FUNDAMENTALS OF DISTRIBUTED SYSTEM

Chapter one
Introduction to Distributed system

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Chapter Outline
• Defining distributed system
• Examples of distributed systems
• Why distribution?
• Goals and challenges of distributed systems
• Where is the borderline between a computer and a distributed
system?

• Examples of distributed architectures

Introduction and Definition

▪ before the mid-80s, computers were

▪ very expensive (hundred of thousands or even millions of dollars)
▪ very slow (a few thousand instructions per second)
▪ not connected among themselves
▪ after the mid-80s: two major developments
▪ cheap and powerful microprocessor-based computers appeared
▪ computer networks
▪ LANs at speeds ranging from 10 to 1000 Mbps
▪ WANs at speed ranging from 64 Kbps to gigabits/sec
▪ consequence
▪ feasibility of using a large network of computers to work for the same
application; this is in contrast to the old centralized systems where there
was a single computer with its peripherals
▪ Definition of a Distributed System
▪ a distributed system is:
a collection of independent computers that appears to its users as a
single coherent system - computer (Tanenbaum & Van Steen)

▪ this definition has two aspects:

1. hardware: autonomous machines
2. software: a single system view for the users
▪ Other Definitions

a distributed system is a system designed to support the development of

applications and services which can exploit a physical architecture consisting of
multiple, autonomous processing elements that do not share primary memory but
cooperate by sending asynchronous messages over a communication network
(Blair & Stefani)
CENTRALIZED SYSTEM CHARACTERISTICS
 One component with non-autonomous parts

 Component shared by users all the time

 All resources accessible

 Software runs in a single process

 Single point of control

 Single point of failure

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DISTRIBUTED SYSTEM CHARACTERISTICS

 Multiple autonomous components

 Components are not shared by all users

 Resources may not be accessible

 Software runs in concurrent processes on different

processors

 Multiple points of control

 Multiple points of failure

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COMMON CHARACTERISTICS OF DISTRIBUTED SYSTEM

 Resource Sharing: is the ability to use any h/w,s/w & data

anywhere in the system.
 Openness: is concerned with extension & improvement of
distributed system.
 Scalability: increase the scale of the system.

 Fault tolerance: if any failure is occurred in the S/W or

H/W the system continue work or it is the reliability of
the system.
 Transparrency:Hides the complexity of distributed
system to the users & application programs.

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❖ OPENNESS
 Openness is concerned with extensions and
improvements of distributed systems.
 Detailed interfaces of components need to be
published.
 New components have to be integrated with existing
components.
 Differences in data representation of interface types
on different processors (of different vendors) have to
be resolved.

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❖ SCALABILITY
 Adaptation of distributed systems to
 accommodate more users
 respond faster (this is the hard one)

 Usually done by adding more and/or faster processors.

 Components should not need to be changed when
scale of a system increases.
 Design components to be scalable!

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❖ FAILURE HANDLING (FAULT TOLERANCE)

 Hardware, software and networks fail!

 Distributed systems must maintain availability even
at low levels of hardware/software/network reliability.
 Fault tolerance is achieved by
 recovery
 redundancy

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❖ CONCURRENCY
 Components in distributed systems are executed in
concurrent processes.
 Components access and update shared resources (e.g.
variables, databases, device drivers).
 Integrity of the system may be violated if concurrent
updates are not coordinated.
 Lost updates
 Inconsistent analysis

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❖ TRANSPARENCY
 Distributed systems should be perceived by users and
application programmers as a whole rather than as a
collection of cooperating components.
 Transparency has different aspects.
 These represent various properties that distributed
systems should have.

