTEM MODELS Architectural Model: Concerned with the

placement of parts and the relationships between them.

Software Layers System Architecture

Fundamental model: Concerned with a more formal description of the properties that are common in all architectural models.

Interaction Model Failure Model Security Model

SOFTWARE LAYERS

SYSTEM ARCHITECTURES

The working of Search engines

CLIENT SERVER MODEL

SERVICES PROVIDED BY MULTIPLE SERVERS

For Replicated data

WEB PROXY SERVER

Proxy servers are used as a cache to store recently used data objects, sharing other web resources, to increase availability and performance, reducing load on a network & web servers.

A DISTRIBUTED APPLICATION BASED ON PEER PROCESSES

All of the processes play similar roles, interacting cooperatively as peers to perform a distributed activity without any distinction between clients and servers.

WEB APPLELTS

FUNDAMENTAL MODELS

Fundamental model: Concerned with a more formal description of the properties that are common in all architectural models.

Interaction Model Failure Model Security Model

INTERACTION MODEL

Performance of communication channel

Latency: delay between the start of a messages transmission from one process and the beginning of its receipt by another process.

Bandwidth: Total amount of information transmitted in a given time.

Jitter: The time taken to deliver a series of messages.

INTERACTION MODEL CONTD…

o Two variants of the interaction model Synchronous Distributed system Asynchronous Distributed system

o Computer clocks & Timing Events

o Event ordering

REAL TIME ORDERING OF EVENT

FAILURE MODEL

Omission failure

Arbitrary failure

Masking failures

Reliability

SECURITY MODEL

Securing processes and their interactions

The enemy

Protecting objects

Defeating Security Threats

PROTECTING OBJECTS

THE ENEMY

SECURE CHANNELS

THIN CLIENTS AND COMPUTE SERVERS

OBJECTS AND PRINCIPALS

THE ENEMY

SECURE CHANNELS

THE WEB

SYSTEM MODELS Architectural Model:

Concerned with the placement of parts and the relationships between them.

Fundamental model:

Concerned with a more formal description of the properties that are common in all architectural models.

SYSTEM MODELSArchitectural Models:

The placements of its parts & relationship between them:

The partition of data or, replication

Requirements to add & remove mobile devices

conveniently

Use of mobile code & agents

Caching of data

Software Layers:

Applications, services Middleware Operating System Computer & Network Hardware

(OS & CN Hardware referd to as Platform)

ARCHITECTURAL MODELS

SYSTEM MODELSArchitectural Models:

The placements of its parts & relationship between them:

The partition of data or, replication

Requirements to add & remove mobile devices

conveniently

Use of mobile code & agents

Caching of data

ORDERING OF EVENTSLamport’s Happened Before relationship:

For two events a and b, a → b if a and b are events in the same process and a occurred

before b a is an event of sending a message m and b is the

corresponding receive event at the destination process If a → c and c → b for some event c, then a → c (transitive relation )

ORDERING OF EVENTSLamport’s Happened Before relationship:

For two events a and b, a → b if a and b are events in the same process and a occurred

before b a is an event of sending a message m and b is the

corresponding receive event at the destination process If a → c and c → b for some event c, then a → c (transitive relation )

ORDERING OF EVENTSLamport’s Happened Before relationship:

For two events a and b, a → b if a and b are events in the same process and a occurred

before b a is an event of sending a message m and b is the

corresponding receive event at the destination process If a → c and c → b for some event c, then a → c (transitive relation )

ORDERING OF EVENTSLamport’s Happened Before relationship:

For two events a and b, a → b if a and b are events in the same process and a occurred

before b a is an event of sending a message m and b is the

corresponding receive event at the destination process If a → c and c → b for some event c, then a → c (transitive relation )

ORDERING OF EVENTSLamport’s Happened Before relationship:

For two events a and b, a → b if a and b are events in the same process and a occurred

before b a is an event of sending a message m and b is the

corresponding receive event at the destination process If a → c and c → b for some event c, then a → c (transitive relation )

ORDERING OF EVENTSLamport’s Happened Before relationship:

For two events a and b, a → b if a and b are events in the same process and a occurred

before b a is an event of sending a message m and b is the

corresponding receive event at the destination process If a → c and c → b for some event c, then a → c (transitive relation )