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EXAMPLES OF DISTRIBUTED SYSTEMS

 Local Area Network and Intranet

 Database Management System

 Automatic Teller Machine Network

 Internet/World-Wide Web

 Mobile and Ubiquitous Computing

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LOCAL AREA NETWORK
email server Desktop
computer s
print and other servers

Local area
Web server network

email server
print
File server
other servers

the rest of
the Internet
router/firewall
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DATABASE MANAGEMENT SYSTEM

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AUTOMATIC TELLER MACHINE NETWORK

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ORGANIZATION OF A DISTRIBUTED SYSTEM

 to support heterogeneous computers and networks and to provide

a single-system view, a distributed system is often organized by
means of a layer of software called middleware that extends over
multiple machines

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a distributed system organized as middleware; note that the middleware layer

extends over multiple machines
INTRODUCTION TO ARCHITECTURAL MODEL

Architecture of distributed system:

 1. shared memory Architecture(Tightly coupled

system)

 2.Distributed memory Architecture(loosly coupled

system)

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 DISTRIBUTED COMPUTING MODELS

 Fundamental model: are based on some fundamental

properties such as: characterstics,failures &security.
1. Interaction models
2. failure model
3. security model
 Architectural models: It deals with organizations of
component across the network of computer & their
interrelationship.
1. client-server model
2. peer-to-peer model
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 ARCHITECTURAL MODEL

 Client-server model
▪ how are processes organized in a system
▪ thinking in terms of clients requesting services from
servers

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general interaction between a client and a server

 Peer-to-peer model
 Unlike client-server,p2p model does not distinguish
between client/server, instead each node can either be
a client or server depending on whether the node is
requesting or providing the service.
 Each component acts as its own process and acts as
both a client and a server to other peer components.
 Any component can initiate a request to any other peer
component.

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 FUNDAMENTAL MODELS

 1. Interaction model:
❖ computation occurs within processes.

❖ the processes interact by passing messages resulting in:

i. communication(information flow)
ii. coordination(synchronization & ordering of
activities)between processes
❖ Interaction model reflects the facts that communication
takes place with delays.
❖ Two variants of the interaction model

❖ I. Synchronized D.S has bounds on:

- process is executing in a known lower/upper bounded

time. 24

-message is received within a known bounded time.

II. A Synchronized D.S has no bounds on:
- process execution speed.
-message transmission delay.
-clocks drift rate.
2. Failure Model:
✓ it defines & classifies the faults.

✓ It is important to understand the kinds of failures that

may occur in a system.
Types of failure model:
*Fail stop
*crash
* omission
*Timing failure
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*Arbitrty
3.Security model:
✓ There are several potential threats a system designer need
be aware of:
1.Treats to processes: an attacker sends a request or response
using false identity.
2.Treats to communication channels: an attacker may listen
to message &save(evasdroping).
3.Denial of service: an attacker may overload a server by
making excessive request.
✓ Cryptography & authentication are often used to provide
security.

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 PROS AND CONS OF DISTRIBUTED SYSTEMS
Advantages of Distributed Systems
 Performance: Very often a collection of processors can provide higher
performance (and better price/performance ratio) than a centralized
computer.
 Distribution: many applications involve, by their nature, spatially
separated machines (banking, commercial, automotive system).
 Reliability (fault tolerance): if some of the machines crash, the
system can survive.
 Incremental growth: as requirements on processing power grow, new
machines can be added incrementally.
 Sharing of data/resources: shared data is essential to many
applications (banking, computer supported cooperative work,
reservation systems); other resources can be also shared (e.g. expensive
printers). 27

 Communication: facilitates human-to-human communication.

DISADVANTAGES OF DISTRIBUTED SYSTEMS

 Difficulties of developing distributed software:

how should operating systems, programming
languages and applications look like?
 Networking problems: several problems are
created by the network infrastructure, which have to
be dealt with: loss of messages, overloading, ...
 Security problems: sharing generates the problem
of data security.
TYPES OF DISTRIBUTION SYSTEMS

 Distributed computing systems

 Distributed information systems

 Distributed pervasive/embedded systems

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 DISTRIBUTED COMPUTING SYSTEMS

 Used for high-performance computing tasks

 Two types: Cluster computing and Grid computing

 Cluster Computing :A collection of similar workstations or

PCs (homogeneous), closely connected by means of a high-
speed LAN
 Each node runs the same operating system

 Used for parallel programming in which a single compute

intensive program is run in parallel on multiple machines
 Grid Computing: “Resource sharing and coordinated
problem solving in dynamic, multi-institutional virtual
organizations” (I. Foster)
 high degree of heterogeneity: no assumptions are made
concerning hardware, operating systems, networks,
administrative domains, security policies, etc. 30
 DISTRIBUTED INFORMATION SYSTEMS
 The vast amount of distributed systems in use today is in the form of
traditional information systems, that now integrate legacy systems.
Example: Transaction processing systems
BEGIN TRANSACTION(server, transaction);
READ(transaction, file-1, data);
WRITE(transaction, file-2, data);
newData := MODIFIED(data);
IF WRONG(newData) THEN
ABORT TRANSACTION(transaction);
ELSE
WRITE(transaction, file-2, newData);
END TRANSACTION(transaction);
END IF;
 Note:
 All READ and WRITE operations are executed, i.e. their effects are made permanent
at the execution of END TRANSACTION.
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 Transactions form an atomic operation.
TRANSACTION PROCESSING SYSTEMS
A transaction is a collection of operations on the state of an object (database, object
composition, etc.) that satisfies the following properties (ACID):
 Atomicity: All operations either succeed, or all of them fail.
- When the transaction fails, the state of the object will remain unaffected by the
transaction.
 Consistency: A transaction establishes a valid state transition.
- This does not exclude the possibility of invalid,
intermediate states during the transaction’s execution.
 Isolation: Concurrent transactions do not interfere with each other.
- It appears to each transaction T that other transactions occur either before T,
or after T, but never both.
 Durability: After the execution of a transaction, its effects are
made permanent:
- Changes to the state survive failures.
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 DISTRIBUTED PERVASIVE SYSTEMS

A next-generation of distributed systems emerging in which the nodes are small,

wireless, battery-powered, mobile (e.g. PDAs, smart phones, wireless surveillance
cameras, portable ECG monitors, etc.), and often embedded as part of a larger
system.
 Three requirements for pervasive applications
Embrace contextual changes: a device is aware that its environment
may change all the time
Encourage ad hoc composition: devices are used in different ways by
different users
Recognize sharing as the default: devices join a system to access or
provide information
 Examples of pervasive systems :
Home Systems
Electronic Health Care Systems
Sensor Networks
HARDWARE AND SOFTWARE CONCEPTS

 Hardware Concepts
different classification schemes exist
 multiprocessors - with shared memory
 multicomputers - that do not share
memory
 can be homogeneous or heterogeneous
 ▪ a single
backbone

Basic
Organizations of a
Node

different basic organizations of processors and memories in distributed systems

Parallel system?
▪ Multiprocessors - Shared Memory (1)
▪ the shared memory has to be coherent - the same value written by
one processor must be read by another processor
▪ performance problem for bus-based organization since the bus
will be overloaded as the number of processors increases
▪ the solution is to add a high-speed cache memory between the
processors and the bus to hold the most recently accessed words;
may result in incoherent memory

a bus-based multiprocessor
▪ bus-based multiprocessors are difficult to scale even with caches
▪ two possible solutions: crossbar switch and omega network
▪ Crossbar switch
▪ divide memory into modules and connect them to the processors
with a crossbar switch
▪ at every intersection, a crosspoint switch is opened and closed to
establish connection
▪ problem: expensive; with n CPUs and n memories, n2 switches
are required

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▪ Omega network
▪ use switches with multiple input and output lines
▪ drawback: high latency because of several switching stages
between the CPU and memory
▪ Homogeneous Multicomputer Systems
▪ also referred to as System Area Networks (SANs)
▪ the nodes are mounted on a big rack and connected through a
high-performance network
▪ could be bus-based or switch-based
▪ bus-based
▪ shared multiaccess network such as Fast Ethernet can be used
and messages are broadcasted
▪ performance drops highly with more than 25-100 nodes
(contention)
▪ switch-based
▪ messages are routed through an interconnection network
▪ two popular topologies: meshes (or grids) and hypercubes

Hypercube
Grid
▪ Heterogeneous Multicomputer Systems
▪ most distributed systems are built on heterogeneous
multicomputer systems
▪ the computers could be different in processor type, memory size,
architecture, power, operating system, etc. and the
interconnection network may be highly heterogeneous as well
▪ the distributed system provides a software layer to hide the
heterogeneity at the hardware level; i.e., provides transparency
 Software Concepts
 OSs in relation to distributed systems
 tightly-coupled systems, referred to as distributed
OSs (DOS)
 the OS tries to maintain a single, global view of
the resources it manages
 used for multiprocessors and homogeneous
multicomputers
 loosely-coupled systems, referred to as network
OSs (NOS)
 a collection of computers each running its own
OS; they work together to make their services
and resources available to others
 used for heterogeneous multicomputers
 Middleware: to enhance the services of NOSs so
that a better support for distribution transparency
is provided 42
▪ Summary of main issues

System Description Main Goal

Tightly-coupled operating system for multi- Hide and manage
DOS processors and homogeneous hardware
multicomputers resources
Loosely-coupled operating system for Offer local
NOS heterogeneous multicomputers (LAN and services to remote
WAN) clients
Provide
Additional layer atop of NOS implementing
Middleware distribution
general-purpose services
transparency

an overview of DOSs, NOSs, and middleware

▪ Distributed Operating Systems
▪ two types
▪ multiprocessor operating system: to manage the resources of a
multiprocessor
▪ multicomputer operating system: for homogeneous
multicomputers
▪ Uniprocessor Operating Systems
▪ separating applications from operating system code through a
microkernel
 Multiprocessor Operating Systems
▪ extended uniprocessor operating systems to support multiple
processors having access to a shared memory
▪ a protection mechanism is required for concurrent access to
guarantee consistency
▪ two synchronization mechanisms: semaphores and monitors
▪ semaphore: an integer with two atomic operations down (if s=0
then sleep; s := s-1) and up (s := s+1; wakeup a sleeping process
if any)
▪ monitor: a programming language construct consisting of
procedures and variables that can be accessed only by the
procedures of the monitor; only a single process at a time is
allowed to execute a procedure

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 Multicomputer Operating Systems
▪ processors can not share memory; instead communication is
through message passing
▪ each node has its own
▪ kernel for managing local resources
▪ separate module for handling interprocessor communication

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general structure of a multicomputer operating system
▪ Distributed Shared Memory Systems
▪ how to emulate shared memories on distributed systems to
provide a virtual shared memory
▪ page-based distributed shared memory (DSM) - use the virtual
memory capabilities of each individual node

pages of address space distributed among four machines

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 situation after CPU 1 references page 10

▪ read-only pages can be easily replicated

situation if page 10 is read only and replication is

used
 Network Operating Systems
 possibly heterogeneous underlying hardware
 constructed from a collection of uniprocessor
systems, each with its own operating system
and connected to each other in a computer
network

general structure of a network operating system

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 Services offered by network operating systems
 remote login (rlogin)
 remote file copy (rcp)
 shared file systems through file servers

two clients and a server in a network operating system

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▪ Middleware
▪ a distributed operating system is not intended to handle a collection
of independent computers but provides transparency and ease of
use
▪ a network operating system does not provide a view of a single
coherent system but is scalable and open
▪ combine the scalability and openness of network operating systems
and the transparency and ease of use of distributed operating
systems
▪ this is achieved through a middleware, another layer of software
general structure of a distributed system as middleware

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 different middleware models exist
 treat every resource as a file; just as in UNIX
 through Remote Procedure Calls (RPCs) -
calling a procedure on a remote machine
 distributed object invocation
▪ middleware services
▪ access transparency: by hiding the low-level message passing
▪ naming: such as a URL in the WWW
▪ distributed transactions: by allowing multiple read and write
operations to occur atomically
▪ security
▪ 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

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▪ a comparison between multiprocessor operating systems,
multicomputer operating systems, network operating systems,
and middleware-based distributed systems

Distributed OS Network Middleware-

Item
Multiproc Multicomp OS based OS

Degree of transparency Very High High Low High

Same OS on all nodes Yes Yes No No

Number of copies of
1 N N N
OS
Basis for Shared Model
Messages Files
communication memory specific
Global, Global,
Resource management Per node Per node
central distributed
Scalability No Moderately Yes Varies
Openness Closed Closed Open Open
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THANK YOU FOR YOUR ATTENTION

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